DNA Tests Revolutionize
Diagnosis of Disease

BY WILLIAM TOWNSEND, O.D.
JUNE 1996

The past 20 years have witnessed tremendous advances in our understanding of genes and DNA. What were highly experimental concepts two decades ago are now practical applications that can be used to identify molecular changes that could result in disease. These same techniques also can be used to identify specific pathogens with amazing accuracy. By isolating specific nucleotide sequences in DNA from a microbe, it's possible to identify that organism. This discovery is the basis of two important tests that have only recently come into widespread use, the Nucleic Acid (NA) probe and the Polymerase Chain Reaction (PCR).

A probe is a piece of nucleic acid, either DNA or RNA, that has a known sequence of nucleotides. It can be labeled and can bind itself to a complementary sequence from an unknown organism. For instance, certain strains of bacteria contain specific genetic sequences that make them resistant to gentamicin. It's possible to identify these organisms by a DNA probe that attaches to the genomes, which give them this resistance.

Recently, automation of this process has vastly improved the accuracy of these tests. The limiting factor is adequate genetic material. The Gen-Probe, an RNA probe, is more sensitive in women, primarily because of the greater amount of specimen available. An RNA probe for chlamydia, for example, has been approved by the FDA and has up to 93 percent sensitivity if adequate genetic material is available.

Polymerase Chain Reaction, a method for cloning specimen material, has been hailed as the "most substantial technical advance in molecular diagnosis in the past decade." By heating and cooling genetic material to split and clone RNA or DNA, a single sample can be multiplied to 100 million copies in less than three hours.

The PCR was cleared for use by the FDA in 1993, and has greatly enhanced our ability to diagnose chlamydia. It's more sensitive than culture or DNA probe tests in diagnosing chlamydial infection. The test is automated, but expensive (approximately $65) because of the technology. It has been approved for genitourinary specimens or conjunctival specimens.

Recently, a PCR for herpetic eye disease was introduced. While conventional culturing and fluorescent antibody tests are available for corneal disease, they aren't effective to detect diseases in which herpes viruses are implicated as the cause, such as acute retinal necrosis (ARN), cytomegalovirus retinitis (CMV) and Epstein-Barr virus keratitis (EBV). PCR, however, will accurately and quickly identify the causes of these conditions.

So which test should you order to determine if a patient has chlamydial eye disease? First, consider these three factors: accuracy (i.e., specificity and selectivity), cost and time. The PCR is rapidly replacing the direct stain/culture as the gold standard, but many labs still don't offer it. When it becomes widely available, this will be the test of choice. If the PCR is not available, choose the direct stain test. It's less expensive than the PCR and in competent hands, is highly accurate. If you opt for the direct or indirect antibody tests, remember that there is documented cross-reactivity between chlamydial antibodies and those of several common organisms found in the genitourinary tract.

In the future, molecular genetic testing procedures will enable us to quickly and confidently identify bacteria, not just by species, but by strain. It's up to us to keep informed as to their availability and application to everyday eye care. CLS

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