HIV Diagnosis and Testing

Introduction to HIV Testing

The primary confirmatory tests for HIV are Western blot (WB) and immunofluorescence assay (IFA), which must be performed after a positive screening test to definitively diagnose HIV infection, as recommended by the Centers for Disease Control and Prevention (CDC) 1, 2
Modern HIV testing typically uses a 4th generation combination test that can detect both HIV-1/HIV-2 antibodies and HIV-1 p24 antigen, offering several advantages, including detecting HIV infection earlier than antibody-only tests (4-7 days after infection versus 7-14 days for 3rd generation tests) 3

Rapid HIV antibody tests are available, providing results in 20-60 minutes, and are useful for initial screening, according to the American Academy of Pediatrics 4
Confirmation with a more specific supplemental test, such as Western blot (WB) or immunofluorescence assay (IFA), is mandatory when the screening test is positive/reactive, as recommended by the CDC 1, 2
The 4th generation HIV test demonstrates excellent performance, with a sensitivity of 100% and specificity of 99.99% in large public health testing populations 3
Rapid HIV tests have shown high sensitivity (100%) and specificity (99.9%) with positive predictive value of 90% in some studies, based on data from the American Academy of Pediatrics 4

For infants <18 months, HIV nucleic acid amplification tests (detecting HIV DNA or RNA) are required for diagnosis due to the presence of maternal antibodies, as recommended by the CDC and the American Academy of Pediatrics 1, 4
Definitive determination of HIV infection in infants should be based on laboratory evidence of HIV in blood or tissues by culture, nucleic acid, or antigen detection, or presence of antibody with compatible immunologic profile and clinical course, according to the CDC 1

If acute retroviral syndrome is suspected but antibody tests are negative, nucleic acid testing (NAT) for HIV RNA should be performed to confirm acute HIV infection before antibodies develop, as recommended by the CDC 2
Routine testing for HIV-2 is not recommended in the US except in blood centers, persons from HIV-2 endemic regions, sexual partners of persons from endemic regions, or cases with clinical evidence of HIV disease but negative HIV-1 tests, according to the CDC 1, 2
If tested beyond the window period (≥60 days after potential exposure), consider the person HIV-negative with high confidence, and no further HIV testing is needed unless new exposures occur 3
If tested within the window period (<60 days, especially <4 weeks), recommend repeat testing at appropriate intervals, and consider risk reduction counseling if appropriate 3

HIV antibody tests cannot rule out infection that occurred less than 6 months before testing, and false-negative results may occur during the "window period", as noted by the CDC 1, 2
A non-reactive HIV 1&2 Ab and p24 Ag test means no detectable HIV-1 or HIV-2 antibodies in the blood and no detectable HIV-1 p24 antigen in the blood, indicating the person is very likely not infected with HIV if tested beyond the window period 3
A repeatedly reactive HIV combination antibody/antigen (ab/ag) test with a negative HIV 1/2 antibody screen suggests possible acute HIV infection, where p24 antigen is detectable but antibodies have not yet developed, and confirmatory testing with HIV-1 RNA nucleic acid testing (NAT) is essential, according to CDC/FDA guidelines 5

Informed consent must be obtained before HIV testing, and positive test results require appropriate medical and psychosocial evaluation and monitoring services, as recommended by the CDC 1, 2
When a 4th generation test is repeatedly reactive, the following confirmatory testing algorithm is recommended:
A positive NAT confirms acute HIV infection, and early antiretroviral therapy improves long-term outcomes, as recommended by the CDC/FDA guidelines 5
If NAT is negative, the result should be considered a false-positive screening test, and no further HIV testing is needed if beyond the window period (60 days post-exposure), with clear communication about the extremely low probability of infection being crucial, as per CDC/FDA guidelines 5
Patients with repeatedly reactive results should be informed that this is a preliminary finding requiring confirmation, and clear communication about the possibility of false-positive results is essential to reduce anxiety 5
For individuals with ongoing risk, discuss prevention strategies, including Pre-Exposure Prophylaxis (PrEP) if appropriate, and recommend regular testing based on risk profile 3
Clear communication about the extremely low (essentially zero) probability of infection is crucial for psychological well-being in patients with occupational exposures, as stated by the CDC guidelines 5