TB Skin Test Reading Time
The tuberculin skin test must be read between 48 and 72 hours after injection, when induration reaches its maximum size. 1
Optimal Reading Window
The standard reading window is 48-72 hours post-injection, as recommended by the American Thoracic Society and CDC guidelines 1
Reading at 72 hours is preferable to 48 hours because induration size is significantly larger at 72 hours, and tests read at only 48 hours may yield false-negative results 2, 3
Research demonstrates that TST measurements at 72 hours are on average 1.7 mm larger than at 48 hours, with 8.5% of test results changing classification based on timing alone 2
Physiologic Basis for Timing
The tuberculin reaction begins 5-6 hours after injection as a delayed-type hypersensitivity response 1
Maximal induration occurs at 48-72 hours due to T-cell recruitment, lymphokine release, and local inflammatory cell accumulation 1
The reaction then subsides over subsequent days 1
Special Circumstances
In elderly patients or first-time test recipients, the reaction may not peak until after 72 hours, though this delayed response does not alter test interpretation 1
Tests read after 72 hours tend to underestimate the true size of induration and should be avoided 1
Immediate hypersensitivity reactions (appearing within hours) disappear by 24 hours and should not be confused with the delayed hypersensitivity reaction that represents true tuberculin sensitivity 1
Critical Reading Technique
Measure only the transverse diameter of induration (perpendicular to the long axis of the forearm), not erythema 1, 4
Record measurements in millimeters; document "0 mm" rather than "negative" when no induration is present 1, 4
Reading should be performed in good light with the forearm slightly flexed, using both inspection and palpation 1, 4
The ballpoint pen method combined with ruler measurement at 72 hours produces the smallest number of false results 3