Sample Pad, Conjugate Pad, and Absorbent Pad Qualification for Lateral Flow IVD Products

Why Pad Qualification Is a Make-or-Break Activity in Lateral Flow Development

A lateral flow immunoassay strip is, at its core, a capillary flow system built from overlapping porous materials mounted on a pressure-sensitive adhesive backing card. The test and control lines on the nitrocellulose membrane get most of the attention during development, but the three pad components — the sample pad, conjugate pad, and absorbent pad — collectively govern whether an assay actually works reliably in production. They control what enters the strip, how the detector reagent is released and carried forward, and whether flow completes fully so that unbound material is washed away from the read zone.

Get any one of these pads wrong, and the consequences are immediate and measurable: elevated background, weak or inconsistent test line intensity, false positives from incomplete washing, lot-to-lot variability that destroys reproducibility, and stability failures during shelf-life studies. The challenge is that pad materials are not inert scaffolds. They are chemically active, hygroscopic, lot-variable, and sensitive to storage conditions. Qualifying them requires a framework that covers incoming material specifications, pretreatment chemistry, functional assay performance, supplier management, and ongoing lot-to-lot consistency verification.

This guide covers the qualification of sample pads, conjugate pads, and absorbent pads for lateral flow IVD products in depth. It addresses material selection, pretreatment chemistry, specification development, supplier qualification, and the regulatory expectations under FDA QMSR and ISO 13485 that govern how these materials must be controlled throughout the product lifecycle.

The Role of Each Pad in the Lateral Flow Strip

Before diving into qualification, it is important to understand what each pad does and why its material properties matter. A lateral flow strip typically consists of four porous materials assembled in sequence on an adhesive backing card: the sample pad, the conjugate pad, the nitrocellulose membrane, and the absorbent pad. Each adjacent pair overlaps by approximately 2 mm, creating a continuous capillary path from the sample application point to the absorbent sink.

The sample pad is where the specimen first contacts the strip. It must receive the sample, filter out particulates and cells, normalize the sample matrix (buffer pH, adjust salt concentration, add blocking agents), and deliver a consistent flow of treated sample onto the conjugate pad.

The conjugate pad holds the dried detector reagent — typically gold nanoparticles, colored or fluorescent latex beads, or magnetic particles conjugated to antibodies or antigens. Upon contact with the flowing sample, it must release the conjugate rapidly, completely, and reproducibly so that the detector particles travel with the sample front through the nitrocellulose membrane and interact with the capture lines. The conjugate pad system is widely recognized as responsible for the majority of variation observed in lateral flow immunoassay performance — variation in pad material leads to inconsistent uptake of pretreatment liquids and conjugates, destabilization of dried conjugates, poor or incomplete release, and nonspecific binding of conjugates to hydrophobic fibers (O'Farrell, 2009). This makes conjugate pad qualification one of the highest-risk activities in lateral flow development.

The absorbent pad acts as the sink at the far end of the strip. It receives all the fluid that has passed through the membrane and, through its wicking action, maintains the capillary pressure that drives continuous flow. It must have enough capacity to absorb the full volume of sample and running buffer while sustaining flow long enough to wash unbound conjugate away from the test and control lines.

None of these functions is optional. A failure in any one pad propagates through the entire assay.

Sample Pads: Qualification from Material Selection to Incoming Specification

Primary Functions

The sample pad is the first point of contact between the specimen and the assay. It performs several critical functions simultaneously:

• Sample reception and distribution. The pad must accept the applied specimen volume and distribute it evenly across its width, ensuring uniform flow onto the conjugate pad. Uneven distribution leads to streaking, patchy flow, and inconsistent test line morphology.
• Filtration. For specimens containing particulates, cells, or debris — whole blood, saliva, nasal swab eluate — the sample pad acts as a depth filter, trapping material that would otherwise clog the nitrocellulose membrane or interfere with the immunoassay chemistry.
• Matrix normalization. The pretreatment chemistry loaded into the sample pad must bring the specimen matrix into the range where the immunoassay performs optimally. This includes adjusting pH, normalizing ionic strength, and introducing blocking agents that prevent nonspecific binding downstream.
• Flow control. The sample pad's wicking characteristics govern how quickly the sample reaches the conjugate pad and the membrane. Too fast, and the detector conjugate may not release completely. Too slow, and the assay running time becomes unacceptably long, or flow may not complete at all.

