Factors affecting detection and quantification of Schistosoma haematobium eggs in pooled urine samples

Building on our previous work showing that pooled urine testing can rapidly detect Schistosoma haematobium infections but has limited sensitivity in low-intensity settings or with large pool sizes, this study evaluated how urine volume, pool size, infection intensity, and diagnostic method affect the detection and quantification of S. haematobium eggs in pooled samples.

Between July 2022 and April 2023, 2,134 urine samples from school-age children living in three regional states of Ethiopia were individually examined by deploying urine filtration microscopy (UFM). Subsequently, 5, 10, 20 and 40 individual samples were strategically pooled and examined by deploying UFM and Fluke Catcher (FC) and varying volumes of urine (10, 20 and 30 mL).

UFM was significantly more sensitive than FC for detecting S. haematobium eggs in pooled urine samples ([Formula: see text] =0.83, p < 0.001). There were significant interactions between the mean log of urine egg count (UEC) and volume of urine ([Formula: see text] =0.01, p = 0.021) or pool size ([Formula: see text] =-0.02, p = 0.007) for detecting S. haematobium eggs in pooled samples. The odds of detecting eggs in pooled samples in which the mean UEC of individual samples equaled 10 eggs/mL were 0.78 (95% CI:0.67-0.90) and 0.60 (95% CI:0.51-0.76) times lower when 10 mL rather than 20 mL and 30 mL was examined, respectively. There was a moderate or strong positive correlation between the mean egg count of pooled samples and the average egg count of the individual samples making the pools when pool sizes were 5, 10 or 20 (r ≥ 0.5).

The diagnostic sensitivity of pooled testing for diagnosing S. haematobium infection is affected by a complex interplay between the pool size, the volume of urine examined, the mean UEC and the diagnostic method. The sensitivity of the pooled testing strategy may increase with larger urine volumes, higher infection intensity, and when UFM (rather than FC) is used for testing. However, sensitivity could decrease as the pool size increases. Use of UFM, larger urine volumes and smaller to moderate pool sizes could improve detection of S. haematobium infection in surveillance and mass deworming programs in low-intensity settings.