Comparison of Sensitivity and Specificity of ZN and Fluorescent Stain Microscopy with Culture as Gold Standard

Introduction: Reports indicate that fluorescent staining of smears increases sensitivity of direct microscopy; so ZN staining is being replaced with fluorescent microscopy in RNTCP in India. Chemical processing and sputum concentration may also improve sensitivity of microscopy. Objective: To compare the sensitivity and specificity of microscopy for AFB using ZN and fluorescent stains in direct and concentrated specimen with culture as gold standard. Methods: Morning sputum specimen of patients, suspected of having pulmonary tuberculosis, over a period of 6 months was subjected to direct microscopy using fluorescent stain; the same slide was over-stained with ZN stain. Same sputum sample was concentrated by Petroff’s method and subjected to fluorescent microscopy followed by ZN microscopy and finally to culture for AFB. Results: Sensitivity of fluorescent stained concentrated sputum samples was maximum and of ZN stained unprocessed sputum samples was minimum. Specificity of three of the methods was equal at 0.96 but of ZN stained concentrated sputum smears was 0.97. Sensitivity of total fluorescent stains was 0.85 (Specificity 0.96) and sensitivity of total ZN stained smears was 0.80 (Specificity 0.96). Discussion: We used same smear for fluorescent and ZN stains, so smear related variability is decreased. Blinding for microscopy was practically complete. Conclusion: The sensitivity of sputum microscopy for AFB can be increased by concentrating the sputum and using fluorescent microscopy. The specificity remains high in all the methods.