298. How long does it take for a PPD to become positive? / Tuberculin skin test procedure / How to read a PPD / Criteria for positive PPD April 9th, 2008 by reuben in TB Back Top Tags: card 297. Relative usefulness of variably-acquired sputum samples for TB / Usefulness of microscopy / Limitation of TB culture 299. PPD false positives / False negatives Leave a Reply Cancel reply Please post any discussion to the emupdates Reddit page.