A Positive TB Skin Test in a Healthcare Worker: Reading the Whole Picture
A 38-year-old nurse is hired at a community hospital. Pre-placement screening shows a 12 mm induration on a tuberculin skin test (TST). She reports no known TB contact, no symptoms, and a normal chest radiograph. She was born in a country with high TB incidence and received BCG as an infant. She has never had a prior TST or IGRA. The employee health nurse asks whether this is a 'positive' test, whether she needs treatment, and whether she can work. The hospital's policy says 'positive TST = chest x-ray and annual symptom review.'
1. Interpret the TST result in this worker and explain the role of BCG and prior testing history.
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A 12 mm induration meets the ≥10 mm threshold for a positive TST in a healthcare worker, and BCG history does not change the interpretation: BCG can cause a false-positive TST, but the effect is mainly on smaller reactions and wanes with time since vaccination, and current guidance is to interpret the TST without adjusting for BCG. Because she has no prior TST/IGRA, this cannot be attributed to boosting. The result therefore indicates TB infection (latent TB infection, LTBI) until proven otherwise, and the normal chest radiograph and absence of symptoms make active TB unlikely. The judgment point is that BCG is not a reason to dismiss a 12 mm reaction in a healthcare worker, and that a single positive test in a never-tested person is interpreted at face value.
2. Recommend the next steps for diagnosis, treatment, and work restriction, and explain the basis for each.
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Diagnosis: confirm with an IGRA (which is not affected by BCG) to support the diagnosis; if IGRA is negative, the TST may still be considered positive given the 12 mm reaction, but the IGRA helps clarify. Exclude active TB with symptom review and chest radiograph (already normal). Treatment: offer treatment for LTBI after excluding active disease, with a regimen chosen for safety and completion (e.g., 4 months rifampin, or 3 months weekly isoniazid + rifapentine, or 6-9 months isoniazid), after baseline liver enzymes and counseling about hepatotoxicity and drug interactions. Work restriction: no restriction is needed for LTBI; she is not infectious. Only active, untreated TB requires exclusion from patient care. The judgment point is that a positive test in an asymptomatic worker with a normal film is LTBI, which is treatable and not a bar to employment, and that the IGRA is used to resolve BCG ambiguity rather than to overrule a clearly positive TST.