What is the optimal timing for reading the Mantoux tuberculin skin test?

Optimal Timing for Reading the Mantoux Tuberculin Skin Test

The Mantoux tuberculin skin test must be read between 48 and 72 hours after injection, when the delayed-type hypersensitivity reaction reaches maximum induration. 1, 2

Physiologic Rationale for the 48-72 Hour Window

The timing is based on the immunologic cascade of the delayed hypersensitivity response:

• The tuberculin reaction begins 5-6 hours after injection with initial neutrophil infiltration 1
• T-cell recruitment and lymphokine release occur progressively, with CD4 T-cells accumulating perivascularly by 48 hours 1
• Maximum induration occurs at 48-72 hours due to peak inflammatory cell accumulation, local vasodilatation, edema, and fibrin deposition 1
• The reaction then subsides over subsequent days 1

Critical Reading Guidelines

Measure only the transverse diameter of palpable induration (perpendicular to the long axis of the forearm), not erythema, and record in millimeters:

• Reading should be performed in good light with the forearm slightly flexed at the elbow 1, 2
• Use both inspection (from a side view against light) and palpation to determine induration boundaries 1, 2
• The ball-point pen method reduces interobserver variability—gently push the pen tip at a 45° angle toward the injection site until it stops at the edge of induration 1, 2
• Record "0 mm" rather than "negative" when no induration is present 1, 2
• Trained healthcare professionals must perform the reading; patient self-reading is inaccurate and strongly discouraged 2, 3

Special Circumstances and Exceptions

Delayed Reactions in Specific Populations

• In elderly patients or first-time test recipients, the reaction may not peak until after 72 hours 1
• This delayed response does not alter test interpretation—the result remains valid 1

Reading After 72 Hours

• Tests read after 72 hours tend to underestimate the true size of induration and should be avoided 1, 2
• Research evidence shows that readings at 72 hours are significantly larger than at 48 hours (median 9.4 mm vs 4.95 mm), with 8.5% of subjects changing from negative to positive based on timing alone 4
• However, one study found that subjects who fail to present at 48-72 hours may still have a reliable reading up to 144-168 hours (days 6-7), particularly useful in real-world settings where patients miss appointments 5

Distinguishing True from False Reactions

• Immediate hypersensitivity reactions appear within hours but disappear by 24 hours and should not be confused with delayed hypersensitivity 1, 2
• If a severe immediate reaction occurs, it may be prudent not to retest 1

Common Pitfalls to Avoid

• Never measure or record erythema alone—only induration counts for interpretation 1, 2
• Never accept patient self-reading—studies demonstrate poor accuracy 3
• Never delay reading beyond 72 hours unless unavoidable—this underestimates induration size 1, 2, 4
• Do not confuse immediate hypersensitivity (resolves by 24 hours) with the delayed reaction 1, 2

If Improper Injection Occurs

• If no wheal forms during injection, immediately repeat the test at another site at least 2 inches away 2
• Circle the second injection site to indicate which site should be read 2