Tuberculin skin testing: determinants and reaction

Mantoux purified protein derivative (PPD) skin testing was performed in schoolchildren who were grouped according to positive (Group I, n = 205) and negative (Group II, n = 79) exposure to recent acid-fast bacilli (AFB) smear-positive tuberculosis (TB) family contact. A prospective case-control study was undertaken to evaluate whether repeat bacille Calmette-Guérin (BCG) vaccination, nutritional state, presence/absence of BCG scar, and degree of AFB positivity of sputum of adult TB cases affect PPD skin reactivity in these two groups. Group I with TB contacts had larger induration (13.00 +/- 11.29 mm) than the Group II control group of 4.52 +/- 6.20; P = 0.000. Purified protein derivative reaction as to the number of BCG vaccination(s) received showed an increase in size as the BCG vaccination is repeated with significantly larger induration in Group I than in Group II (P = 0.048). The nutritional status was subgrouped into A (weight < 10 percentile), B (weight 50-75 percentile), and C (weight > 90 percentile), which were comparable for both groups. The mean PPD induration of subgroup A in Groups I and II was not statistically different. However, the mean PPD induration was highly significant between Groups I and II in subgroup B (12.46 +/- 10.70 vs 3.80 +/- 5.71 mm; P = 0.000) and subgroup C (14.31 +/- 11.54 vs 5.42 +/- 6.70 mm; P = 0.000). Children in group I with the BCG scar were noted to have significantly greater PPD induration size than in group II (14.14 +/- 11.23 vs 5.05 +/- 6.24 mm; P = 0.000). The degree of AFB positivity of sputum of TB adult cases (1+ to 4+ and cavitary TB) has no effect on PPD size (P = 0.766). Close contact with individuals with active TB (AFB smear positive) is a very important factor for PPD skin conversion. Repeat BCG vaccination, malnutrition, and BCG with scars present difficulties in making a diagnosis of TB infection but did not affect PPD reactivity and did highlight the need for thorough clinical evaluation.