The Stability of pH, P_CO_₂, and Calculated [HCO₃] of Urine Samples Collected under Oil

Abstract

In order to determine the stability of directly measured pH and Pco₂, and calculated [HCO₃] in stored urine, 11 alkali-loaded normal subjects provided 33 spot and sixteen 24-hour mineral oil-covered, thymol-preserved, refrigerated urine samples. For the spot samples, pH and Pco₂ were measured immediately and again at 4 and 24 h. In addition, immediately after voiding, 24 of the spot samples were split into oil-covered and no-oil moieties and analyzed immediately and again at 4 and 24 h. pH and Pco₂ measurements of the 24-hour collections were carried out immediately after completion and again 24 h later. The results demonstrated the importance of using oil to limit the escape of CO₂ from stored urine. Thus, after 24 h the oil-uncovered subgroup of 24 spot urine samples sustained a 82% decline in Pco₂ and a 20% fall in [HCO₃]. In contrast, the corresponding percentage decrements in the oil-covered samples were 16 and 1%, respectively. The results also indicated that even with oil there is loss of CO₂, which increases with time and which shows a statistically significant direct correlation with the baseline level of Pco₂· Nevertheless, modest loss of CO₂ usually produces only slight decrements in [HCO₃] because of the countervailing influence of the resultant increase in pH. We conclude that the use of mineral oil is necessary in order to obtain adequate stability of Pco₂, pH and [HCO₃] in refrigerated urine requiring several hours or more of storage.

Estimated Dietary Acid Load Is Not Associated with Blood Pressure or Hypertension Incidence in Men Who Are Approximately 70 Years Old ,

“…Although this is a common assumption in dietary recall research, we must consider it critically in elderly individuals, who from year to year may face growing health problems with consequences to their nutritional behavior. Second, dietary acid load was indirectly estimated on the basis of the ratio of acid-and base-producing food and not by 24-h urine collection (20), difficult to attain in large surveys (21). Both PRAL and NEAP formulas have been validated in free-living children, adolescents, and healthy adults (10,11,22), and been commonly applied in epidemiologic research (12,13).…”

Does the Exposure of Urine Samples to Air Affect Diagnostic Tests for Urine Acidification?

“…Normal urine contains multiple buffers. Urinary buffers that have important influences on urine pH include NH 3 /NH 4 + (pK=9.4), HCO 3 2 /CO 2 (pK=6.1), HPO 4 = /H 2 PO 4 2 (pK=6.8), urate/uric acid (pK=5.5), and creatinine/creatinine + (pK=5.0) (10,11). In a solution containing multiple buffers, the isohydric principle dictates that all buffer pairs in solution must be in equilibrium with the prevailing pH of the solution and also, the pK of the individual buffer pair.…”

“…The new equations will be as follows: pH 5 6.1 1 log (1.2 2 x)/ 0.12 for the bicarbonate and CO 2 buffer system, and pH 5 6.8 1 log (5 1 x)/(25 2 x) for the phosphate buffer system, where x equals the decrease in bicarbonate concentration, which equals the increase in HPO 4 5 , as well as the decrease in H 2 PO 4 2 . Because the two buffer systems are in equilibrium at the same pH, 6.1 1 log(1.2 2 x)/0.12 5 6.8 1 log(5 1 x)/(25 2 x).…”

“…Although the loss of CO 2 from alkaline urine has been shown to cause noticeably higher urine pH in oil-unprotected urine samples, reports about urine pH changes in acidic urine (pH,6.0) after air exposure are rare (4,5).…”

Low urine pH and acid excretion do not predict bone fractures or the loss of bone mineral density: a prospective cohort study

“…Urine pH may change slightly with storage as carbon dioxide (CO 2 ) and ammonia (NH 3 ) evaporate, but the changes are not likely to result in misrepresentation of the measurements. As urine bicarbonate evaporates as CO 2 from alkaline urine (H + and HCO 3 - <--> H 2 O + CO 2 ), pH would increase through the loss of the hydrogen ions [55]. As urine ammonia (NH 3 ) evaporates from more acidic urine, ammonium would converted to ammonia (NH 4 + <--> H + + NH 3 ), hydrogen ions would be liberated and therefore the measured hydrogen ion will increase and thus the pH will decrease.…”