Does the exposure of urine samples to air affect diagnostic tests for urine acidification?
OBJECTIVE
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.


METHODS
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.
Renal Division, Department of Internal Medicine, Hanyang University Guri Hospital, Guri, Gyeonggi-Do, Korea.
Joo-Hark
JH
Yi
Hyun-Jong
HJ
Shin
Sun-Moon
SM
Kim
Sang-Woong
SW
Han
Ho-Jung
HJ
Kim
Man-Seok
MS
Oh
D000328:Adult; D000368:Aged; D000388:Air; D001639:Bicarbonates / Q000652:urine; D002021:Buffers; D002245:Carbon Dioxide / Q000652:urine; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D008875:Middle Aged; D009821:Oils; D010232:Paraffin; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D013048:Specimen Handling; D013997:Time Factors; D016482:Urinalysis / Q000379:methods*; D014556:Urine / Q000737:chemistry*
D001639:Bicarbonates; D002021:Buffers; D009821:Oils; D002245:Carbon Dioxide; D010232:Paraffin; C015418:paraffin oils