This example of the very commonly used ELISA was used to quantitate Thyroid Stimulating Hormone (TSH) in human serum. Anti-TSH antibody is bound to each well of a 96-well plate, and serial dilutions of test serum are then added. After washing, a second (soluble) anti-TSH antibody is added, this one conjugated with an enzyme (e.g. horseradish peroxidase). The final step is to add a substrate for the enzyme which yields a colored product. The presence of color indicates the presence of TSH in the test serum. The first well of each row (labelled "b" is a no-antigen negative control, and the five sera are added at a series of two-fold dilutions beginning at 1:100. The dilution at which the color reaction is about half of maximum is taken as the "titer" of the serum.
In this example, serum #1 shows a titer of 1:1600, while serum #5 has a titer of about 1:400 (and therefore contains about 4 times less TSH than serum #1). This particular form of ELISA is referred to as a "sandwich ELISA" because the antigen which is being detected is "sandwiched" between two layers of antibody.