Urine pH

Alternative names:

pH - urine

How the test is performed:

Child or adult:
Collect a "clean-catch" ("midstream") urine sample. To obtain a clean-catch sample, men or boys should wipe clean the head of the penis. Women or girls need to wash the area between the lips of the vagina with soapy water and rinse well. As you start to urinate, allow a small amount to fall into the toilet bowl (this clears the urethra of contaminants). Then, in a clean container, catch about 1 to 2 ounces of urine and remove the container from the urine stream. Give the container to the health care provider or assistant.

Infant:
Thoroughly wash the area around the urethra. Open a urine collection bag (a plastic bag with an adhesive paper on one end), and place it on your infant. For males, the entire penis can be placed in the bag and the adhesive attached to the skin. For females, the bag is placed over the labia. Place a diaper over the infant (bag and all). Check your baby frequently and remove the bag after the infant has urinated into it. For active infants, this procedure may take a couple of attempts--lively infants can displace the bag, causing an inability to obtain the specimen. The urine is drained into a container for transport back to the health care provider.

How to prepare for the test:

The health care provider may advise you to discontinue drugs that can affect the results of the test.

Eat a normal, balanced diet for several days before the test.

Submit a urine sample to the laboratory.
If the specimen is contaminated by vaginal discharge or bleeding, then collect a clean-catch sample.

If the collection is being taken from an infant, a couple of extra collection bags may be necessary.

How the test will feel:

The test involves only normal urination, and there is no discomfort.

Why the test is 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.

pH is a measure of acidity or alkalinity. pH is defined as the negative log of the hydrogen ion (H+) concentration. This mathematical manipulation allows us to measure acidity 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. pH in the plasma and most body fluids is maintained within a narrow range of 7.35 to 7.45. Only in severe diseases does the pH get slightly below 7.0 or close to 8.0. 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.