Material Options and Trade-Offs

Cellulose fiber filters are the most widely used material for sample pads. They offer high bed volumes, which means they can hold significant quantities of pretreatment solution. This makes them well suited for assays that require extensive matrix conditioning — where the sample pad must deliver substantial amounts of buffer salts, blocking proteins, surfactants, and other reagents into the specimen as it flows through. Cellulose pads are highly absorbent and hygroscopic, which is advantageous for holding pretreatment chemistry but creates handling and storage considerations. Their tensile strength is lower than glass fiber, making them more susceptible to tearing during reel-to-reel processing. Merck SureWick cellulose pads and Whatman/CF series grade pads from Cytiva are common choices, available in strip, sheet, and roll formats.

Glass fiber filters offer lower bed volumes and higher tensile strength. The lower bed volume is a significant limitation when extensive pretreatment is required, because the pad simply cannot hold enough treatment solution. However, glass fiber's more open, porous structure can be advantageous for viscous or limited-volume specimens such as saliva and nasal swab eluate, where the sample must pass through the pad quickly without being excessively retained. Glass fiber is also the material of choice when the sample pad's role is primarily filtration rather than chemical conditioning.

Woven mesh materials provide low bed volume and high tensile strength but limited chemical loading capacity. They are used in specific applications where filtration and mechanical durability are the primary requirements.

Synthetic fiber blends are gaining adoption, driven by the need for more consistent manufactured materials with tighter lot-to-lot uniformity. Porex TruFlow pads, for example, are based on synthetic polyolefin fibers and contain no glass fiber or cellulose. They offer high tensile strength for manufacturing and consistent wicking behavior, though they represent a different chemical surface that may require re-optimization of pretreatment formulations.

The key physical parameters for any sample pad material are:

• Thickness (measured in micrometers at a specified pressure), which affects bed volume and how the pad fits into the strip assembly
• Wicking rate (typically measured in seconds per 4 cm), which determines how quickly the sample passes through and onto the conjugate pad
• Water absorption capacity (mg/cm²), which determines how much pretreatment solution the pad can hold and how much sample volume it can process

These three parameters must be controlled within defined ranges, because they directly affect assay performance.

Pretreatment Chemistry

Sample pad pretreatment is one of the most important and most frequently underestimated steps in lateral flow assay development. The purpose of pretreatment is to condition the specimen into a matrix in which the immunoassay chemistry performs reliably and reproducibly. Without pretreatment, raw specimen variability — in pH, ionic strength, protein content, viscosity, and the presence of interfering substances — would make consistent assay performance impossible.

A typical sample pad treatment solution contains several classes of reagents:

• Buffer salts to normalize sample pH to the assay's optimal range. Phosphate-buffered saline (PBS) is the most common choice.
• Surfactants to reduce surface tension, improve wicking, and prevent nonspecific adsorption of proteins to the pad and membrane surfaces. Tween 20 (polysorbate 20) is the industry standard.
• Blocking proteins to saturate binding sites on the pad and membrane surfaces, reducing background noise and false positives. Bovine serum albumin (BSA) is the most widely used.
• Salts for ionic strength adjustment. Sodium chloride at defined concentrations helps control electrostatic interactions that affect antibody-antigen binding kinetics.
• Polymers such as polyvinylpyrrolidone (PVP) to modify flow behavior and reduce nonspecific binding.
• Preservatives to prevent microbial growth in the treated pad during storage. Sodium azide at 0.1% is common.

