Potential false positive egg-counts through the reuse of polyamide filters in the diagnosis of urinary schistosomiasis.

Nytrel monofilament polyamide filters have recently been strongly advocated as a cheap reusable method for filtering urine in field studies of Schistosoma haematobium endemic areas. We examined 1230 persons in a highly endemic area, filtering 10 ml urine through Nytrel filters and counting the eggs retained. It was found that staining Nytrel filters speeded up the counting process and made the detection of light infections much easier. After very careful washing and rinsing 12.0% of unstained filters and 4.8% of stained filters retained eggs. In view of the implication of these findings for epidemiological studies and control programmes, it is suggested that field workers consider the use of disposable paper filters.