Paper II — 2017 August (Supplementary) (2010 Scheme) — Q2

Paper II

2017 August (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Laboratory diagnosis of HIV infection

Q25 marksShort Essays★✓

Answer

Laboratory diagnosis of HIV infection:

• Screening: ELISA for HIV antibody — 4th-generation combination assays simultaneously detect p24 antigen (appearing earlier, before antibody, narrowing the window period) and antibody, now the standard first-line screening approach.
• Rapid tests: point-of-care immunochromatographic rapid antibody tests, useful for field/bedside screening.
• Confirmatory test: Western blot — detects antibody against specific individual viral proteins (gp41, gp120/160, p24); traditionally used to confirm a reactive screening test, though many current national algorithms now use a second, different ELISA/rapid test or supplemental line immunoassay per WHO guidance.
• CD4 count: flow cytometry, to assess degree of immunosuppression, guide clinical staging, and inform opportunistic-infection prophylaxis decisions.
• Viral load: quantitative HIV RNA PCR, used to monitor disease progression and antiretroviral treatment response.
• Early infant diagnosis: HIV DNA PCR is used in infants born to HIV-positive mothers, since maternal antibody crosses the placenta and can give false-positive antibody-based results in the child for up to 18 months.
• Window period considerations: the interval between infection and detectability by a given test — narrowed by 4th-generation assays (detecting both antigen and antibody) compared to older antibody-only assays.

Related Concepts

1. 1HIV Infection and AIDS (Microbiology)

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