How accurate are water quality test strips in South Africa?

Water quality monitoring is essential for public health and sustainable development, yet laboratory-based analysis remains expensive in resource-limited contexts. Four water quality test strip products available in South Africa were evaluated by comparing test strip measurements of pH, nitrate, nitrite and hardness with reference measurements, with strip readings conducted under indoor (1000 lx) and outdoor (5000 lx) lighting conditions. The results indicate that while pH and nitrate measurements are reasonably accurate, water hardness exhibits the highest error rates (normalised root mean square error up to 43%), highlighting a need for improved assay chemistry. Outdoor lighting improved accuracy by an average of 12% compared with indoor lighting (p = 0.0354), although this improvement was not consistent across all parameters, with hardness remaining a clear exception. Smartphone-based interpretation did not outperform visual reading (p = 0.0382), as digital sensors proved more sensitive to white-balance fluctuations and noise. Furthermore, price was not a reliable predictor of accuracy, with cheaper strips often matching or exceeding the performance of expensive ones. An important limitation of the study is that measurements were conducted by a single user, and insights into smartphone-based analysis were based on a single product (the only one available at the time) and two smartphone devices. While the findings are limited to the products and conditions tested, the results suggest that test strips are not yet suitable for regulatory enforcement but are effective for trend detection when used with standardised protocols. These findings provide a vital framework for making low-cost monitoring technologies robust enough to support Sustainable Development Goal reporting and environmental advocacy.