Urine pH

The pH of a urine sample is affected by a variety of factors including:

renal H+ excretion and HCO3- resorption pathologic abnormalities of systemic acid/base balance. pathologic abnormalities of tubular function: with failure to excrete an acid load or failure to absorb bicarbonate. dietary factors - due to differences in dietary "acid load", herbivores usually have alkaline urine, carnivores tend to have acidic urine. Urine in carnivores does become slightly less acidic after eating, associated with a post-prandial alkaline tide (due to increased secretion of HCl into the stomach). age of specimen (loss of CO2 from the sample to the air raises the pH) presence of contaminant or pathogenic bacteria (some convert urea to ammonia, raising pH)

Knowledge of the urine pH is important in interpreting urine sediment findings. Erythrocytes, leukocytes, and casts tend to disintegrate in alkaline urine (pH > 8.0). In addition, precipitation of urine crystals in supersaturated urine is highly dependent on urine pH (e.g. struvite will precipitate in alkaline not acidic urine). Note that although the kidneys play a central role in the control of acid/base metabolism, the pH of a random urine sample is not a reliable indicator of total body acid/base status. In some conditions, impaired renal tubular function in fact causes or perpetuates the underlying acid/base derangement.

Meaningful evaluation of acid-base status generally requires blood gas analysis and consideration of clinical signs.