We know how to evaluate the results of the tuberculin test: a hard node ≤ 4 mm in diameter is negative, 5-9 mm is weakly positive (+), 10-19 mm is positive (++), ≥ 20 mm or although <20 mm but localized blistering and lymphangitis is a strong positive reaction (++++ to ++++). However, tuberculin tests are not completely accurate and we should not just look at the appearance. False negative tuberculin test can occur for the following reasons: ① Pre-metaplasia, i.e. clinical incubation period of 2-3 weeks on average, and pre-metaplasia of 5-6 weeks, false negative for tuberculin can occur. ②Severe tuberculosis such as cornified tuberculosis, nodular brain, etc. ③ Viral infections or acute infectious diseases such as measles, false negatives occur due to disturbance of the immune system. ④Cancer patients, malnutrition, elderly people, all can show low junctional response performance. ⑤ Technical factors, such as potency, dose, transportation and preservation of conjugate, and misinjection of subcutaneous. ⑥Atypical Mycobacterium avium infection is negative for 5IU human-type-PPD cross-reaction. False positive tuberculin test can occur for the following reasons: ① atypical mycobacterium infection (cross-reactivity); ② overlapping tests at the same site; ③ a few reactions slightly greater than the positive criteria (>5mm) in uninfected persons without BCG vaccination history in the flow transfer, can be retested once, and the second reaction is negative which is a false positive.