Measurement of urine pH for epidemiological studies on bladder cancer.

Methods for efficiently identifying subjects with constantly acidic pH in epidemiological and clinical studies have not been assessed. We recruited 30 volunteers to estimate the minimum number of urine pH measurements using pH strips needed to identify subjects with "constantly acidic urine pH". Spearman's correlation coefficients between urine pH measured with a pH meter and with the four pH strips ranged from 0.94 to 0.95 (p < 0.001 for all four strips). Overall agreement within +/-0.5 pH units between the four strips and the pH meter ranged from 62.2% to 74.4%. When using a spot urine sample from a single morning to classify participants with respect to their urine pH, 80% of individuals fell into the acidic urine pH (pH equal to or lower than 6.0) group. When we required subjects to have urine pH equal to or lower than 6.0 in six consecutive AM spot urine samples and seven spot PM urine samples, only 20% of participants fulfilled this criterion. Measuring urine pH twice a day (early in the morning and early in the evening) during four consecutive days classified individuals in the same way as two daily measurements for one week. A single pH measurement from a spot urine sample is not reliable to identify individuals with constantly acidic pH. Morning and evening urine pH measurements with pH strips during four consecutive days identify individuals with constantly acidic urine pH individuals as well as one week of measurements, and thus might be useful to identify subjects with constantly acidic urine pH in epidemiological and clinical studies.