Diagnostics

Serological assays

Virological testing

Serological assays

Serological assays are commonly used for both screening and diagnosis and may detect HIV antigen, antibodies, or both.

• HIV antigen alone: detects HIV p24 antigen

• HIV antibody assays (i.e., third-generation and below): Detect IgM and IgG antibodies.

  • Laboratory methods

    • Enzyme-linked immunosorbent assays (ELISA)

    • HIV-1 and HIV-2 antibody differentiation immunoassay

      • Laboratory-based test that can differentiate between HIV-1 and HIV-2 (provides separate results for each analyte)

      • Most commonly used confirmatory test in the US

    • Western blot: Detects only IgG antibody to HIV-1

• Combination HIV antibody with HIV antigen test (i.e., fourth-generation and above): Can detect HIV IgG and IgM antibodies and p24 antigen.

Virological tests are most commonly used for screening infants and confirmation of disease in both infants and adults.

• Can detect HIV-1 RNA and/or DNA (depending on the test)

• Laboratory method: nucleic acid testing (NAT)

HIV testing should be offered to patients who have signs that raise concern for HIV infection, prior exposure, and as part of routine screening.

• Routine screening

  • All individuals > 13 years of age in all health care settings

  • Any individuals (even if previously tested):

    • Starting treatment for tuberculosis

    • Attending STI clinics or diagnosed with an STI

  • One-time testing is recommended early in all pregnancies

• All patients with clinical features of acute or chronic HIV infection or opportunistic infections

• All individuals with possible past exposure, especially individuals at high risk of HIV infection (e.g., sex workers, men who have sex with men, individuals who use IV drugs, partners of HIV-positive individuals)

• Annual screening for individuals at high risk of HIV infection

• HIV serology

  • Fourth-generation HIV test (combination HIV antibody with HIV antigen) [21]

    • Timing: Detectable ∼ 14 days after transmission [21]

    • Not recommended for suspected neonatal HIV infection (results may be false positive because of maternally transferred anti-HIV antibodies) [23]

  • Third-generation HIV test (antibody only): frequently used in rapid tests (point-of-care tests)

• Virological tests

  • HIV-1 NAT: Used for screening in infants aged < 18 months with peri- or postnatal HIV exposure.

• Serology

  • HIV-1 and HIV-2 antibody differentiation immunoassay [21]

    • Timing: detectable 3 weeks after infection [21]

    • Positive result: HIV infection confirmed

    • Negative or indeterminate result: Perform HIV-1 NAAT.

      • Positive HIV-1 NAT: HIV-1 infection confirmed

      • Negative HIV-1 NAT: antibody test was likely a false positive

  • HIV-1 western blot: The CDC no longer recommends western blot tests for confirmation of HIV infection. [24]

• Virological tests

  • HIV-1 NAT [24]

    • Timing: Can measure the amount of viral RNA in the blood and detect HIV infection earlier than antibody/antigen-based tests (∼ 10 days after transmission).

    • Indications

      • Neonatal HIV infection

      • Patients with indeterminate results [26][27]

      • Patients presenting before seroconversion

      • Screening of blood donors [28]

      • Disease monitoring

The following studies are recommended to screen for drug resistance, assist in the selection of an appropriate ART regimen, and establish a baseline to monitor the efficacy of therapy.

• HIV drug resistance testing: Genotypic assays are preferred over phenotypic assays.

• CD4+ count: correlates with overall immune function

  • Normal values are > 500 cells/mm3, whereas in the advanced stages of HIV the CD4+ count is often < 200 cells/mm3.

  • Critical measurement to determine when to initiate opportunistic infection prophylaxis

  • CD4+ counts increase in response to successful ART therapy.

• Viral RNA load: indicator of ART response

  • Decrease in viral loads indicates effective treatment.

  • Prognostic marker in long-term treatment (higher viral load → ↑ destruction of CD4+ lymphocytes → more severe immunodeficiency → worse prognosis)

• CD4 cell percentage: used for the assessment of immune function and less variable than CD4+ count

• CD4:CD8 ratios (no longer routinely recommended): an increase in the ratio following ART initiation suggests improved immune system functioning