What is the rationale of collecting three specimens for sputum microscopy?
Ans: Of the total smear positive cases presenting at health Institutions, about 69% can be detected by smear examination of first sputum specimen and another 20% by examination of second specimen. The additional case yield by subsequent specimen is minimal. Therefore, a minimum of two specimens should be examined for obtaining acceptable sensitivity of smear microscopy as a case finding tool. Further, there is likelihood albeit small of single positive sputum smear result to be false positive. However, the chances of two positive results to be false positive are practically non-existent. Therefore, the criteria of at least two smear positive results to label a case as smear positive increases the specificity of the test. Since the bacilli are not excreted consistently in all specimens, at least three specimens should be examined to satisfy the above criterion.
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