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The Missed PPD Reading -- Can This Wait a Week?

Researchers investigated the reliability of delayed skin-test readings, and a newly available assay may clarify response in BCG vaccinees.

The interpretation of purified-protein-derivative (PPD) skin tests for tuberculin sensitivity is difficult when the patient does not return within 48 to 72 hours. QuantiFERON-TB is a blood-based interferon- release assay (IGRA) that measures the response to Mycobacterium tuberculosis and other nontuberculous mycobacteria. To determine the validity of skin-test results at 7 days, Tat and colleagues compared readings on days 2 and 7 for each of the two available tuberculin skin tests and evaluated them against IGRA results.

They tested 116 adults at increased risk for tuberculosis, using one of the skin-test products on each arm, and obtained blood samples for IGRA. Fifty-two had positive skin-test readings on day 2. Agreement between the two skin-test products was 93% on day 2. Overall, 13% of the day-7 skin-test readings were discordant with the day-2 readings; in 10 cases, readings that were initially positive were negative by day 7. Agreement between the IGRA results and the skin-test readings was 88% on day 2 and 85% on day 7.

Comment: These findings demonstrate that although delayed reading of the PPD skin test may be reliable, the standard must remain at 2 to 3 days. The recent commercial availability of the IGRA, which may help clinicians discriminate the response to M. tuberculosis from the response to nontuberculous mycobacteria or BCG vaccination, is an interesting highlight of this report.

— Peggy Sue Weintrub, MD

Published in Journal Watch Pediatrics and Adolescent Medicine February 14, 2005

Citation(s):

Tat D et al. Comparing interferon- release assay with tuberculin skin test readings at 48-72 hours and 144-168 hours with use of 2 commercial reagents. Clin Infect Dis 2005 Jan 15; 40:246-50.

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• 3 - 7 days: 88% accurate reading for a positive Ppd
  Evelyn Clancy, Childcare/Pre-school, 10 Nov 2009 2:37 PM EST
  I remember just mailing in a card with something circled that was closest to what was going on at the... [more]