Application of Strong Ion Difference Theory to Urine and the Relationship Between Urine pH and Net Acid Excretion in Cattle
Urinary net acid excretion (NAE) provides the most sensitive clinical insight into acid-base homeostasis in animals. Measurement of urine pH may also have clinical utility in the assessment of systemic acid-base status, particularly in healthy animals, because urine pH is more easily determined than NAE. The objectives of this study were to develop an equation expressing urine pH in terms of independent variables, to derive an equation relating urine pH to NAE, and to apply this new knowledge to determine the role that measuring urine pH should play in the evaluation of systemic acid-base status in healthy and sick cattle.
A physicochemical strong ion approach was applied to develop a general electroneutrality equation that involved urine pH and urinary strong ion difference (SID = difference between strong cation and strong anion concentrations in mEq/l), the urinary concentration of ammonium ([NH4+]) and phosphate ([P]) in mmol/l, the acidic dissociation constant for H2PO4- (Ka2), urinary Pco2, the dissociation constant (K1) for carbonic acid (H2CO3), and the solubility of CO2 (S) in urine. We validated the general electroneutrality equation using 327 data points from 11 non lactating Holstein-Friesian cows that were fed 11 diets of different dietary cation anion difference.
We determined that urine pH in mammals is dependent on 4 variables, urine SID, [NH4+], Pco2, and [P], and 3 constants, Ka2, K1', and S. In cattle, urine pH is dependent on 3 variables (urine SID, [NH4+], Pco2) and 2 constants (K1', S) because urine [P] 0. The relationship between NAE (in mEq/l) and urine pH for cattle was: NAE = [NH4+] + 2.5 -10(pH - 6.12). A simplified form of the general electroneutrality equation was developed for bovine urine whereby: urine pH 6.12 + log10([K+] + [Na+] + [Mg2+] + [Ca2+] + [NH4+] - [Cl-] - [SO42-]).
We conclude that a change in urine SID, [NH4+], Pco2, or [P] will independently lead to a change in urine pH in mammals. In cattle, urinary [K+] potassium concentration has the greatest effect on urine pH, with high urine [K+] producing alkaline urine, and low urine [K+] producing acidic urine. Whole body potassium depletion should be suspected when aciduria is present in sick cattle that are not consuming an acidogenic diet. Urine pH provides an accurate assessment of systemic acid-base homeostasis in healthy cattle only when urine pH is between 6.3 and 7.6. Urine pH should not be used to predict systemic acid-base status in sick cattle because serum electrolyte abnormalities, such as hypokalemia and hypochloremia, are likely to be present.