False negativity in AFB Smear microscopy: An insight into the caveats of the most widely used screening tool for tuberculosis.

OBJECTIVE: To study the ratio of false negativity in sputum samples in diagnostic smears received for acid fast bacilli smear microscopy.

METHODS: The retrospective cross-sectional study was conducted at the Ojha Institute of Chest Diseases, Karachi, and comprised specimens for microscopy and culture from presumptive tuberculosis patients for 38 months starting from November 2010.All laboratory investigations had been done as per the National Tuberculosis Control Programme guidelines. Statistical analyses were performed on MedCalc and Social statistics calculators, and Open Epi software.

RESULTS: TOf the 2,158 specimens, 1,316(60.98%) were of men and 842(39.02%) of women (male-to-female ratio: 1.56:1). Besides, 843(39.06%) were smear-negative, of which 99(11.74%) were false negative. Of the 1,275(96.88%) men whose age was reported, 808(63.37%) were aged 19-45 years, whereas out of the 792(94.06) women whose age was known, 517(65.28%) were in that age group. Microscopic outcomes were significantly related to gender (p<0.001).

CONCLUSIONS: Smear microscopy cannot be solely relied upon for diagnosis and its results must be correlated with additional clinical information and other diagnostics due to considerable amount of false negativity, especially in female population.