A representative sample pad treatment formulation from the literature is: 1x PBS containing 0.1 M NaCl, 0.2% Tween 20, and 0.1% sodium azide. This formulation provides pH buffering, ionic strength normalization, surfactant activity, and preservation. It is a starting point; actual formulations are optimized for each specific assay during development.

The treatment protocol involves immersing or spraying the pad material with the treatment solution, followed by controlled drying. A typical protocol is drying at 50°C for 2 hours, though this must be optimized for each formulation and pad material to ensure complete drying without degrading heat-sensitive components. Residual moisture in a treated pad is a common source of lot-to-lot variability and stability failures.

Sample Type Considerations

The choice of sample pad material and pretreatment chemistry is strongly influenced by the specimen type the assay is designed to accept.

Urine specimens are relatively low in viscosity and protein content. Cellulose fiber pads are generally adequate, and pretreatment can be less aggressive — primarily buffering and surfactant addition. Filtration requirements are minimal.

Whole blood presents the most demanding filtration challenge. Red blood cells, white blood cells, and platelets must be physically separated from the plasma before the sample reaches the nitrocellulose membrane. Specialized plasma separation pads or dual-layer sample pad configurations are often required. The sample pad must also contain agents that prevent coagulation. Pretreatment chemistry must account for the high protein content and cellular debris in whole blood.

Saliva is viscous due to the presence of mucins and other glycoproteins. Glass fiber pads are often preferred for saliva-based assays because their more open structure allows the viscous sample to pass through without excessive retention. Pretreatment must include mucolytic agents or specific surfactant formulations to break down mucins and prevent them from clogging the membrane. Saliva also has variable pH and ionic strength depending on collection method and individual physiology, requiring robust buffering in the pretreatment.

Nasal swab specimens involve elution of the swab into a buffer, producing a sample with moderate viscosity and variable volume. Volume is often limited — the swab may retain a significant portion of the eluate — so the pad material must efficiently capture and transport small volumes. Glass fiber or specialized low-volume collection pads are common choices.

Conjugate Pads: The Critical Link Between Sample and Signal

Primary Functions

The conjugate pad occupies a uniquely demanding position in the lateral flow strip. It must:

• Hold the dried detector conjugate in a stable, inactive state during storage. The conjugate — whether gold nanoparticles, latex beads, fluorescent microspheres, or magnetic particles conjugated to antibodies or antigens — is the signaling reagent that ultimately produces the visible (or instrument-read) result.
• Release the conjugate rapidly and completely upon contact with the flowing sample. Incomplete release means less conjugate reaches the capture lines, reducing sensitivity. Slow release means the conjugate may lag behind the sample front, degrading assay kinetics.
• Synchronize conjugate release with sample flow. The timing of conjugate release relative to the advancing sample front affects how the detector particles distribute along the membrane and interact with the capture lines. Consistent release kinetics are essential for reproducible test line intensity.
• Maintain conjugate stability over the product's claimed shelf life under defined storage conditions. The pad material and its pretreatment chemistry must protect the conjugate from aggregation, denaturation, and nonspecific adsorption during drying and storage.

Material Options

Glass fiber filters are the most common conjugate pad material. They offer good release characteristics — the conjugate tends to release cleanly from the glass fiber surface upon rehydration — and sufficient mechanical properties for reel-to-reel processing. Glass fiber pads from Merck (SureWick), Ahlstrom (ReliaFlow), and Cytiva are widely used in commercial lateral flow products.

Surface-treated polyester and polypropylene pads offer hydrophilic surfaces engineered for uniform conjugate distribution and release. Ahlstrom's ReliaFlow conjugate pad line includes both glass and polyester options, with ongoing process improvements focused on fiber uniformity and lot-to-lot consistency.

Cellulose filters can be used for conjugate pads but are less common. Their higher bed volume means more conjugate solution can be loaded, but the release characteristics tend to be less uniform than glass fiber.

