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Introduction ⬇

The symbol pH expresses the urine as a dilute acid or base solution and measures the free hydrogen ion (H+) concentration in the urine; 7.0 is the point of neutrality on the pH scale. The lower the pH, the greater the acidity; the higher the pH, the greater the alkalinity. The pH is an indicator of the renal tubules’ ability to maintain normal hydrogen ion concentration in the plasma and extracellular fluid. The kidneys maintain normal acid–base balance primarily through reabsorption of sodium and tubular secretion of hydrogen and ammonium ions. Secretion of an acid or alkaline urine by the kidneys is one of the most important mechanisms the body has for maintaining a balanced body pH.

Urine becomes increasingly acidic as the amount of sodium and excess acid retained by the body increases. Alkaline urine, usually containing bicarbonate–carbonic acid buffer, is normally excreted when there is an excess of base or alkali in the body.

The importance of urinary pH lies primarily in determining the existence of systemic acid–base disorders of metabolic or respiratory origin and in the management of urinary conditions that require the urine to be maintained at a specific pH.

Control of urinary pH is important in the management of several diseases, including bacteriuria, renal calculi, and drug therapy in which streptomycin or methenamine mandelate is being administered.

  1. Renal calculi

    1. Kidney stone formation partially depends on the pH of urine. Patients being treated for renal calculi are frequently given diets or medication to change the pH of the urine so that kidney stones will not form.

    2. Calcium phosphate, calcium carbonate, and magnesium phosphate stones develop in alkaline urine. In such instances, the urine must be kept acidic (see No. 4 “Diet” below).

    3. Uric acid, cystine, and calcium oxalate stones precipitate in acid urines. Therefore, as part of treatment, the urine should be kept alkaline (see No. 4 “Diet” below).

  2. Drug treatment

    1. Streptomycin, neomycin, and kanamycin are effective for treating genitourinary tract infections, provided the urine is alkaline.

    2. During sulfa therapy, alkaline urine should help prevent formation of sulfonamide crystals.

    3. Urine should also be kept persistently alkaline in the presence of salicylate intoxication (to enhance excretion) and during blood transfusions.

  3. Clinical conditions

    1. The urine should be kept acidic during treatment of UTI or persistent bacteriuria and during management of urinary calculi that develop in alkaline urine.

    2. An accurate measurement of urinary pH can be made only on a freshly voided specimen. If the urine must be kept for any length of time before analysis, it must be refrigerated.

    3. Highly concentrated urine, such as that formed in hot, dry environments, is strongly acidic and may produce irritation.

    4. During sleep, decreased pulmonary ventilation causes respiratory acidosis; as a result, urine becomes more acidic.

    5. Chlorothiazide diuretic administration causes acid urine to be excreted.

    6. Bacteria from a UTI or from bacterial contamination of the specimen produce alkaline urine. Urea is converted to ammonia.

  4. Diet

    1. A vegetarian diet that emphasizes citrus fruits and most vegetables, particularly legumes, helps keep the urine alkaline. Alkaline urine after meals is a normal response to the secretion of hydrochloric acid in gastric juice.

    2. A diet high in meat and protein keeps the urine acidic.

Normal Findings ⬆ ⬇

The pH of normal urine can vary widely, from 4.6 to 8.0.

The average pH value is about 6.0 (acidic).

Procedure ⬆ ⬇

  1. Use reagent strips for a dipstick measurement. They produce a spectrum of color changes from orange to green-blue to identify pH ranges from 5.0 to 9.0.

  2. Dip the reagent strip into a freshly voided urine specimen and compare the color change with the standardized color chart on the bottle that correlates color results with pH values.

  3. Maintenance of the urine at a consistent pH requires frequent urine pH testing.

Clinical Implications ⬆ ⬇

To be useful, the urine pH measurement must be used in conjunction with other diagnostic information. For example, with renal tubular necrosis, the kidneys are not able to excrete urine that is strongly acidic. Therefore, if the urine pH is 5.0, renal tubular necrosis is eliminated as a possible diagnosis.

  1. Acidic urine (pH <7.0) occurs in:

    1. Metabolic acidosis, diabetic ketosis, diarrhea, starvation, uremia

    2. UTIs caused by Escherichia coli

    3. Respiratory acidosis (carbon dioxide retention)

    4. Renal tuberculosis

    5. Pyrexia

  2. Alkaline urine (pH >7.0) occurs in:

    1. UTIs caused by urea-splitting bacteria (Proteus and Pseudomonas)

    2. Renal tubular acidosis, chronic kidney disease

    3. Metabolic acidosis (vomiting)

    4. Respiratory alkalosis involving hyperventilation (“blowing off” carbon dioxide)

    5. Potassium depletion

Clinical Alert

The pH of urine never reaches 9, either in normal or abnormal conditions. Therefore, a pH finding of 9 indicates that a fresh specimen should be obtained to ensure the validity of the UA

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose of test, procedure for urine collection, and interfering factors.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Monitor patient appropriately (see Control of Urine pH).

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. With prolonged lapse of time since voiding, the pH of a urine specimen becomes alkaline because bacteria split urea and produce ammonia.

  2. Ammonium chloride and mandelic acid may produce acid urines.

  3. Runover between the pH testing area and the highly acidic protein area on the dipsticks may cause alkaline urine to give an acidic reading.

  4. Sodium bicarbonate, potassium citrate, and acetazolamide may produce alkaline urine.

  5. Urine becomes alkaline after eating because of excretion of stomach acid.

  6. The pH tends to be low following overnight fasting and high following ingestion of a meal.