PPD Test Reading at 80 Hours
Reading a PPD test at 80 hours after placement is not recommended and will likely underestimate the true induration size, potentially leading to false-negative results. 1, 2
Standard Reading Window
The tuberculin skin test must be read between 48-72 hours after injection, when the delayed-type hypersensitivity reaction reaches its maximum induration. 1, 2
- The American Thoracic Society and CDC guidelines explicitly state this 48-72 hour window as the standard for accurate interpretation. 1, 2
- The physiologic basis for this timing is that the delayed hypersensitivity reaction begins 5-6 hours after injection and reaches maximum induration at 48-72 hours due to T-cell recruitment and local inflammatory cell accumulation. 1, 2
What Happens After 72 Hours
Tests read after 72 hours tend to underestimate the true size of induration because the reaction begins to subside, although positive reactions can persist for up to one week. 1, 2
- At 80 hours (3.3 days), you are beyond the optimal reading window and the induration is likely decreasing from its peak. 2
- Reading outside the recommended 48-72 hour window reduces accuracy and should be avoided. 3, 1
Clinical Implications at 80 Hours
If you must read a test at 80 hours:
- Any measured induration should be interpreted with extreme caution as it likely represents an underestimate of the true peak reaction. 1
- A negative or borderline result at 80 hours cannot be considered reliable and the test should be repeated with proper timing. 3, 4
- A clearly positive result (significantly above the threshold for the patient's risk category) may still be valid, as positive reactions can persist beyond 72 hours. 2
Research Evidence on Timing
Research demonstrates that even the difference between 48 and 72 hours is clinically significant:
- One study showed measurements at 72 hours were 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 solely on timing. 4
- This underscores that readings outside the 48-72 hour window are unreliable. 4
Practical Recommendation
If a patient presents at 80 hours for reading, document that the test was read outside the recommended window and strongly consider repeating the test with proper 48-72 hour follow-up. 1, 3 This is particularly important for high-risk individuals where accurate diagnosis is critical for initiating preventive therapy. 2