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REPRODUCIBILITY: Commercially available assays vary based on the lower level of detection and dynamic ranges, as shown in Table 2-5 (J Clin Microbiol 1996;34:3016; J Med Virol 1996;50:293; J Clin Microbiol 1996;34:1058; J Clin Microbiol 1998;36:3392).Two standard deviations (95% confidence limits) with this assay are 0.3 to 0.5 log10 (2- to 3- fold) (JID 1997;175:247; AIDS 1999;13:2269). This means that the 95% confidence limit for a value of 10,000 c/mL ranges from 3,100 to 32,000 c/mL. Quantitative results with the Amplicor (Roche) assay version 1.5 and bDNA assay (Versant 3.0) are comparable except at the low end of the linear range (<1,500 c/mL) (J Clin Microbiol 2000;38: 2837; J Clin Microbiol 2000;38:1113). Comparative testing for the NucliSens (bioMérieux) assay is less extensive but appears comparable (J Clin Microbiol 2000;38:3882; J Clin Microbiol 2000;38: 2837).
SUBTYPES: There is greater performance variation with non-subtype B.In tests with subtypes A-D,F,G,CRF 01 AE and CRF 02_AG, inter subtype recombinants and group O. bDNA (Versant 3.0) was some what superior, with 83% within 0.5 log10 c/mL. The result for Amplicor Monitor 1.5 was 74% and for NucliSens it was 61% (J Clin Microbiol 2005;43:3860). Other studies support these findings (J Med Virol 2006; 78:883; JAIDS 2002;29:330).
SEX DIFFERENCE: There appears to be a modest difference in viral load, averaging 0.23 log10 c/mL (about 2-fold) lower in women compared with men according to a meta-analysis of 12 reports (JAIDS 2002;31: 11). However, these differences disappear at CD4 counts < 300 cells/ mm3 and are unlikely to have an affect on treatment decisions (JAIDS 2000;24:218; JID 1999;180:666; CID 2002;35:313; Lancet 1998;352: 1510; NEJM 2001;344:720).
COST: $100 to $150 per assay (Medicare reimbursement $111 to $130).
USES OF VIRAL LOAD TESTING
- Diagnosis of primary infection: Quantitative HIV RNA is useful for diagnosing acute HIV infection, for counseling on transmission prevention, contact tracing and it probably represents the optimal time for resistance testing (DHHS Guidelines Jan 29, 2008; NEJM 2005;352:1873). Most studies show high levels of virus (105 to 106 c/mL) during acute infection. Note that 2% to 9% of persons without HIV infection have false-positive results, virtually always with low HIV RNA titers (<10,000 c/mL) (Ann Intern Med 1999;130:37; J Clin Microbiol 2000;38:2837; Ann Intern Med 2001;134:25). The alternative is the HIV p24 antigen assay, which is less expensive ($20 vs $100) and highly specific but only 89% sensitive (Ann Intern Med 2001;134:25). A screening method to detect acute HIV infection has been developed in North Carolina. Specimens that are seronegative or indeterminate
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