THE YIELD OF ACID-FAST BACILLI FROM SERIAL SMEARS IN ROUTINE MICROSCOPY LABORATORIES IN RURAL TANZANIA

Routine results of direct examination of sputum smears for acid-fast b acilli from 34 laboratories in Tanzania were analysed. These represent ed 8 regions providing 94 laboratory-years of work; 61 580 tuberculosi s suspects were evaluated with the aid of 141 371 smears. The average proportion of cases found among suspects was 18.9% (range 14.3-23.8% i n the 8 regions). The number of cases missed among suspects with incom plete examinations was calculated based on the number observed among s uspects with a complete set of 3 smears examined, and an incremental y ield of 83.4% with the first, 12.2% with the second, and 4.4% with the third smear was estimated for the total number of expected cases. The se data suggest that (i) the method frequently employed for calculatin g requirements for laboratory supplies in low income countries, based on the estimate that 10 suspects need examination to identify one case of sputum smear-positive tuberculosis, is generous in the concert of Tanzania and (ii) under routine conditions the incremental yield from a third smear examination after 2 negative examinations is relatively small.