TB Skin Test Reading After 72 Hours
If a patient presents after 72 hours for TB skin test reading, the test should still be read and interpreted, though it may underestimate the true induration size; however, in elderly patients or first-time test recipients, delayed reactions peaking after 72 hours do not alter interpretation. 1
Standard Reading Window and Physiologic Rationale
The tuberculin skin test must be read between 48-72 hours after injection when induration reaches maximum size, as recommended by the American Thoracic Society and CDC. 2, 1
The delayed-type hypersensitivity reaction begins 5-6 hours post-injection, with maximal induration occurring at 48-72 hours due to T-cell recruitment, lymphokine release, and inflammatory cell accumulation. 2, 1
Management When Patient Presents After 72 Hours
The test should still be read if the patient presents after 72 hours, with the understanding that measurements may underestimate true induration size. 1
Special Populations with Delayed Reactions
In elderly patients and first-time test recipients, the reaction may not peak until after 72 hours; importantly, this delayed response does not alter test interpretation. 2, 1
These delayed reactions in specific populations are physiologically normal variants and should be interpreted using standard criteria. 2
Critical Reading Technique
Measure only the transverse diameter of induration (perpendicular to the long axis of the forearm), not erythema, using both inspection and palpation in good light. 1
Record measurements in millimeters; document "0 mm" rather than "negative" when no induration is present. 1
Common Pitfall to Avoid
Do not confuse immediate hypersensitivity reactions (appearing within hours and disappearing by 24 hours) with the delayed hypersensitivity reaction that represents true tuberculin sensitivity. 2, 1
Tests read significantly after 72 hours tend to underestimate induration and should be documented as such, though they remain interpretable. 1
When to Consider Retesting
- If the reading after 72 hours shows borderline results or if clinical suspicion for TB infection remains high despite a negative or equivocal late reading, consider repeating the test with proper 48-72 hour timing. 2