Sensitivity and Specificity of a Urine Circulating Anodic Antigen Test for the Diagnosis of Schistosoma haematobium in Low Endemic Settings

Abstract

Background: Elimination of schistosomiasis as a public health problem and interruption of transmission in selected areas are key goals of the World Health Organization for 2025. Conventional parasitological methods are insensitive for the detection of light-intensity infections. Techniques with high sensitivity and specificity are required for an accurate diagnosis in low-transmission settings and verification of elimination. We determined the accuracy of a urine-based up-converting phosphor-lateral flow circulating anodic antigen (UCP-LF CAA) assay for Schistosoma haematobium diagnosis in low-prevalence settings in Zanzibar, Tanzania.

Methodology: A total of 1,740 urine samples were collected in 2013 from children on Pemba Island, from schools where the S. haematobium prevalence was <2%, 2-5%, and 5-10%, based on a single urine filtration. On the day of collection, all samples were tested for microhematuria with reagent strips and for the presence of S. haematobium eggs with microscopy. Eight months later, 1.5 ml of urine from each of 1,200 samples stored at -20°C were analyzed by UCP-LF CAA assay, while urine filtration slides were subjected to quality control (QCUF). In the absence of a true 'gold' standard, the diagnostic performance was calculated using latent class analyses (LCA).

Principal findings: The 'empirical' S. haematobium prevalence revealed by UCP-LF CAA, QCUF, and reagent strips was 14%, 5%, and 4%, respectively. LCA revealed a sensitivity of the UCP-LF CAA, QCUF, and reagent strips of 97% (95% confidence interval (CI): 91-100%), 86% (95% CI: 72-99%), and 67% (95% CI: 52-81%), respectively. Test specificities were consistently above 90%.

Conclusions/significance: The UCP-LF CAA assay shows high sensitivity for the diagnosis of S. haematobium in low-endemicity settings. Empirically, it detects a considerably higher number of infections than microscopy. Hence, the UCP-LF CAA employed in combination with QCUF, is a promising tool for monitoring and surveillance of urogenital schistosomiasis in low-transmission settings targeted for elimination.

Fig 1. Flowchart detailing study participation and urine sampling procedures.

Flowchart indicating the inclusion and exclusion of data for determining the accuracy of different methods for the diagnosis of Schistosoma haematobium in children from Pemba, United Republic of Tanzania, in 2013. UCP-LF CAA: up-converting phosphor-lateral flow assay detecting circulating anodic antigen in urine; UCAA2000: UCP-LF CAA prepared with 1.5 ml of urine; UCAA250: UCP-LF CAA prepared with 250 μl of urine.

Fig 2. Maps indicating Schistosoma haematobium prevalence levels according to different diagnostic tests.

The two maps indicate different S. haematobium prevalence levels as identified with a single urine filtration method (A) and a urine-based up-converting phosphor-lateral flow circulating anodic antigen (UCAA2000) assay (B) in 16 schools on Pemba island, United Republic of Tanzania, in 2013. UCAA2000: up-converting phosphor-lateral flow assay detecting circulating anodic antigen in urine and prepared with 1.5 ml of urine; green spot: school with a prevalence of <2%; yellow spot: school with a prevalence of 2–5%; orange spot: school with a prevalence of 5-<10%; red spot: school with a prevalence of ≥10%.

Fig 3. Correlation of circulating anodic antigen (CAA) levels and S. haematobium egg counts or microhematuria grading.

(A) Correlation of CAA levels (pg/ml) in 1.5 ml of urine and the number of S. haematobium eggs detected in 10 ml of urine (Spearman’s rho = 0.24; P <0.001); (B) correlation of CAA levels (pg/ml) and the microhematuria grading (Spearman’s rho = 0.23; P <0.001); and (C) correlation of S. haematobium eggs detected and microhematuria grading (Spearman’s rho = 0.57; p<0.001), in urine samples from children from Pemba, United Republic of Tanzania, collected in 2013. The horizontal continuous red line indicates the cut-off value of >0.4 pg/ml for samples clearly indicated as S. haematobium-positive by the UCAA2000 (A and B). The horizontal dotted red line indicates the cut-off value of <0.2 pg/ml for samples clearly indicated as S. haematobium-negative by the UCAA2000 (A and B). Values right from the vertical continuous red line (A) and above the horizontal continuous red line (C) indicate egg-positive urine filtration tests