A reactive HIV combo assay with a negative NAT in a repeat blood donor
A blood bank tests 80 donations daily with a fourth-generation HIV-1/2 antigen–antibody combination ELISA and, for reactive units, an individual-donation nucleic acid test (ID-NAT) for HIV, HCV and HBV. A 26-year-old repeat donor's donation is reactive with an S/CO of 4.8 on the combination assay; the ID-NAT is negative and a point-of-care rapid immunochromatographic test is also negative. He reports a flu-like illness 12 days ago and a new sexual partner three weeks ago. He also donated 60 days earlier; that unit was processed into red cells and fresh frozen plasma, both already issued to hospitals. The laboratory SOP states that any repeat-reactive sample must be confirmed by a line immunoassay (Western blot) and that donors are notified only after confirmation; the line immunoassay is negative.
1. Interpret the combination of results. What is the most likely explanation, and how much weight should the negative line immunoassay and the negative rapid test carry?
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The combination - repeat-reactive fourth-generation combo assay (which detects p24 antigen plus IgG/IgM antibody), negative ID-NAT, negative rapid test and negative line immunoassay - is most consistent with a biological false-positive reaction, with early HIV infection still a possibility that cannot be excluded on a single sample.
The reasoning turns on the kinetics of the markers. After infection, HIV RNA becomes detectable at about day 10, p24 antigen around days 15–20, and antibody around days 20–30. The reported exposure (new partner three weeks ago; febrile illness 12 days ago) falls inside the window in which a true infection could be present, but by day 12–21 RNA would already be expected to be positive and ID-NAT is the most sensitive of the four assays. A reactive antigen/antibody screen with a negative NAT therefore points away from acute infection. Non-specific reactivity is well recognised after acute febrile illness and in other inflammatory or autoimmune states, and S/CO values below about 10 are the range where false reactivity concentrates.
Weight of the other tests: the rapid test is less sensitive than the laboratory assay and a negative result adds little. The negative line immunoassay must not be read as 'confirmed negative' - in early seroconversion it is expected to be negative or indeterminate, which is exactly why a negative Western blot cannot be used to exclude recent infection. The SOP's algorithm (line immunoassay as the sole confirmatory step) is out of date for a fourth-generation era, when NAT and repeat sampling are the decisive tests.
2. What must happen to this donation and to the earlier one, and what follow-up testing and deferral do you recommend? Identify the main defect in the SOP.
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Current donation: do not release. The unit and all co-components must be quarantined and discarded regardless of the negative NAT and line immunoassay, because the donor is repeat-reactive and infectivity is not excluded. The donor is deferred pending resolution.
Earlier donation (60 days ago): this is a mandatory lookback, triggered by the donor's repeat reactivity, not by confirmation. Both components have been issued, so the blood bank must immediately notify the receiving hospitals, request that any untransfused components be quarantined and returned, and trace the recipients so that they can be tested (HIV combo, NAT, and full transfusion-transmitted infection screen). The earlier donation predates the reported exposure, so transmission from it is less likely, but the donor may have been infected before the reported partner; lookback is not optional on probability grounds.
Follow-up: repeat sampling at about 6 weeks and 3 months with fourth-generation combo plus ID-NAT; if both are negative and no other risk emerges, the donor can be re-entered according to written criteria. Add an HIV-1/HIV-2 discrimination assay if any later sample is reactive.
SOP defect: requiring line immunoassay confirmation before donor notification, and treating a negative line immunoassay as final, delays counselling and deferral for a donor who may be in the window period. The SOP should specify that repeat-reactive donors are notified of the need for follow-up (without being labelled positive), that NAT is part of the resolution algorithm, and that lookback is initiated on repeat reactivity. It should also state that a negative Western blot in a recent-exposure context is an indeterminate result, not a negative.
3. Draft the counselling message for this donor, including what he must do in the interval and what he must not be told.
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What to say: 'Your screening test for HIV was reactive, but the follow-up tests we ran (the nucleic acid test and the confirmatory blot) were negative. That pattern is often caused by a false reaction - for example after a recent viral illness - but it can also occur very early after infection, before the tests become positive. We cannot call this a negative or a positive result today. We need to repeat the tests on a fresh sample at about six weeks and again at three months. Please do not donate blood, plasma, platelets or semen in the meantime, and use condoms with any partner until the follow-up is complete.'
Offer referral to a confidential ICTC/voluntary counselling and testing centre for independent testing and support, explain the window period in plain language, and give a written appointment. Explain that his earlier donation is being reviewed and that recipients are being contacted as a precaution - this is standard practice and is not an accusation.
What not to say: do not tell him he is HIV-positive, and do not tell him he is definitely negative. Do not reassure him that 'the blot was negative, so you are fine'. Do not disclose the result to family, employer or partner without his consent; partner notification, if needed later, must be handled through the counselling service. Record the discussion, the deferral and the follow-up plan in the donor record.