Accuracy of urine pH testing in a regional metabolic renal clinic: is the dipstick accurate enough?

Abstract

Urine pH is a useful marker for assessing treatment need and efficacy in patients with nephrolithiasis. Though the gold standard of measurement is with a pH electrode, dipsticks offer the convenience of cost, ease of use, and the possibility of patients measuring their own values outside the clinic. The aim of this study was to determine whether dipsticks offer the same accuracy as the electrode. Paired measurements of freshly voided urine pH with both electrode and dipstick were analysed in a multidisciplinary renal clinic. We found that although there was a high Pearson correlation between the samples (0.89, p = 0.001), urine dipstick measurements carried an approximately 1 in 4 risk of producing clinically significant differences (pH differences > 0.5 pH unit) from meter values. We also found that at high and low urine pH, the dipstick tended to over- and underestimate true pH readings, respectively. Examining the values in the 98 patients where a need for pharmacological urinary pH manipulation was indicated by the true pH, we found 14 who would not have been appropriately treated, and 5 who would have been unnecessarily medicated, if the stick pH value had been used. We conclude that dipstick pH measurement is insufficiently reliable for guiding clinical decision-making.

Fig. 1

Scatter graph demonstrating range of electrode pH measurements per stated dipstick pH measurement. Line of unity is dotted; regression line is solid