16.3.1: Introduction

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Indirect serologic testing is the procedure whereby antibodies in a person's serum being made by that individual against an antigen associated with a particular disease are detected using a known antigen.

1. The concept and general procedure for indirect serologic testing.

The concept and general procedure for this type of serological testing are as follows:

Concept:
This type of testing is based on the fact that antibodies are only produced in response to a specific antigen. In other words, a person will not be producing antibodies against a disease antigen unless that antigen is in the body stimulating antibody production.
General Procedure:
A sample of the patient's serum (the liquid portion of the blood after clotting and containing antibodies against the disease antigen if the person has or has had the disease) is mixed with the known antigen for that suspected disease. One then looks for an antigen-antibody reaction.

Examples of serologic tests to diagnose disease by the detection of antibodies in the patient's serum include the various serological tests for syphilis or STS (such as the RPR, the VDRL, and the FTA-ABS tests), the tests for infectious mononucleosis, the tests for the Human Immunodeficiency Virus (HIV), the tests for systemic lupus erythematosus, and tests for variety of other viral infections.

2. Qualitative and quantitative serologic tests.

Indirect serologic tests may be qualitative or quantitative. A qualitative test only detects the presence or absence of specific antibodies in the patient's serum and is often used for screening purposes. A quantitative test gives the titer or amount of that antibody in the serum. Titer indicates how far you can dilute the patient's serum and still have it contain enough antibodies to give a detectable antigen-antibody reaction. In other words, the more antibodies being produced by the body, the more you can dilute the person's serum and still see a reaction. Quantitative serological tests are often used to follow the progress of a disease by looking for a rise and subsequent drop in antibody titer.

3. Examples of indirect serologic tests to detect antibodies in the patient's serum

We will discuss:

The RPR Test for Syphilis
Serologic Tests for Infectious Mononucleosis
Serologic Tests for Systemic Lupus Erythematosus (SLE)
Detecting Antibody Using the Indirect Fluorescent Antibody Technique: The FTA-ABS test for syphilis
The EIA and Western Blot serologic tests for antibodies against the Human Immunodeficiency Virus (HIV)
Diagnosis of COVID-19 infection by the detection of antibodies made against SARS-CoV-2 antigens

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)