Using the home pregnancy test as an example, the sample, in this case urine, is introduced to the sample pad, which holds the fluid like a sponge. The fluid then moves (flows) to the conjugate pad, which stores the conjugated antibodies and labels used to detect the analyte. There are several different labels but in most tests the label is gold, which presents as red in colour. In pregnancy tests the analyte tends to be latex, which is blue.
In a pregnancy test the analyte being detected is human chorionic gonadotropin (hCG), a hormone produced by the placenta. If this target analyte is present, the conjugated antibodies and labels will bind to it and continue to move along the test. A coloured line will form at the test line, giving a simple visual confirmation that the analyte has been detected. An independent control line contains an antibody that captures the free labels to confirm the test is working properly.