Key material selection criteria for conjugate pads include uniform fiber distribution (which affects conjugate distribution across the pad width), consistent release kinetics between lots, and mechanical properties suitable for the chosen manufacturing process.

Cytiva provides detailed specifications for its conjugate pad materials that illustrate the range of available properties:

• LF1: 247 µm thickness, 35.6 s/4cm wicking rate, 25.3 mg/cm² water absorption
• MF1: 367 µm thickness, 29.7 s/4cm wicking rate, 39.4 mg/cm² water absorption
• Fusion 5: 370 µm thickness, 43.9 s/4cm wicking rate, 42.3 mg/cm² water absorption
• VF2: 785 µm thickness, 23.8 s/4cm wicking rate, 86.2 mg/cm² water absorption

These specifications matter because they define how much conjugate solution can be loaded per unit area, how quickly the conjugate pad rehydrates upon sample contact, and how the pad integrates into the strip assembly. Selecting the right material is an optimization that must be done empirically during development.

Conjugate Pad Treatment and Drying

The conjugate pad buffer formulation serves a different purpose than the sample pad treatment. Where the sample pad formulation conditions the incoming specimen, the conjugate pad formulation must stabilize the dried conjugate and enable its rapid, complete rehydration and release.

A representative conjugate pad buffer from the literature is: 20 mM sodium borate containing 2% BSA, 3% sucrose, 0.6 M NaCl, 0.2% Tween 20, and 0.1% sodium azide. Each component plays a specific role:

• Sodium borate provides pH buffering in the alkaline range, which is often optimal for gold nanoparticle conjugate stability.
• BSA serves as a blocking and stabilizing agent, preventing nonspecific adsorption of the conjugate to the pad fibers and protecting the antibody or antigen on the conjugate surface during drying.
• Sucrose is a critical stabilizer. During drying, sucrose forms an amorphous glassy matrix that protects the protein component of the conjugate from denaturation and aggregation. The 3% concentration is typical, though this is optimized for each conjugate.
• NaCl at relatively high concentration (0.6 M) helps control the electrostatic interactions between the conjugate particles and the pad fibers, promoting clean release.
• Tween 20 ensures the rehydrated conjugate solution has appropriate surface tension for capillary flow through the membrane.
• Sodium azide prevents microbial growth in the dried conjugate pad.

The conjugate is mixed with the storage buffer, and the resulting solution is applied to the pad material by immersion, spraying, or dispensing. After application, the pad is dried under controlled conditions. A typical drying protocol is incubation at 37°C for 1 hour, though this varies based on the pad material, the conjugate volume loaded, and the stability requirements of the specific conjugate. Overdrying at excessive temperature can degrade the conjugate; insufficient drying leaves residual moisture that accelerates conjugate degradation during storage.

Release Qualification

Qualifying the conjugate pad is fundamentally about qualifying conjugate release. The key questions are:

• Is the active detector particle released completely upon sample contact? Incomplete release directly reduces assay sensitivity.
• Are release kinetics consistent across lots? Variability in release timing produces variability in test line intensity.
• Does the rehydrated conjugate maintain its functional properties — binding activity, colloidal stability, absence of aggregation — after release from the pad?
• Does the released conjugate produce the expected capture line intensity and signal-to-background ratio in the finished assay?

Release qualification is assessed functionally by testing conjugate pads from multiple material lots in the actual assay and measuring test line intensity, control line intensity, and background. Physical parameters (wicking rate, absorption capacity) are necessary but not sufficient — they are incoming material specifications, not predictors of functional performance on their own.

