Evaluation and Treatment of Classic Hypokalemic Distal Renal Tubular Acidosis

Since normal anion gap hyperchloremic metabolic acidosis is a feature in patients with classic dRTA, diagnosis begins with a history and physical examination (PE) to rule out (TRO) other conditions. The condition of the urine gives a lot of clues to the underlying cause and of RTA.

Lab investigation

STEP 1

The laboratory evaluation should be initiated with the examination of the urine.

If the acidosis is the result of an extrarenal disorder, such as diarrhea, the urine will be rich in ammonium. This can easily be disclosed by measuring the urinary electrolytes. There will be considerably more chloride than sodium (plus potassium), usually more than 50 mmol/L. In other words, the anion gap will be minus 50 or more. The missing cation is ammonium. Patients with dRTA or related syndromes typically have more cation than chloride in the urine when they are acidemic, indicating reduced ammonium excretion and, hence, defective acidification. Once the diagnosis of an RTA syndrome has been made, the urinary anion gap has no further use, since it is abnormal in all RTA syndromes.

STEP 2

The next step is to categorize patients according to serum potassium -- those with a low (or normal) serum potassium and those in whom it is elevated. The first group can be further subdivided into those patients in whom the urine pH can be lowered below 5.5 and those in whom it cannot. When proximal RTA has been excluded by measuring fractional bicarbonate excretion at a serum HCO₃ greater than 20 mmol/L, distal RTA can be diagnosed with certainty in those whose urine pH is greater than 5.5 at an acid systemic pH.

STEP 3

Patients in whom urine pH can be lowered below 5.5 and in whom a proximal lesion has been ruled out can then be given sodium bicarbonate intravenously; if the urine PCO ₂ fails to rise normally, the diagnosis of rate-dependent dRTA can be made.

Children will need alkali therapy for normal growth.

Maintenance on alkali therapy can be for indefinite period in some cases.

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