|
Med 2000;109:568). Positive tests should be confirmed with repeat tests or with corroborating clinical or laboratory data.
FALSE NEGATIVE RESULTS : False negative results are usually due to testing in the "window period." The rate of false negatives ranges from 0.3% in a high-prevalence population (JID 1993;168:327) to < 0.001% in low-prevalence populations (NEJM 1991;325:593). Causes
of false-negative results include:
- Window period: The time delay from infection to positive EIA averages 10 to 14 days (CID 1997;25:101; Am J Med 2000;109:568). Some do not seroconvert for 3 to 4 weeks, but virtually all patients seroconvert within 6 months (Am J Med 2000;109:568). Newer tests detect HIV RNA as well as HIV antibody and should correct for this.
- Seroreversion: Rare patients serorevert in late-stage disease (JAMA 1993;269:2786; Ann Intern Med 1988;108:785). Seroreversion has also been reported in patients who achieve prolonged immune reconstitution (<5 yrs) due to highly active antiretroviral therapy (HAART) (NEJM 1999;340:1683; AIDS 2006;20:460) or with initiation of HAART during acute HIV infection (CID 2006;42:700).
- "Atypical host response" accounts for rare cases of false negative serology and is largely unexplained (AIDS 1995;9:95; MMWR 1996;45:181; CID 1997;25:98 JID 1997;175:955; AIDS 2004;18: 1071; AIDS 1999;13:89; CID 2008;46:785). The diagnosis in these cases is made with viral load testing but there needs to be awareness of the potential for false positives at low titer (Ann Intern Med 2001;134:25).
- Agammaglobulinemia (NEJM 2005;353:1074) Confirm with HIV viral load.
- Type N or O strains or HIV-2: Standard serologic tests detect M subtypes (subtypes A-H) of HIV-1, and some detect both HIV-1 and -2. EIA screening tests may fail to detect the O and N subtypes (Lancet 1994;343:1393; Lancet 1994;344:1333; MMWR 1996;45:561; J Clin Microbiol 2006;44:1856; J Clin Microbiol 2006;44:662). Only two patients with strain O HIV infection were detected in the United States through March, 2000 (MMWR 1996;45:561; Emerg Infect Dis 1996;2:209; AIDS 2002;18:269). The N group is another rare variant that causes false-negative EIA screening tests but may be positive by WB (Nat Med 1998;4:1032). There have been no recognized infections with the N strain in the United States through March 2000 (JID 2000; 181:470). Standard EIA screening tests are falsely negative in 20% to 30% of patients infected with HIV-2. Detection may require tests specifically for HIV-2. HIV-2 is now most frequently seen with dual infection with HIV-1 in patients in West Africa (AIDS 1999;13:701). Risks for HIV-2 are summarized above (pg 7).
|
