- (Scand JID 2003;35:155). The time to VL nadir is dependent on pretreatment VL as well as potency of the regimen, adherence, pharmacology, and resistance. Patients with high baseline VL take longer to achieve maximum suppression. Failure to reduce VL by 1 log10 c/mL (90%) at 4 weeks suggests virologic failure due to nonadherence, pre-existing resistance, or inadequate drug exposure. The expectation is to reduce the VL to <50 c/mL by 16-24 weeks (JAMA 2006;296:827; J Clin Invest 2000;105:777). The VL should then be measured in patients on treatment every 3-4 months to assure a VL <50 c/mL (JID 1999;180:1347). Blips (See pg ____) are usually inconsequential but can also indicate non-adherence (JID 2007;196:1773) and sustained levels >50 c/mL indicate virologic failure (JAMA 2005;293:817; J Antimicrob Chemother 2008;61:699).
- Interpretation: Changes of ≥50% (0.3 log10 c/mL) are considered significant.
- Factors that correlate with increase viral load
- Switch from R5-tropic to X4-tropic HIV.
- Progression of disease
- Failure of antiretroviral therapy due to inadequate potency, inadequate drug levels, nonadherence, and/or resistance.
- HIV superinfection (JID 2005;192:438)
- Active infections; active TB increases viral load 5- to 160-fold (J Immunol 1996;157:1271); pneumococcal pneumonia increases viral load 3- to 5-fold.
- Immunizations such as influenza or pneumococcal vaccine (Blood 1995;86:1082; NEJM 1996;335:817; NEJM 1996;334: 1222). Increases are modest and last (2 to 4 weeks).
- Relative merit of tests: The Versant version 3.0 assay has good reproducibility for viral load levels of 75 to 500,000 c/mL. The linear range for Amplicor is 50 to 100,000 c/mL for the ultrasensitive test. It should not be used in patients expected to have higher viral loads (J Clin Microbiol 2000;38:2837). The NucliSens assay has a broad dynamic range (176 to 3,500,000 c/mL) and can be used for HIV quantification on blood or on various body fluids or tissue such as seminal fluid, CSF, breast milk, saliva, and vaginal fluid (J Clin Microbiol 2000;38:1414).
CD4 Cell Count
This is a standard test to assess prognosis for progression to AIDS or death, to formulate the differential diagnosis in a symptomatic patient (Table 1-2, pg 3), and to make therapeutic decisions regarding antiviral treatment and prophylaxis for opportunistic infections. It is the
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