The test was considered successful, if after injection of 0.1 ml of tuberculin, a wheal of at least 6 to 10 mm in diameter was produced. Necessary precautions were taken to avoid any bias due to quality, quantity, or method of administration. Skin tests were read after 48 hours by the same experienced pulmonologist. In each test, the patient was standing behind a screen through which only the forearm was exposed through a hole. In that way, the identity of the person tested was unknown to the reader. Induration, not erythema, was first measured by the palpation technique, and the average of the measurement in two axes at right angles, was recorded.
Later the same day, the measurement was repeated, in a random sequence, by the same reader, in the same manner, using the pen technique. Using a medium ballpoint pen, a line was drawn from a point 5 to 10 mm away from the margin of the skin induration towards its center, until resistance was felt to further movement. The procedure was repeated on the opposite site and in a right angle axis. Again the average of the measurements in the two axes was recorded. Induration <5 mm was considered as negative, ^10 mm as positive, and 5-9 mm as doubtful. Statistical analysis was performed using the two-tailed, paired Students t-test, and linear regression was computed by the least squares method and a p<0.05 was considered as showing a statistically significant difference.