Improved Ziehl-Neelsen Microscopy: Bleach Sputum Smear Negative Specimens after Centrifugation

Abstract

Background: Direct ZN (Ziehl-Neelsen) sputum smear microscopy for diagnosis of TB (tuberculosis) has low sensitivity, especially in TB/HIV co-infected patients. Sputum concentration by bleach (NaOCl) with sedimentation has been used to increase the sensitivity of sputum smear microscopy in many settings but with varying results. Objective: To determine whether bleach plus centrifugation significantly improves the detection of AFB (acid-fast bacilli) in ZN smear-negative sputum specimens. Methods: Three hundred and seventy sputum specimens were collected from new TB suspects attending a Nairobi referral district hospital and processed for direct microscopy using ZN technique and culture on Lowenstein Jensen Media. All smear-negative specimens were treated with 3.5% bleach and left to stand for 30 min before centrifugation. The bleach treated smears were processed and examined using ZN technique. Results: Of the 370 specimens, 200 (54%) were positive culture. The number of sputum samples that were smear-positive by direct ZN was 138 (37.2%), with a sensitivity of 66%. After treatment of direct ZN smear-negative specimens with 3.5% bleach and centrifugation, the total number of AFB smear-positive samples increased to 171 with an increase in sensitivity of 66% to 81.1% (15.1%). Conclusion: In this study, bleach with centrifugation significantly increased the yield of sputum smear microscopy. Further evaluation of these techniques in routine programmes is required especially in settings where the burden of TB/HIV is high.