Which statement about PPD interpretation is correct?

• A. PPD can differentiate active TB from latent TB.
• B. PPD results should be read within 24 hours after administration.
• C. A positive PPD in all individuals is defined as ≥15 mm.
• D. PPD interpretation cannot differentiate active from latent TB.

Answer: (D)

PPD interpretation shows exposure or infection with Mycobacterium tuberculosis, but it does not tell you whether the infection is latent or active disease. To decide if someone has active TB, you need the whole clinical picture—symptoms such as cough, fever, night sweats, weight loss—plus chest radiography and microbiologic testing (sputum smear/culture, NAAT).

Reading the test is done 48 to 72 hours after administration, not at 24 hours, and the result is read as induration in millimeters, not just redness; thresholds for a positive result vary by risk factors: small indurations have different cutoffs for high-risk groups (for example, ≥5 mm in HIV) versus lower-risk individuals (≥15 mm for those with no risk factors). This is why a universal cutoff of ≥15 mm for everyone isn’t correct.

So the statement that PPD interpretation cannot differentiate active from latent TB is the correct one.