There are two general approaches to diagnosing diseases by immunoassays:  testing for specific antigens or testing for antigen-specific antibodies.  Enzyme linked immunosorbent assays (ELISA), also known as enzyme immunoassays (EIA), are tests designed to detect antigens or antibodies by producing an enzyme triggered color change. 

All of the EIAs performed in the Diagnostic Serology Section are known as solid-phase assays.  This assay requires the immobilization of antigens or antibodies on solid surfaces such as plastic beads or the wells of microtiter plates.

The following table lists the types of EIAs for each of the tests 
performed in the Medical Serology Branch.

EIAs for Antigen Detection

The only tests performed in the Diagnostic Serology Section for antigen detection are Hepatitis B EIAs.  They are direct EIAs which have four steps:

1. Antigen-specific antibody is attached to a solid-phase surface (plastic beads)
2. Test specimen is added, which may or may not contain the antigen
3. An enzyme-labeled antibody specific to the antigen is added (conjugate)
4. Chromogenic substrate is added, which in the presence of the enzyme, changes color.  The amount of color that develops is proportional to the amount of antigen in the test specimen.

EIAs for Antibody Detection

All other EIAs performed in the Diagnostic Serology Section are for antibody detection.  There are three types of antibody detection EIAs:   noncompetitive EIAs, competitive EIAs, and capture EIAs. 

Noncompetitive EIA

1. Specific antigen is attached to a solid-phase surface (plastic bead or microtiter well)
2. Test specimen is added, which may or may not contain the antibody
3. An enzyme-labeled antibody specific to the test antibody is added (conjugate)
4. Chromogenic substrate is added, which in the presence of the enzyme, changes color.  The amount of color that develops is proportional to the amount of antibody in the test specimen.

Competitive EIA

1. Specific antigen is attached to a solid-phase surface (plastic bead or microtiter well)
2. Test specimen, which may or may not contain the antibody, and an enzyme-labeled antibody specific to the test antibody (conjugate) are added together
3. Chromogenic substrate is added, which in the presence of the enzyme, changes color.  The amount of color that develops is inversely proportional to the amount of antibody in the test specimen.

Capture EIA

A capture EIA is designed to detect a specific type of antibody, such as IgG or IgM.

1. Antibody specific for IgG or IgM is attached to a solid-phase surface (plastic bead or microtiter well)
2. Test specimen containing IgG or IgM is added
3. Specific antigen is added
4. An enzyme-labeled antibody specific to the test antigen is added (conjugate)
5. Chromogenic substrate is added, which in the presence of the enzyme, changes color.  The amount of color that develops is proportional to the amount of antigen-specific IgG or IgM in the test specimen.