How to interpret the smear?
At least 100 oil immersion fields must be viewed before declaring the smear as negative. The sensitivity of smear is low because it requires the presence of 104 bacilli/ml to be smear positive. If the number of bacilli is less than this, the chances of detecting them are less. In such a case, the sample should be subjected to concentration techniques such as Petroffs method. If the smear is positive for AFB, it should be counted/graded. Failure to detect any AFB does not rule tuberculosis. Grading of smears has prognostic value.
