How many hours after administration of the PPD (Purified Protein Derivative) test for tuberculosis should the reaction be assessed?

PPD Test Reading Time

The PPD (tuberculin skin test) must be read between 48 and 72 hours after injection, when induration reaches its maximum size. 1, 2

Standard Reading Window

• The American Thoracic Society and CDC guidelines establish 48-72 hours post-injection as the mandatory reading window for tuberculin skin testing 1, 2
• Tests should be read by trained healthcare personnel only—patient self-reading is unacceptable 1, 3
• Reading after 72 hours will underestimate the true size of induration and should be avoided 2, 3

Physiologic Rationale for This Timing

The 48-72 hour window is based on the immunologic cascade of delayed-type hypersensitivity:

• The tuberculin reaction begins 5-6 hours after injection as sensitized T cells are recruited to the injection site 1, 2
• Maximal induration occurs at 48-72 hours due to lymphokine release, local vasodilatation, edema, fibrin deposition, and inflammatory cell accumulation 1, 2
• The reaction then subsides over subsequent days 1

Special Circumstances

Elderly or First-Time Test Recipients

• In elderly patients or those being tested for the first time, the reaction may not peak until after 72 hours 1, 2
• This delayed response does not alter test interpretation—the standard cutoffs still apply 1, 2

Immediate Hypersensitivity Reactions

• Immediate reactions (appearing within hours) can occur to tuberculin or diluent constituents 1, 2
• These disappear by 24 hours and should not be confused with the delayed hypersensitivity reaction that represents true tuberculin sensitivity 1, 2
• If severe immediate reactions occur, retesting may not be prudent 1

Critical Reading Technique

When reading at 48-72 hours:

• Measure only the transverse diameter of induration (palpable, raised, hardened area), NOT erythema (redness) 1, 2, 3
• Record measurements in millimeters; document "0 mm" rather than "negative" when no induration is present 2
• Reading should be performed in good light with the forearm slightly flexed, using both inspection and palpation 2
• The transverse diameter should be measured perpendicular to the long axis of the forearm 2

Common Pitfalls to Avoid

• Do not read tests before 48 hours or after 72 hours—this compromises accuracy 2, 3
• Do not measure or record erythema—only induration matters for interpretation 1, 2, 3
• Do not accept patient self-readings 1, 3
• Do not delay reading beyond 72 hours, as this reduces measured induration size 2, 3