Kidney Week

Abstract: PUB085

Multifactorial Elevation of Anion Gap in Chronic Respiratory Alkalosis: A Case of Rett Syndrome

Session Information

• Publication Only

Category: Fluid, Electrolytes, and Acid-Base Disorders

• 1102 Fluid, Electrolyte, and Acid-Base Disorders: Clinical

Authors

• Jahanshahi, Atousa, University of California Los Angeles David Geffen School of Medicine, Los Angeles, California, United States

Atousa Jahanshahi, MD

• Nguyen, Minhtri K., University of California Los Angeles David Geffen School of Medicine, Los Angeles, California, United States

Minhtri K. Nguyen, MD

Introduction

Elevated anion gap (AG) traditionally indicates accumulation of unmeasured organic acids. However, chronic alkalemia can increase AG through pH-dependent mechanisms. We present a case demonstrating multifactorial AG elevation in chronic respiratory alkalosis associated with Rett syndrome, including decreased bicarbonate without corresponding increase in chloride level, increased albumin negative charge, reduced ionized calcium (iCa2+) and ionized magnesium (iMg2+), and increased phosphate charge—without true metabolic acidosis.

Case Description

A 22-year-old woman with Rett syndrome presented for aspiration pneumonia. Baseline AG was 11 mEq/L. She developed worsening respiratory alkalosis (pH 7.53, pCO2 22 mmHg), with AG rising to 19 mEq/L with normal lactate (1.3 mmol/L), negative ketones, undetectable salicylate, and stable albumin (3.2 g/dL). Labs also showed hypocalcemia (6.5 mg/dL) and hypomagnesemia (1.2 mEq/L). Urine studies showed positive urine AG (+76 mEq/L) with alkaline pH (8.0), confirming suppressed ammonium excretion characteristic of chronic respiratory alkalosis. As pH normalized (7.44), AG decreased to 10 mEq/L without targeted intervention.

Discussion

The AG elevation (11 → 19 mEq/L) was multifactorial, (Table 1). Decreased bicarbonate was the dominant factor (+6.00 mEq/L). Chloride remained stable at 110 mEq/L, providing no offsetting effect. The ΔHCO3- was proportional to ΔPaCO2 (≈0.4–0.5 × ΔPaCO2), excluding superimposed AG metabolic acidosis. Alkalemia reduces iCa2+ by 0.36 mmol/L per pH unit and iMg2+ by 0.12 mmol/L per pH unit through pH-dependent albumin binding, yielding a combined divalent cation contribution of +0.19 mEq/L. The Figge-Jabor equation predicts an albumin charge contribution of +0.80 mEq/L, with phosphate adding +0.15 mEq/L. Hemoconcentration and significant lactate elevation were absent, and the positive urine AG with alkaline urine were consistent with chronic respiratory alkalosis vs metabolic acidosis.

Quantitative Contributions to Anion Gap Elevation