A new test for infection with the AIDS virus allows scientists to pinpoint small pieces of the virus' genetic material, or DNA, hidden within the much larger amount of DNA from chromosomes of the patient's blood cells, according to a report published Friday.
The test is a promising new way of confirming infection in people whose blood contains antibodies to the human immunodeficiency virus or HIV, a sign of past exposure. The same technique can also be used to detect other retroviruses related to HIV as well as herpes viruses, the Epstein-Barr virus, hepatitis viruses and others that can cause chronic, silent infections, according to researchers at the Centers for Disease Control and California's Cetus Corp. who performed the study.
The test works by multiplying the amount of viral DNA in a blood sample, allowing scientists to detect genes from the AIDS virus even if they are present only in one cell out of every 8,000, said Gerald Schochetman, chief of the laboratory branch of the CDC's AIDS program. Schochetman said it could also be used to test cells from semen, brain tissue and other organs or body fluids for the virus.
Because so few cells in an infected patient actually harbor the HIV virus, searching for viral DNA is "like looking for a needle in a haystack," Schochetman said. "The best way to do it is if you can multiply the number of needles."
In the new test, researchers extract the DNA from a sample of a patient's blood. They add primers, small chains of DNA common to all known strains of the AIDS virus. Then, in a process called polymerase chain reaction, any viral DNA present in the sample is increased approximately 200,000-fold. Finally, they add a radioactively labeled "probe" that reacts with viral DNA to produce a positive result.
In the study, published in the journal Science, the new test was positive in 11 infected men who also had positive antibody tests and in whom the HIV virus had been cultured. It was negative in 11 healthy, uninfected men.
In a third group of 11 men with positive antibody tests but in whom efforts to culture the virus had failed, the new test was positive in seven, suggesting that it is more reliable than cultures for confirming HIV infection. Also, the test takes three days; cultures can take weeks.
Schochetman said the strength of a positive result can be used to estimate how much virus is present in a patient, which may help doctors monitor contagiousness, predict progression of the disease or test effectiveness of drugs.
"From a scientific standpoint, I'm very excited about it," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease.