A tuberculin skin test is read 72 hours after intradermal injection in a 52-year-old renal transplant recipient on tacrolimus and prednisolone, who was recently exposed to a sputum smear-positive relative. The induration measures 8 mm. How should this result be interpreted?
- A Negative, because less than 10 mm induration is required in any adult
- B Positive, because an induration of 5 mm or more qualifies in transplant recipients and recent contacts ✓
- C Positive only if vesiculation or necrosis is present
- D Interpretation is invalid because corticosteroids always abolish the response
Correct answer: B. Positive, because an induration of 5 mm or more qualifies in transplant recipients and recent contacts
Explanation
Cut-offs are tiered by risk: an induration of 5 mm or more is read as positive in persons living with HIV, recent close contacts of infectious cases, organ transplant recipients, and those on equivalent of 15 mg/day or more prednisolone for a month or longer, because their probability of true infection is high while their skin test responsiveness may be blunted. The 10 mm threshold applies to moderate-risk groups such as intravenous drug users and laboratory staff, and the 15 mm threshold to persons with no known risk factors.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.