Discordant HIV Serology in an Antenatal Woman
A 26-year-old woman at 28 weeks' gestation attends an antenatal clinic. She has no history of fever, weight loss, or risk exposure, but had a transient flu-like illness three weeks ago and received an inactivated influenza vaccine ten days ago. Screening by a rapid immunochromatographic HIV-1/2 antibody test is reactive. A repeat rapid test from a different manufacturer is negative. A laboratory-based HIV-1/2 antibody ELISA is reactive with a signal-to-cutoff ratio of 3.8. HIV-1 RNA quantitative PCR is below the limit of detection (<20 copies/mL). CD4 count is 860 cells/µL and she is clinically well. The clinic is busy; a health worker suggests starting lifelong antiretroviral therapy immediately and telling the woman she is HIV-positive. The woman asks whether her baby is at risk.
1. Do these results confirm HIV infection? Explain your interpretation and the next steps.
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The results do not confirm HIV infection. The serology is discordant: one rapid test is reactive, a second rapid test from a different manufacturer is negative, and the ELISA is reactive. In a testing algorithm, discordant antibody results require resolution with a third assay or a confirmatory test, not an immediate diagnosis.
The undetectable HIV-1 RNA is important. In an untreated adult, a negative NAT does not formally exclude very recent infection, but three weeks after a flu-like illness and with a reactive ELISA, a completely negative NAT makes false-positive serology more likely. Recent inactivated influenza vaccination and pregnancy are recognised causes of non-specific antibody reactivity, although they do not explain every false positive.
Next steps:
- Repeat HIV-1/2 serology in 4 weeks using a different platform.
- Request an HIV-1/2 antibody differentiation or confirmatory immunoblot assay.
- Consider p24 antigen testing and repeat HIV-1 RNA PCR.
- Provide counselling, confidentiality, and a clear explanation that the result is inconclusive, not a diagnosis.
- Do not start lifelong antiretroviral therapy on these results alone.
2. What should be communicated to the woman and recorded in the report while confirmation is pending?
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The report should state that the HIV screening result is inconclusive and requires confirmation, rather than 'HIV-positive' or 'HIV-negative'. It should list the discordant rapid test, the reactive ELISA, and the undetectable HIV-1 RNA, with a comment that recent vaccination, pregnancy, and cross-reactivity can cause false-positive antibody results.
Communication to the woman:
- Explain in plain language that one test was reactive but the additional tests do not agree, so infection is neither confirmed nor excluded.
- Emphasise that she should not start treatment until confirmatory testing is complete.
- Offer repeat testing at a specified date and explain the window period.
- Discuss partner testing and prevention counselling without disclosing her results to others.
- If she were later confirmed positive, explain that prevention of mother-to-child transmission is highly effective; at present, the baby's risk cannot be estimated from an unconfirmed result.
Document the lot numbers, assay names, dates, and the counselling provided. A busy clinic is not a reason to skip confirmation.
3. If repeat ELISA remains reactive at 4 weeks but HIV-1 RNA is still undetectable, how would you resolve the case?
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A persistently reactive ELISA with undetectable HIV-1 RNA still requires a confirmatory antibody assay. The key question is whether the antibody reactivity is specific.
Resolution pathway:
- Perform an HIV-1/2 antibody differentiation or immunoblot assay. A clear positive pattern supports HIV infection; a negative or indeterminate pattern supports a false-positive ELISA.
- Repeat HIV-1 RNA PCR with a sensitive assay. If it remains undetectable off antiretroviral therapy, active HIV replication is very unlikely.
- Repeat serology at 3 months from the original exposure or illness. If the ELISA becomes negative or remains indeterminate and NAT stays negative, the episode is best classified as a false-positive serology.
- If the confirmatory antibody assay is positive but NAT is repeatedly undetectable, discuss with a reference laboratory or HIV specialist; rare possibilities include a false-positive confirmatory assay or an unusual control of viral replication.
Do not use a single reactive ELISA as the basis for lifelong therapy. The final report should state the resolution and the tests used.