The concentration of HIV RNA strands in the plasma of infected individuals is a highly reliable measure of viral activity and an accurate predictor of disease progression. The HIV RNA assay, recently approved by the F.D.A., is a precise, practical, swift and sensitive means of determining the level of viral activity in all HIV-infected patients. Because this new test is accurate at viral loads as low as 400 HIV RNA copies per milliliter of plasma -- and at levels as high as 750,000 copies -- it can be used at all stages of infection, and it will register changes in viral load long before that destructive activity is reflected in diminishing CD4 cell counts.
Although there is no specific threshold of HIV RNA that signals disease progression, concentrations above 100,000 copies per µL are believed to herald rapid deterioration, whereas concentrations below 10,000 copies are thought to predict a more favorable course. A growing number of studies suggest that women with high copy numbers are more likely to transmit infection to their infants, and RNA quantification is therefore recommended for all HIV-positive pregnant women. Because the evidence suggests that zidovudine therapy exerts its protective effect by lowering viral burden in HIV-infected women, more aggressive therapy may be indicated for those with high copy numbers.
Important as HIV RNA testing may be in interrupting vertical transmission of infection, its larger clinical application will be in optimizing antiretroviral therapy. We know, for example, that patients with similar CD4 counts often have widely different concentrations of plasma RNA -- and these patients have widely different risks of disease progression and death. HIV RNA testing enables the clinician to identify patients with high viral loads, and to treat those patients with whatever combination of antiretroviral agents is required to reduce their viral burden.
The particular clinical value of HIV RNA testing is that it permits true individualization of therapy: The practitioner can determine, through serial assays, when a treatment has taken effect -- and also when it ceases to be effective. Falling RNA copy numbers indicate therapeutic efficacy; rising numbers indicate the development of viral resistance, and suggest that it is time to switch agents or add another to the patient's regimen.
The usefulness of the RNA assay cannot be overestimated in the era of combination therapy. The practitioner has an ever-expanding range of therapeutic options -- and no firm guidelines on which drugs or drug combinations are likely to yield the best results. Only a few of these potential combinations have been studied in large-scale trials; some may never be. Moreover, the results of clinical trials cannot predict how a particular patients will respond to a particular therapy. HIV RNA testing can do precisely that.
How should clinicians apply HIV RNA testing in their practices? Although the potential utility of RNA quantification is considerable, the data are still very preliminary. Nonetheless, it is possible to offer practitioners some general guidelines:
Whichever assay is chosen for initial testing should be used for all future testing, to ensure uniformity of results.
At baseline, HIV RNA quantification should be obtained in duplicate, after which single determinations will suffice.
Antiretroviral therapy, if it is effective, should cause a drop in viral burden of at least 0.5 log after 2-4 weeks; a RNA assay taken at that time will tell the clinician if a chosen therapy is working. If not, another agent or combination of agents should be tried.
During therapy, serial RNA measurements should be obtained at intervals of 3-4 months, to determine if efficacy is sustained or is diminishing as a result of viral resistance.
When HIV RNA levels approach or exceed baseline values, the clinician should consider alternative drugs or combinations, should use subsequent measurements of RNA level to gauge the efficacy of the new therapy, and should obtain those values at monthly intervals following the change of regimen.
The color diagram below shows how the HIV RNA assay is obtained. With this method of testing -- which can produce billions of DNA copies from the RNA of a single segment of the HIV genome -- highly reproducible results can be obtained in less than six hours by a single trained technician.