Random blinded rechecking of AFB smears in a pilot project at an intermediate reference laboratory

In most developing countries, sputum smear microscopy for acid-fast bacilli remains the front line and often the only diagnostic tool for the diagnosis of tuberculosis (TB), making quality assurance of smear microscopy an important activity.

To evaluate the results of a pilot study, where the random blinded rechecking for the entire state of Delhi was conducted at a reference laboratory.

Slides from 25 Revised National Tuberculosis Control Programme designated districts (200 peripheral microscopy centres) in Delhi were re-read after proper coding by all the Senior Tuberculosis Laboratory Supervisors (STLS) at an intermediate reference laboratory under proper supervision.

Of 12,162 re-read slides, 204 discrepant results were found. Of these, 150 (73.5%) errors were attributed to the peripheral microscopy centres and 54 (26.5%) to STLS. High false-positive errors were observed at a frequency of 12/150 (8%), and high false-negative errors at a frequency of 38/150 (25%). Minor errors, i.e., low false-negative, low false-positive and quantification errors, were observed at frequencies of respectively 68/150 (45.3%), 17/150 (11.3%) and 15/150 (10.0%).

Greater stringency in the supervision of random blinded rechecking at the district level is essential to make smear rechecking more efficient and effective.