Abstract: A sample applicator for use in performing immunoassays comprises a tube defining an internal lumen and having a filter matrix at one end thereof. Sample is drawn into the applicator tube, either using a vacuum bulb or an automated assay device. The sample is filtered as it enters the tube, and is again filtered as it discharged from the tube in the opposite flow direction. By removing particulates from the liquid sample, assay performance can be greatly improved. The applicator may also comprise a reagent dispersed within a permeable matrix, which is useful for introducing reagent to a sample or other liquid assay component. As a liquid assay component is drawn into the applicator tube, the reagent is reconstituted by and mixed with the liquid, thereby reducing the number of steps required for the assay.