Blinded rechecking of sputum smears for acid-fast bacilli to ensure the quality and usefulness of restaining smears to assess false-positive errors

Selvakumar, N., Prabhakaran, E.; Rahman, F.; Chandu, N.A.; Srinivasan, S.; Santha, T.; Chauhan, L.S.; Narayanan, P.R.

International Journal of Tubercle and Lung Diseases; 2003; 7; 1077-1082.

Setting : Microscopy Centres, Velliyur Tuberculosis Unit, Tiruvallur District, Tamil Nadu, India.

Procedures : Twelve microscopy centre laboratory technicians examined 41 978 direct sputum smears for acid-fast bacilli. Senior Tuberculosis Laboraotry Supervisors (STLS) checked all positive smears (4696) and 10% of negative smears (4776) in an unblended fashion as per Revised National Tuberculosis Control Programme (RNTCP) guidelines. Ten per cent of the positive and negative slides and another 10% of unchecked negative slides were selected systematically for blinded rereading at the Tuberculosis Research Centre (TRC); 422 slides were reread without and with restaining.

Results : Unblinded checking by STLS of the smears read by the laboratory technicians yielded 95 to 100% agreement. Blinded rereading at the TRC revealed that false-negative errors were greater among the laboratory technicians (2-7%) than the STLS (0-3%). Restaining and blinded rereading of slides reduced false-positive errors from 27% to 7%.

Conclusion : Blinded rereading at the reference centre facilitates assessment of laboratory technicians and STLS. Restaining before rereading the smears was found to be useful for precise estimation of false-positive errors.

Keywords : pulmonary tuberculosis; quality assurance; sputum smears; acid-fast bacilli; microscopy

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