Does bicarbonate accumulate as acid in kidney failure?

Bicarbonate and Acid Balance in Kidney Failure

Bicarbonate does not turn to acid in kidney failure; rather, the kidneys fail to excrete acid and regenerate bicarbonate, leading to acid accumulation and decreased serum bicarbonate levels. 1, 2

Pathophysiology of Acid-Base Imbalance in Kidney Failure

The kidneys play a critical role in acid-base homeostasis through three main mechanisms:

• Reabsorption of filtered bicarbonate
• Excretion of titratable acids
• Production and excretion of ammonium

In chronic kidney disease (CKD), these processes become impaired:

1. Reduced Acid Excretion: As kidney function declines, the ability to excrete acid in the form of ammonium decreases 3
2. Bicarbonate Handling: Contrary to the question's premise, bicarbonate is not converted to acid; instead, the kidneys fail to regenerate enough bicarbonate to buffer the daily acid load 2
3. Acid Retention: This leads to accumulation of acids in the body, resulting in metabolic acidosis 1

Clinical Manifestations

Metabolic acidosis typically develops when glomerular filtration rate (GFR) falls below 20-25% of normal 4, 2. Key characteristics include:

• Serum bicarbonate concentrations typically range from 12-22 mEq/L (mmol/L) 4
• Both high anion gap and normal anion gap (hyperchloremic) acidosis can occur 1, 2
• Severity of acidosis generally correlates with the degree of renal impairment 4

Diagnostic Considerations

• Laboratory evaluation should include arterial blood gases, serum electrolytes with calculated anion gap, and renal function tests 1
• The urine anion gap (UAG) is a valuable tool for evaluating hyperchloremic metabolic acidosis and assessing renal ammonium excretion 1
• UAG calculation: (Na⁺ + K⁺) - Cl⁻ 1
  • Positive UAG indicates impaired renal acid excretion
  • Negative UAG suggests adequate renal ammonium excretion

Clinical Implications and Complications

Metabolic acidosis in kidney failure is associated with several adverse consequences:

• Muscle wasting and protein catabolism
• Bone disease and impaired growth
• Progression of kidney disease
• Increased mortality
• Abnormalities in hormone secretion
• Impaired insulin sensitivity 4, 2

Treatment Approach

Treatment is recommended when serum bicarbonate falls below 22 mmol/L 2, 3:

• Focus on correcting the underlying cause when possible
• Administer base supplementation (sodium bicarbonate or citrate)
• Target serum bicarbonate levels carefully - evidence suggests maintaining levels between 22-24 mEq/L 2, 3
• Avoid excessive correction (>26 mEq/L) due to potential cardiovascular risks 3

Special Considerations

• Diabetic patients with advanced kidney failure tend to have less severe metabolic acidosis compared to non-diabetic patients with similar kidney function 5
• Extrarenal buffering may help maintain near-normal acid-base status even with marked reduction in GFR 6
• Monitoring for complications of bicarbonate therapy is essential, including volume overload, hypertension, and potential vascular calcifications 4

Clinical Pearls

• Acidosis is rarely seen until GFR decreases to less than 20-25% of normal 4, 2
• The acidosis of kidney failure is primarily due to reduced functional renal mass rather than bicarbonate wastage 6
• Per-nephron bicarbonate reabsorption and ammonium production are actually enhanced in the remaining functional nephrons 6
• The decision to initiate dialysis may be influenced by uncontrollable acidosis 6