Maximum Time Frame for Reading TB Skin Test
The tuberculin skin test should be read between 48 and 72 hours after injection, when induration reaches its maximum, though tests read after 72 hours tend to underestimate the true size of induration. 1
Standard Reading Window
- The optimal reading time is 48-72 hours after PPD injection, as this is when the delayed-type hypersensitivity reaction reaches peak induration 1
- Tests should be read by designated, trained personnel during this window, and patient self-reading should not be accepted 1
- The transverse diameter of induration (perpendicular to the long axis of the forearm) should be measured and recorded in millimeters 1, 2
What Happens After 72 Hours
- Tests read after 72 hours tend to underestimate the true size of induration, making them less reliable for accurate diagnosis 1
- While the reaction typically subsides over a period of days, positive reactions often persist for up to 1 week 1
- In some individuals, particularly the elderly and those being tested for the first time, the reaction may not peak until after 72 hours, though such delayed reactions do not alter the interpretation of the test 1
Clinical Implications of Timing
- Research comparing different reading times found that the adjusted mean TST size was significantly larger at 72 hours compared to both 48 hours and 96 hours (13.4 vs. 11.8 vs. 10.1 mm, P < 0.05) 3
- The smallest number of false results was observed when tests were measured at 72 hours 3
- One study of Southeast Asian refugees found that reading tests at 5-7 days captured additional positive reactions that were negative at 48 hours, suggesting some populations may benefit from extended reading windows 4
Practical Considerations
- The 48-72 hour window requires patients to return for a second visit, which can be a barrier to completion 5
- Reading should be performed in good light with the forearm slightly flexed at the elbow 1, 2
- Only induration (hard, raised, palpable swelling) should be measured—redness or erythema alone should not be measured 1, 2
- The absence of induration should be recorded as "0 mm," not "negative" 1, 2
Common Pitfalls to Avoid
- Do not read tests before 48 hours, as the reaction may not have fully developed 1
- Avoid reading tests significantly beyond 72 hours when possible, as this increases the risk of underestimating induration size 1
- Do not accept patient self-readings, as proper measurement requires trained personnel 1
- Immediate hypersensitivity reactions (which disappear by 24 hours) should not be confused with delayed hypersensitivity reactions 1