Background and objectives: For accurate measurement of pH, urine collection under oil to limit the escape of CO(2) on air exposure is recommended. This study aims to test the hypothesis that urine collection under oil is not necessary in acidic urine in which bicarbonate and CO(2) are minor buffers, because loss of CO(2) would have little effect on its pH.
Design, setting, participants, & measurements: One hundred consecutive random urine samples were collected under oil and analyzed for pH, pCO(2), and HCO(3)(-) immediately and after 5 minutes of vigorous shaking in uncovered flasks to allow CO(2) escape.
Results: The pH values in 97 unshaken samples ranged from 5.03 to 6.83. With shaking, urine pCO(2) decreased by 76%, whereas urine HCO(3)(-) decreased by 60%. Meanwhile, urine baseline median pH (interquartile range) of 5.84 (5.44-6.25) increased to 5.93 (5.50-6.54) after shaking (ΔpH=0.12 [0.07-0.29], P<0.001). ΔpH with pH≤6.0 was significantly lower than the ΔpH with pH>6.0 (0.08 [0.05-0.12] versus 0.36 [0.23-0.51], P<0.001). Overall, the lower the baseline pH, the smaller the ΔpH.
Conclusions: The calculation of buffer reactions in a hypothetical acidic urine predicted a negligible effect on urine pH on loss of CO(2) by air exposure, which was empirically proven by the experimental study. Therefore, exposure of urine to air does not substantially alter the results of diagnostic tests for urine acidification, and urine collection under oil is not necessary.