Absorbent Pads: The Unsung Driver of Flow Completion

Primary Functions

The absorbent pad is the simplest pad conceptually but critically important. It must:

• Act as a reservoir and sink for all the fluid that passes through the nitrocellulose membrane — the sample, the pretreatment reagents released from the sample pad, the conjugate released from the conjugate pad, and any running buffer introduced upstream.
• Maintain continuous capillary flow through the membrane via sustained wicking action. As long as the absorbent pad has unused absorption capacity, capillary pressure drives flow from the sample pad end toward the absorbent pad end. When the absorbent pad saturates, flow stops.
• Ensure complete sample transport past the capture lines. If flow stops before the sample front has fully traversed the test and control lines, unbound conjugate remains in the reading zone, producing elevated background and potential false positives.
• Provide a wash function. The volume of fluid that enters the strip after the initial sample front passes through the membrane acts as a wash, carrying unbound conjugate and sample components away from the capture lines. This wash effect is essential for achieving the signal-to-background ratio that the assay requires.

Material Options

Cellulose fibers are the most common absorbent pad material. They offer high bed volumes and high absorption capacity at relatively low cost. Merck SureWick absorbent pads, Cytiva CF-series cellulose pads, and Ahlstrom absorbent materials are all widely used. Cellulose pads are available in various thicknesses, allowing the developer to match absorption capacity to the assay's volume requirements.

Glass fiber is an alternative for specific applications where the chemical compatibility or mechanical properties of glass fiber are preferred, though cellulose's higher absorption capacity makes it the default choice for most assays.

Synthetic polyolefin fibers represent a newer category. Porex TruFlow absorbent pads are synthetic, containing no glass fiber or cellulose. They offer high tensile strength, which is advantageous for reel-to-reel manufacturing processes where the pad material must withstand tension without tearing. The absorption characteristics differ from cellulose and must be characterized during development.

Key parameters for absorbent pad selection are:

• Absorption capacity (mg/cm² or total volume per unit area), which must substantially exceed the total fluid volume the assay generates
• Thickness, which determines both capacity and how the pad integrates into the cassette housing
• Tensile strength, which affects manufacturability in high-speed reel-to-reel processing

Capacity Requirements

The absorbent pad's absorption capacity must be significantly larger than the total volume of all fluids that enter the strip during the assay. This includes the applied sample volume plus the volume of any pretreatment reagents released from the sample pad and conjugate pad. If the absorbent pad reaches capacity before flow completes, the assay will exhibit incomplete washing, high background, and potentially invalid results.

A common mistake is assuming that a thicker pad automatically provides more absorption capacity. Thickness and capacity are related but not directly proportional — the actual absorption capacity must be measured for each material grade, not estimated from dimensions. A thinner cellulose pad with a high absorption coefficient may outperform a thicker pad made from a less absorbent material.

Both total volume capacity and sustained wicking rate must be evaluated. A pad may have sufficient total capacity but a wicking rate that slows as it approaches saturation, leading to incomplete washing in the final seconds of the assay's running time.

Qualification Framework: From Incoming Material to Functional Performance

Incoming Material Qualification

Incoming material qualification for pads involves establishing and verifying specifications at three levels:

Identity testing confirms that the material received is the material ordered. For pads, this typically involves verifying the product grade, lot number, and supplier certificate of conformance. In some cases, Fourier-transform infrared spectroscopy (FTIR) or similar analytical methods may be used for material identity confirmation.

Physical specification testing measures the quantifiable properties of the pad material against established acceptance criteria. The core physical parameters are:

• Thickness at a defined pressure, measured in micrometers. Thickness affects how the pad fits into the strip assembly, the overlap with adjacent materials, and the total fluid volume the pad can hold. Measured with a micrometer or thickness gauge under a specified load.
• Wicking rate, typically measured as the time in seconds for a colored solution to advance 4 cm along the material. This is a direct measure of capillary flow performance and is one of the most important incoming specifications.
• Water absorption capacity, measured in mg/cm². This defines how much fluid the pad can hold per unit area and directly relates to the pad's ability to perform its function — whether that is holding pretreatment chemistry (sample pad), conjugate solution (conjugate pad), or acting as a fluid sink (absorbent pad).

These three parameters must be measured for each incoming lot and compared against statistically derived acceptance criteria based on development and historical data.

