Why should we examine at least 200 oil-immersion fields to declare a sputum smear as negative?
Ans: A sputum smear is spread over an area of 200 mm2 and contains about 0.01 ml of the specimen. Each oil-immersion field has an area of 0.02 mm2. Therefore, there are 10,000 fields on the slide. If a specimen contains 5000 bacilli/ml, there will be 50 bacilli distributed over entire smear and 1 bacilli over 200 fields. Considering that the bacilli are not distributed evenly over the smear, there must be at least 10,000 bacilli/ml to detect 1 bacillus over 200 fields.
