Historically, CD4⁺ cell counts were first recognized as prognostic indicators of the short-term risk for the development of new opportunistic infections or death, yet their ability to predict disease progression over a more prolonged period of time was limited. In the early 1990s, new technologies, including quantitative reverse transcriptase polymerase chain reaction (PCR) [2, 3], branched DNA (bDNA) [4], and nucleic acid sequence-based assay (NASBA) techniques [5], allowed accurate measurement of virally associated circulating HIV RNA in plasma. The amount of HIV RNA detected is directly related to the production of HIV in tissue sites within the body, most notably the lymphatic system. On the basis of the highly dynamic response to initiation of or change in antiretroviral therapy, HIV RNA plasma levels were initially used to determine the relative activity of antiretroviral drugs in clinical trials [2, 6, 7]. The HIV RNA assays also enabled investigators to delineate the extraordinary rate of viral replication in vivo (production of 10 billion virions per day in an infected person), leading to dramatic new insights into HIV pathogenesis [7-9]. Early in 1996, when the prognostic value of HIV RNA became evident, physicians quickly incorporated the use of HIV RNA assays into routine clinical practice [10]. Although serial measurements of HIV RNA levels can clearly show when an antiretroviral regimen is working and-more important-when it is failing, much uncertainty remains about the best use of this marker in conjunction with CD4⁺ cell counts.

Investigators from the Multicenter AIDS Cohort Study previously demonstrated the predictive value of HIV RNA plasma levels on long-term survival in a subset of 181 patients [11]. In this issue, these investigators extend their observations to 1604 patients and demonstrate that the predictive value of HIV RNA measurements improves when these measurements are combined with CD4⁺ cell counts [12]. For example, among patients with low viral load (<500 copies/mL) and CD4⁺ cell counts greater than 750 cells/mm³, 3.6% had progression to the acquired immunodeficiency syndrome (AIDS) over 9 years compared with 76.3% of those with CD4⁺ cell counts greater than 500 cells/mm³ and viral load levels greater than 30 000 copies/mL. Similarly, among patients with CD4⁺ cell counts of 750 cells/mm³ or less, 22.3% of those with a viral load less than 500 copies/mL developed an AIDS-defining illness over 9 years and 59.7% of those with only 3000 to 10 000 copies/mL.

Also in this issue, Hughes and O'Brien and their colleagues [13, 14] confirm the ability of baseline viral load measurements and CD4⁺ cell counts independently to predict clinical outcome. They note that short-term changes in these markers after initiation of antiretroviral therapy can predict outcome. Using data from a virologic substudy of AIDS Clinical Trials Group Protocol 241, Hughes and colleagues [13] demonstrate that baseline viral load and a 10-fold reduction in viral load at 8 weeks of therapy have comparable predictive values. Moreover, the 10-fold reduction seen at 8 weeks was associated with an increase in CD4⁺ cell count of 15 cells/mm³ at 48 weeks. O'Brien and colleagues [14] conducted a substudy of Veterans Administration Collaborative Study 298 and showed that the magnitude and durability of both viral load and CD4⁺ cell count responses contributed to clinical outcome. Each threefold (0.5-log) reduction in viral load (averaged over a 6-month period) was associated with a 30% reduction in the risk for clinical progression; each 10% increase in CD4⁺ cell count was associated with a 15% reduction in risk. If neither a threefold reduction in viral load nor a 10% increase in CD4⁺ cell count was seen, the risk for progression increased 2.3-fold.

Taken together, the three reports underscore the value of routinely using both viral load and CD4⁺ cell counts in the clinical care of HIV-infected patients. The additional cost associated with viral load testing [approximately $100 per assay] is well justified because the information provided by this testing can be used 1) to assess the activity of antiretroviral regimens [which can cost as much as $800 to $1000 per month] and thus determine when a regimen is failing and 2) to initiate changes in therapy before the development of clinical failure, which can be expensive (for example, cytomegalovirus retinitis can cost as much as $100 000 per year). Previously published guidelines suggest that plasma HIV RNA levels be measured at baseline (two measurements, taken 2 to 4 weeks apart, are preferable) and every 3 to 4 months thereafter [10]. It is recommended that plasma HIV RNA be measured at shorter intervals as critical decision points with regard to antiretroviral therapy are approached and 3 to 4 weeks after initiating or changing antiretroviral therapy. CD4⁺ cell counts should be measured at baseline and at 3- to 4-month intervals.

