Need for Different Cutoff Values for Reading Mantoux Test with 2TU and 5TU PPD

To compare the tuberculin reaction of 2 tuberculin unit (TU) with 5TU purified protein derivative (PPD) (both calibrated against RT 23) in healthy children.

This was a cross sectional study done in the pediatric outpatient department of a tertiary care teaching hospital. Seventy healthy siblings of the children attending pediatric outpatient department in the age group of 1 to 12 y were enrolled. The exclusion criteria included previously diagnosed tuberculosis patients, malnutrition diagnosed according to the WHO classification, history of drug intake like steroids, recent history of measles, any skin lesions over forearm, history of fever, contact with tuberculosis and previous mantoux testing. The study was conducted wherein each child was subjected to simultaneous testing with 2TU and 5TU by standard technique. The reactions to both the tests was read at 48-72 h. Children with induration ≥10 mm were evaluated for tuberculosis by taking chest x-ray, gastric lavage or sputum smear examination for acid fast bacilli (AFB).

Considering ≥10 mm induration as positive, subjects positive with 5TU were 7 (10%) and 2TU was 1(p value = 0.031); thus, there is no agreement between the two methods (McNemar's test). Comparing the mean diameter of induration of 2TU and 5TU (p < 0.001, Wilcoxon test), signified no agreement between the two strengths. Bland-Altman plot and kappa statistic showed no agreement between the two strengths.

Cutaneous hypersensitivity to 2TU PPD is not comparable to that of 5TU PPD.