Urine pH

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Why is the Test Performed?

Certain medications are more effective in acidic or alkaline environments, for example, streptomycin, neomycin, and kanamycin. These are more effective in treating urinary tract infections when the urine is alkaline.

Certain kinds of urine crystals are in acidic urine. Urine crystals suggest a predisposition to form renal stones. This predisposition can be subverted by modifying urine pH. Acidic urine is associated with xanthine, cystine, uric acid, and calcium oxalate stones. Alkaline urine is associated with calcium carbonate, calcium phosphate, and magnesium phosphate stones.

The pH is a measure of acidity or alkalinity, and pH is defined as the negative log of the hydrogen ion (H+) concentration. This mathematical manipulation allows acidity to be measured on a convenient scale of 0 to 14. Neutral pH (that is, the same number of hydrogen ions as hydroxyl, OH-, ions are present in the solution) is 7.0. The pH in the plasma is maintained within a narrow range of 7.35 to 7.45.

Only in severe life-threatening disease states does the pH fall below 7.2 or above 7.6. More dramatic variation is incompatible with life. The body maintains the pH within this narrow range by using buffers, that is, a chemical such as a weak acid or base which can exist in 2 forms. These alternative forms are interconvertible through the absorption or release of hydrogen ions.

The most important buffer in the body is the bicarbonate buffer system: H+ + HCO3- <--> CO2 + H2O. HCO3- is controlled by the kidneys, and CO2 is controlled by the lungs. If there is a problem with the kidneys, the lungs will try to compensate and visa versa. However, this balance can be upset, for example, in the presence of excessive organic acids which neutralize HCO3-.

Urine pH is not as rigidly controlled as blood pH, and can be affected by various factors after collection. Standing in an uncovered container allows CO2 to escape. Bacteria usually increases the pH because they contain enzymes which can break down urea to ammonia, which then combines with hydrogen ions.

In most cases serum pH is a better measure of acid-base imbalance than urine pH, but in some cases urine pH is also helpful. For example, in a respiratory acidosis resulting from chronic obstructive pulmonary disease, the kidneys (if they are normal) will attempt to counterbalance the increased pH by re-absorbing and synthesizing more bicarbonate, which will then result in a more acidic urine.