AIDS TREATMENT NEWS No. 060 - July 15, 1988
John S. James
The new test, called PCR (polymerase chain reaction), often detects HIV infection months before the antibody test; in one case reported at the AIDS conference in Stockholm, it detected HIV three and a half years before seroconversion. Some persons who are negative on the antibody test, P24 antigen, viral culture, and all other tests have been found to be positive on the PCR. PCR will prove especially useful for testing infants, because only about 40 percent of the infants who test antibody positive for HIV are really infected; the others are only carrying their mother's antibodies temporarily, and the usual antibody tests cannot tell the difference.
No one knows how many people will be found to be positive on the PCR even though they are negative on the antibody and all other tests. And no one knows whether everyone who is PCR positive will become antibody positive eventually, or whether some people who are positive only on the PCR might have controlled the virus successfully.
The PCR detects very small amounts of a particular sequence of DNA, which contains the hereditary information of cells. For HIV diagnosis, the PCR test is used to detect genetic information inserted into the DNA of human cells by HIV.
Unfortunately the current version of the PCR only tells whether or not the DNA created by HIV was detected--it does not give any indication of how much. Therefore the test now available cannot be used to monitor how well patients are responding to treatment--or as an indicator of how well experimental drugs are working--because once a person is positive on the PCR, he or she will presumably remain positive, short of being completely cured. Later improvements in PCR technology may enable the test to give quantitative results, so that it could be used to monitor therapy and drug trials.
How It Works
The PCR (polymerase chain reaction) is a major scientific advance which will be an important tool for many kinds of basic research, and for diagnosis of other diseases as well as AIDS.
The PCR test works by taking advantage of the fact that DNA can make a copy of itself, as it does during normal cell division. The sample being tested is treated in such a way that each piece of the DNA being tested for will double into two pieces. The treatment is repeated, and the two pieces become four, then eight, then 16, etc. After 20 or more repetitions, the DNA (if any) will have multiplied more than a million times, and then it can be detected by ordinary biochemical tests.
The PCR can detect HIV infection even if it is completely latent, in the DNA of macrophages or other cells, not causing the replication of virus or the production of antibodies.
Persons who are antibody positive would presumably always test positive with the PCR. Those who are antibody negative despite high risk of exposure to HIV could use the PCR for reassurance if they are negative, or to start early infection control, periodic blood work, or early treatment if they test positive.
For more background on the PCR, see "Multiplying Genes by Leaps and Bounds", Science, June 10, 1988.
Where To Get the PCR
The PCR is now available with a physician's prescription everywhere except New York State, which requires special licensing the companies have not yet obtained. The cost for the test itself is $145., and it takes about five days to get the results.
Cetus Corporation, a biotechnology company in Emeryville, CA, developed the PCR. Last week Cetus licensed two companies to provide the test commercially. Physicians who want to order the test should call customer service at either (1) Pathology Institute, Berkeley, CA, (800) 438-8674 or (415) 540-1638, or (2) Specialty Laboratories, Inc., Santa Monica, CA, (800) 882- 1345 from within California, (800) 421-7110 from other states, or (213) 828-6543. The PCR might also be available through whatever laboratory the physician uses already, by subcontract with the one of these two companies.
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