Point-of-Care HBsAg Testing in Antenatal Care
A subdistrict hospital in a low-resource setting plans to introduce a point-of-care rapid test for hepatitis B surface antigen (HBsAg) in antenatal clinics so that positive mothers can be referred for antiviral prophylaxis and their newborns vaccinated within 24 hours. Local HBsAg prevalence among pregnant women is 4%. The manufacturer reports sensitivity 92% and specificity 95%. The laboratory can confirm positives with ELISA (sensitivity 98%, specificity 99%), but has capacity for only 40 confirmations per month and a 3-day turnaround. In a pilot of 300 women, 36 tested positive on the rapid test and 12 were later confirmed. The clinic has no electronic record, and one midwife runs the antenatal session alone with 45 women per day. The hospital board asks whether to roll out the rapid test, restrict it, or wait.
1. Calculate the positive and negative predictive values of the rapid test at 4% prevalence and explain what they mean for counselling a woman who tests positive.
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Per 1,000 women at 4% prevalence: 40 truly have HBsAg and 960 do not. True positives = 40 × 0.92 = 36.8; false negatives = 3.2; false positives = 960 × 0.05 = 48; true negatives = 912. PPV = 36.8/(36.8 + 48) = 43%. NPV = 912/(912 + 3.2) = 99.7%.
So a positive rapid test means roughly a 43% chance of true infection - more likely false than true. A negative result is highly reassuring but not perfect, missing about 3 infections per 1,000 women. Counselling must therefore avoid words such as infected or carrier at this stage: explain that the result is a screening result, that a second test is needed, and that she should return for confirmation. A negative result does not remove the need for the birth-dose vaccine, which should be given to every newborn regardless of maternal status.
In the pilot, 12 of 36 positives were confirmed (33%; 95% CI roughly 19–51%), consistent with the predicted 43%. The pilot positivity of 12% is higher than the 8.8% expected from 4% prevalence plus false positives, so local prevalence should be re-estimated. The test is useful for triage and rule-out, not for declaring a woman infected.
2. Given the ELISA capacity of 40 confirmations per month and a 3-day turnaround, recommend how the hospital should use the rapid test and the ELISA.
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Estimate demand: 45 women per day × about 26 clinic days = roughly 1,170 tests per month. At 12% positive that is about 140 confirmations per month; at 8.8% it is about 103. Capacity is 40, so a screen-all-then-confirm-all model creates a queue, and a 3-day turnaround with paper records means women will be lost before treatment.
Options: test at the first antenatal visit, ideally before 28 weeks, so confirmation and tenofovir can start in the third trimester; prioritise ELISA for women at or beyond 28 weeks, those who are HBeAg-positive on a rapid test, and those with other high-risk features; expand ELISA capacity through additional runs, pooled testing or a regional laboratory; do not use a second rapid test as confirmation, because the errors are correlated and it adds little. Give birth-dose vaccine to every newborn irrespective of maternal status, so the rapid test is not on the critical path for preventing perinatal transmission.
Recommend a restricted roll-out with triage and a named tracker rather than a blanket programme that overwhelms the laboratory. Monitor positivity, confirmation rate, time from test to result, treatment initiation and the proportion of infants vaccinated within 24 hours. Stop and redesign if the system cannot close the loop from test to treatment.
3. Identify the implementation and ethical safeguards needed if the programme is rolled out.
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Consent and counselling: explain that the test is for her care and her baby's, that a positive result needs confirmation, and that she may decline. Confidentiality: do not share results with her husband or family without her consent, because disclosure can trigger stigma or violence; support her to disclose where she wishes. Equity: no fee at the point of care, and no public marking or separate queue that identifies positive women. Quality: train the midwife in the test's limits, keep a register, and audit confirmations against ELISA. Linkage: a named contact and appointment for confirmation and treatment, combined with later antenatal or delivery visits for women who travel far. Workload: 45 women per day with one midwife is unsafe for counselling and testing; add a second provider or a trained counsellor. Data protection: a locked register, unique identifiers, and no HBsAg status written on cards carried openly. Monitoring: positivity, confirmation, treatment initiation, birth-dose vaccination and loss to follow-up. Ethical review and community engagement before roll-out, with a complaints route. Finally, check that the programme does not crowd out other antenatal care; if it does, phase it in by facility.