Effect of urine storage conditions on circulating anodic antigen detection using the UCP-LF CAA test

Schistosomiasis is a parasitic neglected tropical disease that remains a major public health problem in many African countries. Accurate diagnosis is essential for control, surveillance and elimination efforts. Conventional parasitological methods have limited sensitivity, particularly in low-intensity infections commonly observed after treatment or in low-endemic settings. Circulating anodic antigen (CAA), released by live Schistosoma worms, is a highly sensitive and specific marker of active infection. The up-converting reporter particle lateral flow (UCP-LF) CAA test enables non-invasive urine-based diagnosis and is increasingly applied in low-endemic and near-elimination settings. For operational feasibility, urine samples may need to be stored before testing, but the effect of different storage conditions on CAA detection has not been systematically evaluated.

We investigated the effect of freezing and non-frozen storage on urine CAA detection using the UCP-LF CAA test, compared with freshly tested urine. The study was conducted among school-aged children in a near-elimination Schistosoma haematobium setting on Pemba, Tanzania. A total of 272 urine samples were tested fresh and after freezing at -20 °C for 5-7 days. For a subset of 153 samples with sufficient urine available, an additional aliquot was stored under non-frozen conditions (4-25 °C) and tested approximately 5 weeks after collection.

CAA positivity decreased from 44/272 (16.2%) in fresh urine to 35/272 (12.9%) after freezing. In the subset, positivity was 20/153 (13.1%) in fresh, 16/153 (10.5%) in frozen, and 24/153 (15.7%) in non-frozen stored urine. CAA concentrations showed strong correlation between storage conditions (Spearman's ρ = 0.86-0.93, p<0.0001). Freezing was associated with a small but significant reduction in CAA positivity (p=0.004), while CAA concentrations showed a non-significant trend towards lower values after freezing. Non-frozen stored samples showed generally comparable results to fresh urine.

In conclusion, these findings suggest that flexible urine storage strategies may be feasible for delayed centralized CAA testing, although freezing may result in loss of detection among samples with CAA concentrations close to the assay cut-off.