Functional testing assesses the pad's performance in the actual assay. This is the ultimate qualification: the pad is assembled into test strips and evaluated for test line intensity, control line intensity, background signal, flow completion time, and overall assay sensitivity and specificity. Functional testing is the most resource-intensive but also the most meaningful specification, because it directly measures what matters — whether the pad produces acceptable assay performance.

The relationship between physical specifications and functional performance is not always linear or predictable. A pad lot that is within specification for thickness, wicking rate, and absorption capacity may still produce suboptimal assay performance due to factors not captured by physical testing — surface chemistry variations, fiber distribution differences, or residual processing chemicals from manufacturing. This is why functional testing, at least on a periodic or skip-lot basis, is essential even when physical specifications are met.

Lot-to-Lot Consistency

Lot-to-lot consistency is the central challenge in pad qualification. The porous materials used for lateral flow pads are manufactured by wet-laying or dry-laying fibers, and the resulting products exhibit inherent variability between production lots. This variability is not a defect — it is a characteristic of the material category. The qualification framework must accommodate and control it.

To establish lot-to-lot consistency:

1. During development, test a sufficient number of material lots (at minimum three, ideally five or more) to characterize the range of variability in physical properties and functional performance.
2. Use this data to establish incoming acceptance criteria that are tight enough to ensure consistent assay performance but not so tight that they reject functional lots based on physical parameters alone.
3. For each new incoming lot, perform physical testing against these criteria. If the lot passes physical testing, perform functional testing in the assay to confirm performance.
4. Track functional performance data over time using control charts or similar statistical tools to detect trends, shifts, or gradual drift in material properties that might not trigger a single-lot failure but could indicate a supplier process change.

Supplier Qualification Under ISO 13485

Pad materials are critical raw materials in a lateral flow IVD product. They are not commodity items that can be purchased on specification alone without supplier oversight. ISO 13485 clause 7.4 and FDA QMSR require that suppliers of critical materials be qualified, monitored, and periodically re-evaluated.

Initial supplier assessment should evaluate the supplier's quality management system, manufacturing controls, ability to provide consistent material, willingness to share detailed material specifications and lot data, and capacity for change notification. On-site audits may be warranted for sole-source or high-risk materials.

Quality agreements with pad material suppliers must include specific provisions for critical raw materials. At minimum, the agreement should require the supplier to:

• Notify the device manufacturer before making any changes to the material composition, manufacturing process, equipment, or manufacturing site
• Provide lot-specific certificates of analysis with measured physical properties
• Maintain traceability records sufficient for the device manufacturer's regulatory obligations
• Cooperate with the device manufacturer's supplier audits and corrective action requests

Change notification requirements are particularly important for pad materials. A supplier's decision to change a fiber source, modify a binder formulation, adjust a web-forming process parameter, or move production to a different line can alter the pad's performance in ways that are not apparent from the supplier's standard physical testing. The device manufacturer must evaluate any material change through functional testing before the changed material enters production.

Periodic re-evaluation of pad suppliers should include a review of delivery performance, quality metrics (lot rejection rates, complaint trends), responsiveness to quality issues, and the results of any audits or assessments conducted during the review period.

Specification Development

Developing specifications for pad materials requires balancing several competing considerations:

Physical specification ranges should be based on statistical analysis of development and production data. The acceptable range for each physical parameter (thickness, wicking rate, absorption capacity) is the range within which functional assay performance remains acceptable. Setting specifications based solely on the supplier's nominal range, without correlating to functional performance data, is a common mistake that leads to accepting out-of-range lots or rejecting functional lots.

Functional specifications define acceptable assay performance when a pad lot is assembled into test strips. These specifications — minimum test line intensity, maximum background, flow completion time, sensitivity — are the true acceptance criteria. Physical specifications are surrogate tests that predict functional performance but do not replace it.

The hierarchy is: functional performance is the primary acceptance criterion. Physical parameters are secondary, incoming screening criteria used for efficiency. When physical and functional results conflict — a lot passes physical testing but fails functional testing, or vice versa — the functional result governs.

