The kidney in acid-base balance

Renal tubular acidosis: a new look at an old problem

Although the definition of renal tubular acidosis (RTA) is simple, understanding the physiologic basis underlying the various types of this clinical entity is much more difficult. The pathophysiology of this disorder is reviewed using the normal acid-base functions of the involved segments of the nephron as a guide to understanding. Clinical and laboratory features of the subtypes of RTA are addressed, and diagnosis and treatment discussed. New developments in the knowledge and understanding of the associated growth disturbances, mineral metabolism, and molecular biology of RTA are also reviewed to provide the most current view of this relatively common pediatric entity.

Serum anion gap in the differential diagnosis of metabolic acidosis in critically ill newborns

OBJECTIVES

To determine in critically ill newborn infants (1) the range of the serum anion gap without metabolic acidosis and (2) whether the serum anion gap can be used to distinguish newborns with lactic acidosis from those with hyperchloremic metabolic acidosis.

STUDY DESIGN

Umbilical arterial blood gases and serum electrolyte and lactate concentrations were measured simultaneously in 210 samples from 63 infants over the first week of life. Metabolic acidosis was defined as a blood base deficit (BD) >4 mmol/L. The anion gap was calculated as [Na(+)] - [C1(-)] - [TCO (2)]. Lactic acidosis was defined as a serum lactate concentration >2 SD above the mean serum lactate concentration in samples without metabolic acidosis.

RESULTS

In 89 blood samples with BD <4 mmol/L, serum lactate concentration decreased with postnatal age (r = 0.51). The upper limit of serum lactate concentration was 3.8 mmol/L at less than 48 hours, 2.4 mmol/L between 48 and 96 hours, and 1.5 mmol/L for infants greater than 96 hours of age. The mean serum anion gap +/- 2 SD in 174 samples without lactic acidosis was 8 +/- 4 mmol/L; in 36 samples with lactic acidosis it was 16 +/- 9 mmol/L (P <.0001). Serum anion gap and lactate concentration were poorly correlated for samples without lactic acidosis (r = 0.04) but highly correlated in those with lactic acidosis (r = 0.81, P <.0001). None of the 85 samples with metabolic acidosis but without lactic acidosis had an anion gap >16 mmol/L; only 4 of 36 samples with lactic acidosis had an anion gap <8 meq/L. However, 25 of 36 samples with lactic acidosis had serum anion gaps of 8 to 16 mmol/L.

CONCLUSION

In the presence of metabolic acidosis, a serum anion gap >16 mmol/L is highly predictive of lactic acidosis; a serum anion gap <8 is highly predictive of the absence of lactic acidosis; an anion gap = 8 - 16 mmol/L has no use in the differential diagnosis of metabolic acidosis in the critically ill newborn.