Metabolic acidosis is a condition which occurs when either the body is producing excess acid or the kidneys are not able to remove the acid efficiently. It is caused by various metabolic disorders which cause buildup of excess acids in the body. Metabolic acidosis causes acidemia (excess acid in the blood) if not corrected in time. This leads to decrease in pH of the blood (7.35). It has diverse causes and its consequences can be serious including coma and death.
Causes of Metabolic Acidosis
- Diabetic ketoacidosis due to uncontrolled diabetes
- Acidosis in severe diarrhea due to loss of sodium bicarbonate from the body
- Lactic acidosis due to alcohol, liver failure, cancer, etc.
- Kidney disease
- Cyanide poisoning
- Severe dehydration
- Aspirin (salicylate) poisoning
Urine Tests to Diagnose Metabolic Acidosis
Diagnostic urine tests for metabolic acidosis are frequently done as preliminary tests for the following reasons:
- Easy to perform.
- Specimen is easily procured.
- It is not an invasive procedure.
The following tests are done to diagnose metabolic acidosis:
- Urinary pH – Normally the urine is acidic with pH of less than 5. In case of metabolic acidosis, the pH decreases further and the urine becomes more acidic. In case of metabolic acidosis due to distal renal tubular acidosis, the pH is never less than 5.5 whereas in proximal renal tubular acidosis, the pH becomes less than 5.
- Calcium oxalate crystals – This is seen as needle shaped crystals in the urine in the case of ethylene glycol toxicity. The level of oxalate is more than 150 mg/24 hours.
- Ketone bodies – Normally ketone bodies are absent in the urine. The presence of ketone bodies in the urine indicates diabetic, alcoholic and starvation ketoacidosis. This is detected by the nitroprusside test.
- Urine chloride – This is done along with sodium and potassium as a 24 hr urine test. The urinary anion gap [(sodium+potassium)-(chloride+bicarbonate)] is useful in evaluation of metabolic acidosis in which the chloride levels are raised. The urine is collected for 24 hours in a container and kept refrigerated until analyzed. Normal adult value is 140-250 mEq/24 hours.
- Urine sodium – This is also done as a 24 hr urine test. Sodium is a major ion maintaining the acid base balance in the body. Urine is collected for 24 hours and sent for analysis. Normal values are 40-220 mEq/L. Urinary sodium is increased in renal tubular acidosis and diabetic acidosis. It is decreased in case of excessive sweating and diarrhea.
- Urine glucose – This is elevated in case of diabetic ketoacidosis.
Thus diagnostic urine tests for metabolic acidosis are done as an initial step to have an idea of the causes of metabolic acidosis. Further confirmatory tests are required to find out the exact cause.
References and Further Reading
- A manual of laboratory and diagnostic tests by Frances Talaska Fischbach, Marshall Barnett Dunning
- Cases in chemical pathology: a diagnostic approach by R. N. Walmsley, L. R. Watkinson, H. J. Cain
- Essentials of pathophysiology: concepts of altered health states by Carol Porth