Counts of viable tubercle bacilli in sputum related to smear and culture gradings.

Pairs of sputum specimens obtained pre-treatment from 166 smear-positive patients with pulmonary tuberculosis were examined by direct smear, culture on Löwenstein-Jensen medium after decontamination by the Petroff method, and by quantitative colony counting on selective 7H11 medium after digestion with dithiothreitol. The selective medium counts ranged from no growth to 8.3 log10 cfu/ml with the largest numbers in the range 3.5-7.0 log10 cfu/ml. Although there was overlap in counts between specimens with negative and positive direct smears, a specimen with a count of 4.0 log10 cfu/ml or more was likely to have a positive smear while a negative smear was likely if the count was lower. This demarcation value should be increased to 4.5 log10 cfu/ml to account for under-estimation in the selective medium counts. The corresponding demarcation estimate for LJ cultures was 80 colonies. In distinguishing between patients in the bacterial contents of their sputum, the selective medium counts were better than the gradings of either LJ cultures or direct smears.