Sensitivity is a function of the position of the line on the membrane (10). Because sample flow is unidirectional, the formation of the immunocomplex is a nonequilibrium reaction. Interaction of the detector particle with the capture reagent is related to the length of time that the two are sufficiently close at the molecular level. Once a detector particle has passed beyond the capture reagent, it no longer has the potential to contribute to a signal. Therefore, increasing the flow rate of the membrane increases the speed with which the detector particle passes the capture reagent, reducing the probability of an interaction and, consequently, the sensitivity.

Sensitivity is also affected by the change in flow rate as the sample moves along the membrane (10). The flow rate decays exponentially as the liquid front moves through the membrane (Fig. 3). When the sample front reaches the capture reagent line, the overall rate at which the detector particles pass is dictated by the flow profile distal to the test line, not by the faster flow profile between the conjugate pad and the test line. Placement of the line closer to the conjugate pad results in lower sensitivity because, in total, the detector particles move past the capture reagent more rapidly. This aspect of sensitivity presents a major challenge when a single test strip is being used to screen for several drugs of abuse. For each capture reagent line, the corresponding detector particles have a different flow rate profile, requiring that the concentration of each pair be optimized for its position on the membrane.

Also, variation in the release of the detector particle from the conjugate pad between test strips introduces variation into the sensitivity (10). A test strip exhibiting rapid release of the detector particle will produce lower sensitivity because most of the particles pass the test line at a relatively fast rate. If release of the detector particles is delayed, they pass the test line at a slower rate, resulting in a greater probability of interaction with the capture reagent. Although this phenomenon can increase sensitivity, it can also cause false positives resulting from nonspecific interactions. For these reasons, it is important to design the strip so that detector particle release is consistent.

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