Recently published guidelines for the use of antiretroviral therapy help further define the use of viral load and CD4⁺ cell counts in clinical practice [15, 16]. The overall goal of antiretroviral therapy should be to reduce the viral load as much as possible, preferably to below the limit of detection of the assay (currently <400 copies/mL), for as long as possible. Although some patients respond well to antiretroviral therapy and promptly achieve undetectable levels, other patients are unable to achieve undetectable status because the drug regimen is not potent enough, because adequate drug levels are not achieved (because of drug-drug interactions, poor absorption, or rapid metabolism), because the patient does not adhere to the regimen, or because viral resistance to the drugs develops. Although there is some consensus about the ideal goal of antiretroviral therapy, no clear consensus has been reached about the definition of treatment failure. Although a "return of viral load toward baseline" is generally considered evidence of the failure of antiretroviral therapy, it is unclear whether regimens should be changed as soon as viral levels become detectable again (that is, at approximately 600 copies/mL) or whether it is preferable to wait until the viral load returns to a higher level (such as >5000 to 10 000 copies/mL). What is clear is that each patient needs to be assessed individually with regard to baseline viral load; CD4⁺ cell count; previous antiretroviral regimens; initial response to therapy; and, most important, commitment to antiretroviral therapy and adherence to the regimen.

As viral load measurements are used routinely in clinical practice, several notes of caution are warranted. First, no clinical decision should be made on the basis of a single test result. There is inherent intraassay and biological variability (generally 0.3-log [twofold] variation), and concomitant illnesses or vaccination may result in transient elevations in viral load. In addition, specimens must be processed promptly (within 2 to 4 hours), and the plasma must be frozen immediately after processing. If whole blood or plasma is left at room temperature overnight, as much as 70% or more of the signal may be lost, leading to spuriously low measurements, depending on the anticoagulant used. Consistent use of either acid citrate dextran (ACD; "yellow top") or EDTA ("purple top") tubes should be ensured when blood is collected; the use of heparin ("green top") tubes should be avoided because HIV RNA is less stable in heparin [12].

Second, because of the dynamic nature of viral replication in vivo, patients who miss even a few doses of drug before their visit may already be experiencing viral rebound and their antiretroviral therapy might be judged to be failing. Before ordering an HIV RNA test, clinicians should review the patient's adherence to the regimen and should postpone testing if recent doses have been missed. Any unexpectedly high HIV RNA level warrants repeated testing and reevaluation of patient adherence before the therapy is changed.

Finally, with the increasing focus on achieving undetectable viral levels as an ideal goal of antiretroviral therapy, sufficient time should be allowed for the patient to achieve undetectable status. When potent therapy is initiated, a profound (usually 100-fold), rapid decrease in plasma viral load occurs over the first 7 to 14 days. This is followed by a slower decline in viral load over the subsequent 4 to 16 weeks. Given the two-phase decay of HIV replication kinetics, achieving undetectable status often requires as much as 12 to 16 weeks after an initial rapid decrease in HIV RNA levels.

Existing technology allows us to perform rapid genotypic analyses to determine the presence of specific mutations within the HIV genome that confer resistance to existing antiretroviral drugs. Despite the superficial attractiveness of using such information to select new antiretroviral regimens when a therapy is failing, no data show that virologic or clinical outcomes are improved when a genotypic profile rather than a simple medication history is used. Moreover, because many genotypic assays report only the most prevalent viral genotype, potentially important subpopulations of HIV may not be reported in the results. Until clinical trial data demonstrate clear evidence of the utility of genotypic analyses, these tests should not be used in clinical practice.

The establishment of HIV viral load as a critical component of HIV care, combined with the recent advances in our understanding of HIV pathogenesis and the availability of more potent antiretroviral therapy, has revolutionized the management of HIV-infected patients. However, like most new advances, the improvements in clinical outcome are associated with many new challenges. The paradigm of treating HIV-infected patients has changed from attempting to keep patients alive from this year to the next to attempting to keep patients alive from this decade to the next. Therefore, it is incumbent upon clinicians to establish a long-term plan for each patient as antiretroviral therapy is initiated. When therapy is considered, physicians should 1) prospectively establish the target viral load that the regimen should achieve; 2) select the best initial regimen that is likely to achieve the targeted virologic response; 3) specify the viral load level that will be considered evidence of antiretroviral failure; and 4) specify the subsequent regimen or regimens that will be used when the initial therapeutic regimen fails. By incorporating this long-term management strategy into the routine practice of HIV care, physicians can once again help their patients return their focus "back to the future."