A practical approach is to use physical specifications for routine incoming lot acceptance, supplemented by periodic functional testing (e.g., every third lot, or when a physical parameter is near its specification limit) to verify the correlation between physical and functional performance.

Overlap and Assembly Considerations

The physical assembly of the lateral flow strip — how the pads overlap on the backing card — directly affects flow behavior and assay performance. The standard overlap between adjacent pads is approximately 2 mm. This overlap creates the capillary bridge that transfers fluid from one material to the next.

Overlap dimensions are a controlled assembly parameter. Too little overlap, and flow may not transfer reliably between materials, leading to intermittent flow failures. Too much overlap, and the flow characteristics of the junction change, potentially altering flow rate and conjugate release timing. The overlap for each junction — sample pad to conjugate pad, conjugate pad to membrane, membrane to absorbent pad — must be specified, controlled during manufacturing, and verified during in-process inspection.

The backing card itself matters. The adhesive must not migrate into the porous materials (which would block capillary flow), and the card must be dimensionally stable under the temperature and humidity conditions of manufacturing and storage. Pad compression during lamination affects thickness and flow properties and must be controlled.

Stability Implications

Pad pretreatment chemistry and conjugate loading introduce reagents that have their own stability profiles. The treated pads are not inert — they contain buffers, proteins, surfactants, preservatives, and dried conjugate that can degrade over time.

Pre-treated pad shelf life must be established through stability studies. The key stability concerns for treated pads are:

• Degradation of pretreatment components (protein denaturation, buffer pH drift, preservative depletion)
• Absorption of environmental moisture (hygroscopic pads can absorb water from ambient humidity, altering their performance)
• Oxidation or chemical degradation of the conjugate on the conjugate pad

Stability studies should evaluate treated pad performance at defined intervals under the intended storage conditions (temperature, humidity), using functional assay performance as the primary endpoint. Accelerated stability data may supplement but should not replace real-time stability data.

Impact on finished product stability claims is a system-level consideration. The shelf life of the finished lateral flow test is limited by the least stable component. If the conjugate pad degrades at 18 months while all other components are stable at 24 months, the product's shelf life claim is 18 months unless the conjugate pad formulation is improved.

Storage conditions for both treated pads and finished products must be defined and controlled. Most lateral flow products specify storage at 4-30°C with specific humidity limits. Including desiccant in the foil pouch is standard practice and directly addresses the hygroscopic nature of the pad materials.

Regulatory Expectations for Pad Material Control

Under FDA QMSR (effective February 2, 2026), which incorporates ISO 13485:2016 by reference, pad materials used in lateral flow IVD products are purchased components subject to purchasing controls under clause 7.4. As critical raw materials that directly affect device performance, they require a higher level of control than non-critical purchased items.

Specific regulatory expectations include:

• Documented procedures for evaluating and selecting pad material suppliers based on their ability to meet specified requirements (ISO 13485 clause 7.4.1)
• Purchasing information that describes the pad material with sufficient detail to ensure the correct material is received, including product grade, physical specifications, and any special requirements (clause 7.4.2)
• Verification of purchased product through incoming inspection and testing, with the extent of verification based on the risk associated with the material and the supplier's evaluation results (clause 7.4.3)
• Records of supplier evaluations, incoming inspection results, and any corrective actions taken in response to nonconforming material
• Change control procedures that ensure any change in pad material (supplier change, grade change, manufacturing process change) is evaluated for its impact on device performance before implementation

For FDA-submitted products (510(k), De Novo, PMA), the pad materials and their specifications are part of the Device Master Record. Changes to pad materials or suppliers may require regulatory assessment under 21 CFR 807.81 (for 510(k) devices) or PMA supplement requirements.

Under EU IVDR, Notified Bodies conducting conformity assessments will examine purchasing controls for critical raw materials as part of the quality system audit. The expectation is that the manufacturer has established appropriate specifications, conducts adequate incoming inspection, maintains supplier quality agreements, and can demonstrate lot-to-lot consistency through objective evidence.

