Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1381 page)

   The IgG antibody assay differentiates between type 1 and 2 HSV antibodies. The presence of IgG antibodies specific for HSV type 1 or 2 indicates previous exposure to the corresponding serotype of the virus.

   Limitations

   A clinical diagnosis of genital herpes should be confirmed with laboratory testing. The diagnosis can be made by viral culture, PCR, DFA, Tzanck preparation, and type-specific serologic tests. The choice of test varies with the clinical presentation.

   Both the CDC and the International Union against Sexually Transmitted Infections (IUSTI) recommend using molecular methods for typing all patients with first-episode genital herpes.

   The prevalence of HSV antibodies can vary depending on a number of factors such as age, gender, geographic location, socioeconomic status, race, sexual behavior, testing method used, specimen collection and handling procedures, and the clinical and epidemiologic history of individual patients.

   A negative result does not necessarily rule out a primary or reactivated infection, since specimens may have been collected too early in the course of disease, when antibodies have not yet reached detectable levels, or too late, after IgM levels have declined below detectable levels.

   False-positive test results may occur in patients infected with EBV, in primary or reactivated varicella virus infection, and in the presence of rheumatoid factor antibodies.

HUMAN IMMUNODEFICIENCY VIRUS 1/2 ANTIBODY SCREEN

   Definition

   HIV is a highly variable virus that mutates readily, and numerous virus strains may be classified into types, groups, and subtypes. There are two types of HIV: HIV-1 and HIV-2. Both types are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2.

   The diagnosis of HIV infection is established by one of the following methods: detecting antibodies to the virus; detecting the viral p24 antigen; detecting viral nucleic acid (NAT); or culturing HIV. The most widely used test is the detection of IgG antibody against HIV-1 and HIV-2 antigens in serum. HIV-1 antigens include p24 (a nucleocapsid protein) and gp 120 and gp 41 (envelope proteins). Antibodies to gp41 and p24 antigens are the first detectable serologic markers following HIV infection. IgG antibodies appear 6–12 weeks following HIV infection in the majority of patients and by 6 months in 95% of patients. IgG antibodies to HIV generally persist for life. Positive tests should be confirmed with repeat antibody testing, testing to differentiate HIV-1 and HIV-2, and confirmatory testing (e.g., type-specific HIV RNA, western blot assays). Assays for IgM antibodies are not used because they are relatively insensitive.

   Fourth-generation HIV tests can detect both antibody and p24 antigen. The main advantage of these assays is the ability to detect HIV infection during the “window period,” where antibody may not be detected.