Lab Testing for Chlamydia
and Other Diseases

BY WILLIAM TOWNSEND, O.D.
MAY 1996

After the February article about diagnosis and management of AIC was published, we received a number of inquiries about lab tests with which we were unfamiliar. Because this is a sexually transmitted disease, and a positive diagnosis can impact people's lives, lab testing is crucial. We felt another look at the subject was justified.

To understand the positive and negative characteristics of a laboratory test, we need to have some idea of how each test works as well as the pitfalls inherent to each test. Sensitivity measures how many people with a disease will be detected by a test. Specificity measures how well a test predicts that a person is not infected. Due to the nature of chlamydial disease, we need to select the most sensitive test with the best specificity.

One time-tested method for diagnosing chlamydia and other intracellular parasites is to obtain a sample directly from infected epithelial tissue, and stain it with iodine. If inclusion bodies are present, the test is positive.

A more sensitive means of diagnosing chlamydia is by laboratory culture, but this technique is not easy or foolproof. Specimen collection is critical and somewhat difficult because chlamydia does not parasitize stratified epithelium, but only grows on squamocolumnar cells. To obtain ocular samples, the conjunctival surface is briskly rubbed with a calcium alginate swab. If a genitourinary culture is obtained, it must be collected from the endocervix. Specimens are collected, sent to the lab and inoculated into columnar cells that contain special growth media. After 48 to 72 hours, the cultures are evaluated by direct or fluorescent staining of the cells for inclusion bodies. These cultures can also be used to perform indirect immunologic testing to confirm chlamydia. The two biggest disadvantages of direct culture tests are time and cost. These factors led to the development of non-culture tests.

DFA AND ELISA TESTS

A well-known example of an antigen-antibody test is the direct fluorescent antibody (DFA) test. In this test, frozen tissue or media containing specimen are exposed to antibody specific for a given organism. The Fab portion of the antibody adheres to the corresponding antigen. The Fc portion of the antibody is attached to fluorescein or another fluorescent material. After exposure for the recommended length of time, the tissue is rinsed to remove all antibody that is not adherent to the test material. When the test material is viewed under a fluorescent microscope, the fluorescein glows confirming the presence of the antigen.

The Enzyme-linked Immunoassay test (ELISA) works in a similar manner. Antigen is inoculated onto a plastic or agar surface. Antibody known to be specific to a particular species is introduced. After a specified time, the plate is rinsed to remove any superfluous antibody. A ligand (a molecule that adheres to antibody and is coupled to an enzyme) is introduced and after a given time, is rinsed. Then a chromogen (a substrate that takes on color under the influence of an enzyme) is added and the results are read.

One drawback with both tests is that there is some cross-reactivity between chlamydial antibodies and A calcoacetius, E coli, G vaginalis, K pneumoniae and N gonorrhoeae. CLS

Dr. Townsend is in private practice in Canyon, Texas, and is a consultant at the Amarillo VA Medical Center.