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Gay Men's Health Crisis
Variations in Viral Load Still Under Investigation
Theo Smart
September 1, 1995
GMHC Treatment Issues 1995 Sep 1; 9(9): 7

Viral load, the concentration of HIV particles in the blood, may be a less stable marker in individual patients than some think, according to one study presented at the ICC Conference (abstract 0538) by Janet Raboud, Ph.D., and a group from the University of British Columbia in Vancouver. The researchers suggest that more must be known about the variability of assays like PCR (polymerase chain reaction) before they are used to manage patient care (initiating treatment or switching therapies), as many doctors are already doing.

The Vancouver group obtained samples from 30 clinically stable patients who had no apparent infections during the four week study and for at least three months prior to the study. Researchers took three samples from each patient over the course of one week. Four weeks later, they took three more samples from each patient. Then they noted the highest and lowest viral load results for each person (as measured by Roche's Amplicor PCR kit, which measures the concentration of HIV RNA) and calculated his or her maximum variation over the study period.

The median variation for all the patients was .83 log (a log is a factor of ten -- one log equals a ten-fold variation and .83 log equals a 6.76-fold variation). The median range for patients with more than 500 copies of viral RNA per ml was .76 log (5.75-fold), and the median range for those with fewer than 500 copies of HIV RNA per ml was 1.7 log (50.1- fold). (Roche does not claim that the assay is very accurate below 400 copies per ml.) Although many laboratories have reported that the viral load assay is highly precise (that different laboratories testing the same sample will get the same result), there is only a small amount of published data on the stability of viral load in individual patients. The Vancouver study is the largest one to address that question.

One other study, by Robert Coombs, M.D., of the University of Washington, found that four patients had stable viral load results over 40 to 60 days.1 A study by Mark Holodniy, M.D., of the Palo Alto (California) Veterans Affairs Hospital, reported a less than 0.2 log (1.6-fold) variation in viral load for seven out of nine patients who had two tests performed one to three weeks apart.2 Another study by Dr. Holodniy reported less than a 0.5 log (three-fold) variation in results from six out of eight patients tested weekly over four weeks.3 Holodniy claims that the four other people in these studies are exceptional "outliers." Roche Molecular Systems presented data from a study of its assay in a poster at the Second National Conference on Human Retroviruses (this poster was omitted from the abstract book). Roche found a maximum two-fold variation in HIV RNA results in weekly samples drawn over three to six weeks in eleven out of nineteen clinically stable patients (not on antiretroviral therapy and with more than 400 CD4 cells). In four of the nineteen patients there was a maximum three-fold variation. In three others there was a maximum variation of three- to six-fold, and one patient had a 6.5-fold jump in viral load in one week.

Such findings have inspired some clinicians to use a three- fold (or half log) rule of thumb for monitoring their patients' progression and the effectiveness of antiviral therapy. But according to Dr. Raboud, "The variation in viral load and its causes needs to be better understood before viral load results can be used for individual patient management." She does not question the use of the tests in clinical trials, just the use of a major change detected by a single viral load assay as the basis for initiating or switching therapy. A large viral load variation probably should be confirmed by a subsequent test before any action is taken.

In the New York area, there is a wide range of opinion among doctors about how and whether these tests can be used. Treatment Issues' medical consultant, Gabriel Torres, M.D., believes that running two tests to establish that there has been a real change in viral load might be prohibitively costly for most clinicians.

He continues, "The same variability occurs in CD4 cells, and people have been using individual CD4 tests to make clinical decisions for ages, and they are one-sixth the cost. Doctors are using these [viral load] tests to make clinical decisions, but whether they are switching on the basis of a 0.5 log, one log or 1.5 log change in viral load depends upon the person." Meanwhile, James Braun, D.O., president of the Physicians' Research Network (PRN) in New York, says that "people who are using the tests to make clinical decisions are jumping the gun. We really don't know how to use these tests." Conflicting Studies Other researchers who have worked with the viral load assays are somewhat critical of Raboud's findings. "I don't believe that there is a .83 log variation in clinically stable patients," says Dr. Holodniy, who argues that viral load is a very stable marker in the majority of patients. He noted that there are a number of variables that could affect the test results, for example how the samples were stored or the particular technician who ran the assay.

In Roche's and Dr. Holodniy's studies, researchers batched the samples (stored the samples and sent them in to be tested at the same time). In the Vancouver study, the samples were tested as they became available. Dr. Holodniy observes, "If they didn't batch the samples, that could increase the variation." In the real world, of course, physicians do not batch samples from the same patient that are taken at different times.

Dr. Holodniy also mentioned the possibility that subclinical infections transiently affected viral burden in the Vancouver study. Dr. Coombs agrees, "You have to consider the biologic variability in the patient. The precision of the assay is very good, at about 0.2 log, but when you look at viral load over time in patients you've introduced a new variable. We know that things like respiratory infections, herpes infection, and flu vaccination can dramatically affect viral load." Dr. Raboud believes that her patients were clinically stable, though, so the variations must have arisen from the problems inherent in the technology of PCR amplification. She claims that the variations "were basically just noise." Dr. Holodniy thinks that this variation might not be observed using other measurement techniques such as bDNA (the branched DNA test produced by Chiron) or NASBA (Nucleic Acid Sequence Based Amplification, recently developed by Organon). "I'd love to able to run these samples through one of the other [commercial] assays," Dr. Holodniy said.

If the variation is due to the amplification technology, as Dr. Raboud believes, the other assays may be more reliable for patient management, but not if the variation is caused by biologic variability. And if there is this much variation in clinically stable patients, how is anyone to interpret viral burden changes in people who are not stable, such as those with active opportunistic infections? Future Studies A number of ongoing and upcoming studies will help resolve how advisable it is to alter individual patients' therapy on the basis of viral load results.

The PRN together with the Aaron Diamond Institute is currently running a pilot study in New York to assess the strengths of Chiron's bDNA assay. The researchers will track how helpful it is in making therapy decisions as well as the correlation between viral load and the course of a patient's disease. This study will require at least two viral load tests, performed at least three days apart to establish a baseline value when changing or initiating therapy.

The Community Programs for Clinical Research on AIDS (CPCRA) is opening a controlled multicenter study evaluating the benefits of switching therapies when there is at least a three-fold change in viral load as compared to switching therapies as a result of changes in CD4 cell counts. Currently, the plan is to use Chiron's bDNA assay, but Roche has asked the CPCRA to use its assay as well. One of the chief criticisms of this study is that it does not require a second, confirmatory viral load test before a patient's treatment regimen is changed. "Whether it's CD4 or some other parameter, you always need two baselines [at least one confirmatory test] for making clinical decisions," reiterates Dr. Coombs.

Through the AIDS Clinical Trial Group (ACTG), Dr. Coombs is running a study with Roche's test that requires two successive tests showing at least a five-fold change in viral load before modifying therapy. Even so, he believes that a smaller change in viral load can be significant based upon his analysis of the ACTG 116B/117 study (which evaluated whether it was better for people with AIDS to switch to ddI or remain on AZT). His analysis, to appear in a forthcoming paper, found that "even a two-fold [decrease] can have clinical benefit [improve symptoms or survival] if it is sustained." 1. Coombs RW et al. Journal of Clinical Microbiology. Aug 1993; 31(8):1980-6.

2. Holodniy et al. Journal of Clinical Investigation. Nov 1991; 88(11):1755-9.

3. Holodniy et al. Journal of Acquired Immune Deficiency Syndromes. Apr 1994; 7(4):363-368.