Chin Med J (Taipei) 1998;61:S174.

Clinical Practice on Electrolyte Disorders: Renal Tubular Acidosis

Shih-Hua Lin, M.D.

Division of Nephrology, Department of Medicine, Tri-Service General Hospital

Abstract

The daily load of H+ is eliminated mainly by titration of bicarbonate. To mainatain acid-base balance, the kidney must not only reclaim all the filtered bicarbonate, but also generate new bicarbonate by excreting ammonium (NH4⁺) and H2PO4^-. Renal control of the rate of excretion of metabolizable organic anions has been recently emphasized to play an important role in the acid-base because their loss in the urine represents the loss of "potential bicarbonate". RTA is a term used to describe a patient who has chronic hyperchloridemic metabolic acidosis (HCMA) with a normal plasma anion gap, characterized by defective H⁺ secretion by the kidney or by the renal loss of bicarbonate or "potential bicarbonate". Based on the components of net acid excretion (NAE = NH4⁺ + H2PO4^- - HC03^- -potential HC03^-) that is abnormal, three major groups of RTA are recognized: excessive excretion of bicarbonate, low rate of NH4⁺ excretion and excessive excretion of potential bicarbonate. Because a low rate of excretion of NH4⁺ is present in patients with both distal and proximal RTA, the first clinical step to differentiate three major causes of RTA is to evaluate the rate of excretion of NH4⁺ which can be determined by direct measurement of NH4⁺ or indirectly estimated by using urine anion gap (UAG) and urine osmolal gap (UOG). The basis for a low rate of excretion of NH4⁺ shown by examining the urine pH. A low urine pH < 5.3 indicates that the major problem is with NH4⁺ production or transfer to the urine; further studies are necessary to determine why the availability of NH3 is low. A high urine pH > 6.0 suggests that there is a significant impairment of H⁺ secretion; further investigations are initiated to examine if the defect in H⁺ secretion involves the proximal or distal nephron or both. Conversely, if the rate of excretion of NH4⁺ is high in a patient with HCMA, a component of the degree of acidosis could be attributable to a higher rate of excretion of metabolizable organic anions (D-lactate, beta-hydroxybutyrate, hippurate). Case examples are provided to illustrate this pathophysiological approach. In addition, diagnostic tools used at the bedside for RTA, including fractional excretion of bicarbonate, carbon dioxide partial pressure in alkaline urine, urine pH, NH4⁺ in the urine, citrate excretion, anions in the urine, NH4Cl acid loading, loop diuretic tests, will be introduced.

[Chin Med J (Taipei) 1998;61:S174.]