Metabolic Acidosis

Metabolic acidosis is heralded by a decreased concentration of plasma bicarbonate.

Etiology

• Production of organic acid exceeds rates of elimination
  • Beta-hydroxybutyrate and acetoacetic acid production during diabetic acidosis
  • Lactic acid production during lactic acidosis
• Reduced excretion of acids due to:
  • Renal failure
  • Renal tubular acidosis
• Excessive loss of bicarbonate due to:
  • Renal losses or gastrointestinal losses (eg, diarrhea)

Classification

• Type is based on anion gap calculation
• Anion gap = [Na] – [Cl ]+ [HCO₃^-]
  • Na = sodium, Cl = chloride, HCO₃ = bicarbonate
  • Normal = 7-16 mmol
• High anion gap acidosis (organic acidosis)
  • Methanol poisoning
    • Osmolar gap –  high
    • Retained acid – formate
    • No crystals in urine
  • Ethylene glycol poisoning
    • Osmolar gap –  high
    • Retained acids – hippurate, glycolate, oxalate
    • Oxalate crystals in urine
  • Diabetes mellitus – ketoacidosis
    • Osmolar gap – normal
    • Retained acids – acetoacetate, beta-hydroxybutyrate
  • Ethanol poisoning
    • Osmolar gap – high
    • Renal failure
    • Retained acids – sulfuric, phosphoric, organic
  • Starvation
    • Retained acid – beta-hydroxybutyrate
  • Salicylate toxicity
    • Retained acids – salicylate, organic
  • Lactic acidosis
    • Retained acid – lactate
  • Paraldehyde toxicity
    • Pyruvate may be helpful in differentiating inborn errors and cardiac ischemia
• Normal anion gap acidosis (inorganic acidosis)
  • Gastrointestinal fluid loss
    • Severe diarrhea – results from loss of Na, K (potassium), HCO₃^-
    • Pancreatitis – loss of HCO₃^- production
    • Intestinal fistula – loss of Na, K, HCO₃^-
  • Renal tubular acidosis (RTA)
    • Proximal (type II) RTA – loss of HCO₃^- due to decreased tubular secretion of H⁺
    • Distal (type I) RTA – decreased reabsorption of HCO₃^-
    • Type IV RTA – inhibited Na reabsorption with abnormal K⁺ and H⁺ retention; decreased renal ammonia formation with reduced elimination of H⁺
  • Drug-induced hyperkalemia
    • Potassium sparing diuretics
    • Angiotensin-converting enzymes (ACE) inhibitors
    • Cyclosporine
    • Trimethoprim

Diagnosis

• Laboratory testing
  • Initial suspicion that acidosis exists from values found on a basic metabolic laboratory panel (sodium, potassium, chloride and bicarbonate)
    • Decreased bicarbonate level along with arterial blood gases which demonstrates acidosis
  • Based on clinical scenario and anion gap calculation, further testing may be appropriate
    • Glucose serum or plasma to rule out diabetes mellitus
    • BUN/creatinine to rule out renal failure
    • Lactate/pyruvate levels to rule out lactic acidosis
    • Ethanol levels to rule out alcohol poisoning
    • Microscopic examination of urine for crystals to differentiate methanol from ethylene glycol
      • Methanol and ethylene glycol serum levels may also be necessary
    • Salicylate levels to rule out salicylate poisoning
    • Serum osmolality to calculate osmolar gap
    • Anion gap may also be elevated by toxicants such as iron, toluene, phenformin, paraldehyde and arsenic

Indications for Ordering

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

• Click on number for test-specific information in the ARUP Laboratory Test Directory