Practical Recommendations

Based on the considerations above, the following practical recommendations emerge for qualifying sample pads, conjugate pads, and absorbent pads in lateral flow IVD products:

Establish tight internal specifications that are derived from your own development data and functional performance correlation, not simply adopted from the supplier's nominal ranges. Your internal specification for wicking rate, for example, may be narrower than the supplier's published range because you have identified the subrange that produces acceptable assay performance in your specific product.

Use functional assay performance as the primary acceptance criterion. Physical parameters are useful for incoming screening, but the final determination of whether a pad lot is acceptable must be based on whether it produces acceptable assay performance. Build functional testing into your incoming inspection protocol, at minimum on a skip-lot basis.

Qualify more than one material source when possible. Sole-source dependence on a single pad material supplier creates significant supply chain risk. If the sole supplier experiences a manufacturing disruption, makes an unannounced material change, or discontinues the product grade, the device manufacturer has no qualified alternative. Qualifying a second source requires investment but provides insurance against supply chain disruption.

Monitor supplier performance over time using statistical process control tools. Track incoming physical parameters and functional performance data on control charts. This allows early detection of gradual shifts in material properties before they cause out-of-specification results or assay performance failures.

Include pad material stability in your product stability program. Do not assume that pad materials are stable indefinitely. Establish shelf life for treated pads through formal stability studies and monitor treated pad performance as part of ongoing product stability testing.

Document everything. In a regulatory audit, the question is not whether you qualified your pad materials — it is whether you can demonstrate, through documented evidence, that your qualification was adequate, your specifications are appropriate, your incoming testing is sufficient, your supplier controls are effective, and your change management process is robust. Records of supplier evaluations, incoming inspection results, functional test data, stability data, and change assessments must all be maintained and accessible.

Key Suppliers and Available Materials

The pad materials market for lateral flow IVD products is served by several established suppliers, each offering a range of materials with different properties:

• Merck/Millipore (SureWick): Offers cellulose and glass fiber pad materials in strip, sheet, and roll formats, covering sample pads, conjugate pads, and absorbent pads. Well-established supplier with broad adoption in the lateral flow industry.
• Ahlstrom (ReliaFlow): Offers conjugate pad materials in both glass and polyester options, with ongoing process improvements focused on uniformity and lot-to-lot consistency. Also provides sample pad and absorbent pad materials.
• Whatman/Cytiva: Offers a comprehensive range of materials including the CF (cellulose), GF (glass fiber), Fusion 5 (integrated pad/membrane), LF1, MF1, and VF2 product lines. Detailed specifications for thickness, wicking rate, and absorption capacity are available for each grade.
• Porex (TruFlow): Offers synthetic polyolefin-based pads containing no glass fiber or cellulose, with high tensile strength suitable for demanding manufacturing processes. A newer entrant to the lateral flow pad market with a differentiated material platform.

Selecting a supplier is not just a procurement decision — it is a quality and regulatory decision. The supplier's manufacturing consistency, quality system maturity, willingness to provide detailed lot data and change notifications, and long-term commitment to the product line all factor into the qualification.

Conclusion

The sample pad, conjugate pad, and absorbent pad are individually complex components that collectively determine whether a lateral flow assay performs reliably. Qualifying these materials requires a multi-layered approach: understanding the functional requirements of each pad, selecting the right material grade, developing and validating pretreatment chemistry, establishing specifications that correlate physical parameters to functional performance, qualifying and monitoring suppliers, and maintaining control throughout the product lifecycle.

The investment in rigorous pad qualification pays dividends in manufacturing consistency, regulatory compliance, and — ultimately — the reliability of the diagnostic result that reaches the patient or clinician. Skipping or shortchanging this activity during development inevitably leads to problems during technology transfer, manufacturing scale-up, stability studies, or regulatory review. The pads are not peripheral components. They are the foundation on which assay performance is built.