Academic journal article Bulletin of the World Health Organization

Quality Control of Kato Slide Counts for Schistosoma Mansoni: A Review of 12 Years' Experience in Kenya

Academic journal article Bulletin of the World Health Organization

Quality Control of Kato Slide Counts for Schistosoma Mansoni: A Review of 12 Years' Experience in Kenya

Article excerpt


Few would question the desirability of checking the performance of technical staff involved in the monotonous and repetitive task of counting Schistosoma mansoni eggs in faecal preparations. For all their imperfections, such counts remain the only means of estimating, however crudely, the intensity of schistosome infections and the magnitude of any changes following interventions such as treatment. Jordan [1] illustrated a method developed on Saint Lucia for calculating the "false-negative rate" (FNR), based on extra infections detected on re-examination of 10% of slides "negative" at the first examination. Later, the method was modified, but still re-examined only a fraction of slides declared to be negative, using a continuous graphical evaluation of the performance of the microscopists over a period of time against each other and a laboratory supervisor [2]. An added refinement was the surreptitious inclusion of known positive preparations at the first examination [3]. In Saint Lucia, the FNR was in the range 8-14% [1].

The continuous presence of supervisory staff which this type of quality control requires may be possible in large, well-endowed research programmes or dedicated diagnostic laboratories, but is rarely possible in smaller research programmes with limited funds. Furthermore, although the FNR tackles the principal problem of "false negatives" (missing true infections, usually at the lower threshold of sensitivity of any particular diagnostic technique), it ignores the possibility, however improbable, of "false positives".

In the course of long-term studies on schistosomiasis mansoni in Kenya [4-6], a different form of quality control was developed, more suitable for projects with limited resources. It depends on the fact that properly stored Kato smears remain countable for many months after preparation [7-10]. Thus, it is possible to "audit" randomized subsamples to compare recounts with the original counts. This article summarizes 12 years of experience using this system.

Materials and methods

Preparation and counting of Kato slides; selection and recounting of a random subsample

Studies, approved by the appropriate Kenyan authorities and conforming with the prevailing ethical requirements, involved numerous faecal surveys from 1981 onwards on whole communities, individual schools, or specified study populations or cohorts from the study areas in the Machakos and Makueni Districts [4-6]. The following system of quality control audits was developed during the initial years of these studies.

At each survey, depending on the objectives of the study, one or more (to a maximum of five) stools were collected from each individual by mobile field teams comprising a driver/supervisor and two assistants, if possible within a 5-7-day period although sometimes longer. The field team plus an extra assistant prepared duplicate 50mg Kato smears from each faecal sample. The slides were labelled with the name and unique identity number (ID) of the subject, as well as the date and, if appropriate, the study code. Duplicate slides, labelled A and B, were stored in labelled, closable wooden or plastic slide boxes, together with a complete list (Form I) of the slides with columns for entering the observed counts. The A and B slides were placed sequentially in boxes holding 50 slides in one row, or side by side in adjacent rows in larger boxes holding 100 slides. The boxes were stored in a large plastic bag to minimize desiccation and, in later years, sprayed with a commercial aerosol insecticide before closing. With this system, a field team could collect and process 100 to 120 stool samples a day.

The boxes were transported to a local laboratory for microscopical examination by which time the slides were at least 24h old. The basic sampling unit was the slide box. Two microscopists, initially designated A and B, counted the A and B slides, respectively. …

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