Background and Objective: Intradermal Purified Protein Derivative PPD test is a reliable test assessment of primary mycobacterial infection. In this test the specific antigen is Purified Protein Derivative (PPD). The reaction were assessed by touching, the induration's diameter after 48 &48hours. We compared the induration's size 48 and 72 hour in this study.

  Methods: At this semi experimental study a 5 unit PPD was administered to 120 healthy medical students.

  Results: The measurements made at 72 h were significantly (4.22 mm) (p<0.001) higher than those made at 48h (2.79 mm). The reading taken at 72 h were 1.47mm larger than at 48 h recovery. There were significant differences between induration size of 48 and 72h between male and female.

  Conclusion: This study demonstrates that, in adults, the size of the 5 U Monteux reaction is significantly larger at 72h compared to the reaction at 48h. We suggest to read PPD after 72 hours if the PPD is negative after 48 hours.