What is the best method to evaluate urine pH? A trial of three urinary pH measurement methods in a stone clinic.

BACKGROUND: Monitoring of urinary pH is an important part of the assessment of patients with urinary tract stones. It provides valuable information about the future stone risk of certain patients and further allows the effective tailoring of medical intervention. Accurate measurement is therefore essential in these patients.
OBJECTIVE: The purpose of this study was to determine the most accurate method of measuring urinary pH in an outpatient setting. Materials, Methods, and Participants: Urine samples were collected from 200 patients attending stone clinics at The University Hospital of South Manchester. pH was measured by three commonly used methods: Siemens Clinitek Status pH meter, a hand-held pH meter, and litmus paper read visually. Results were compared with readings simultaneously obtained from a bench-top laboratory pH machine, which is the reference method for pH measurement. STATISTICAL ANALYSIS: The pH readings obtained were analyzed using the Bland-Altman plot. RESULTS AND LIMITATIONS: When compared with the reference method, the hand-held pH meter differed the least with a mean bias of 0.0073 and a maximum under-read of -0.2 pH units and maximum over-read of +0.2 pH units. The Siemens Clinitek pH meter differed most with a mean bias of -0.108, with a maximum over-read of +0.99 pH units and a maximum under-read of 0.78 pH units. The pH values obtained with the litmus paper gave similar results to that of the Clinitek pH meter with a mean bias of -0.069, with a maximum over-read of 0.96 and maximum under-read of 0.82 pH units.
CONCLUSION: The hand-held pH device gave urinary pH readings that most closely and consistently matched those of the reference bench-top laboratory machine. This method of pH measurement should be considered in stone clinics in patients with pH-dependent stone risk.