Laboratory Diagnosis and Monitoring of HIV Infection

Abstract

In the majority of UK laboratories initial testing for HIV is now performed using a fourth generation test, which is a combination test for antibody to HIV and p24 antigen. These tests should be able to detect antibody to both HIV-1 and HIV-2. In addition, due to the heterogeneity of the virus they should be able to reliably detect antibody to the main circulating subtypes of HIV-1, i.e. group M (Major), O (Outlier), and N (non-M, non-O). The p24 antigen is an HIV capsid protein which is produced in large quantities during initial infection, prior to seroconversion. The sensitivity and specificity of fourth generation tests is typically > 99%. However, all positive results need further confirmation tests, as discussed below. Third generation laboratory assays only test for the presence of antibody to HIV. Though it includes the detection of IgM (which is not included in second generation assays), they do not detect early infection with isolated HIV antigen prior to seroconversion. Point-of-care testing for HIV is performed in the clinic or at bedside. Like laboratory based assays these tests can be either third or fourth generation. The sensitivity and specificity of point-of-care tests is considered lower than that of laboratory tests, and all positive results require confirmation with a laboratory assay. The window period is the length of time following infection with HIV until the appearance of laboratory markers of HIV infection in the blood. This period varies depending on which marker, i.e. antibody or antigen, is being tested for. The window period for fourth-generation tests is between eleven days and one month. Patients being counselled prior to this testing should be advised that a negative result does not cover risk exposures in the preceding month. These patients should be advised to have repeat testing if they have any further exposure risks in the preceding month prior to testing. For third-generation tests the window period is up to three months, correlating with the amount of time it may take for antibodies to HIV to develop.