Arguably, the most important aspect of membrane performance in a lateral flow test is capillary flow time—the time required for a liquid sample to migrate through the pores in the lateral direction (10). Capillary flow time is synonymous with wicking time (2). The flow time affects the appearance of the lines when they are applied during strip manufacture and the sensitivity of the final test when the strip is run. Capillary flow time is expressed in terms of time per distance, with the length of the strip kept constant. (Millipore reports capillary flow time as s/4-cm distance .) One end of the strip is placed in a liquid reservoir, and the time that it takes for the liquid to wet out the strip by capillary flow is measured. The flow front of the liquid should be uniform across the strip. Unevenness of the flow front indicates uneven wetting of the pores, which could arise from variation in wettability or pore structure. Many lateral-flow tests incorporate membrane strips that are 2 to 2.5 cm in length. Although lateral flow times can be measured on strips this short, the precision of the measurements is typically lower because of difficulties in handling small coupons. Longer strips are easier to handle, and the endpoint is easier to visualize.
Capillary flow times span a wide range. The fastest membranes have flow times of approx 60 s/4 cm; the slowest membranes have flow times of 240300 s/4 cm (10). The choice of flow time for a given assay depends on the requirements for sensitivity and specificity, the availability of the critical immunoreagents, and the required time to reach endpoint. Faster-flowing membranes reach endpoint more quickly but require more reagents and may lack the required sensitivity. Slower membranes can be used when reagents are in limited supply or costly, with the tradeoff that it will take longer to run the test.
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