Metabolic Acidosis

Diagnosis

Indications for Testing

• Patient with altered mentation
• Patient with initial laboratory results that indicate the presence of acidosis

Laboratory Testing

• Metabolic panel (Na, K, Cl, HCO3-) and arterial blood gases
  • Expect decreased bicarbonate level on both tests, along with acidosis on the arterial blood gases
  • Calculate anion and osmolar gaps to further aid in differential
    • Anion gap = [Na] - ([Cl] + [HCO3-])
      • Normal = 7-16 mmol/L
    • Osmolar gap = calculated plasma osmolality - measured plasma osmolality (2[Na+] + [glucose]/18 + [BUN]/2.8)
      • Normal = -10 to +10 mOsm/kg
• Based on clinical scenario and anion gap calculation, further testing may be appropriate
  • Glucose – rule out diabetes mellitus (DM)
  • BUN/creatinine – rule out renal failure
  • Lactate/pyruvate levels – rule out lactic acidosis
  • Beta-hydroxybutyrate acid – suggests DM, starvation
  • Ethanol levels – 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 – rule out salicylate poisoning
  • Anion gap may also be elevated by toxicants such as acetaminophen, iron, toluene, phenformin, paraldehyde, arsenic
    • Other testing (serum drug levels) based on results of above testing

Differential Diagnosis

• Refer to the different types of metabolic acidosis in the Background tab

Clinical Background

Metabolic acidosis, a condition in which there is excess buildup of acid in the body fluids, is heralded by a decreased concentration of plasma bicarbonate.

Classification

• Type of metabolic acidosis is based on anion/osmolar gap calculation
  • Anion gap = [Na] - ([Cl ] + [HCO₃^-])
  • Osmolar gap = calculated plasma osmolality - measured plasma osmolality (2[Na⁺] + [glucose]/18 + [BUN]/2.8)
• High anion gap acidosis (organic acidosis)

  Osmolar gap may be used to differentiate between different types within high anion gap acidosis Diabetes mellitus – ketoacidosis Osmolar gap – normal Retained acids – acetoacetic acid, beta-hydroxybutyric acid Ethanol poisoning Osmolar gap – high Retained acids – sulfuric, phosphoric, organic Renal failure Ethylene glycol poisoning Osmolar gap –  high Retained acids – oxalic acid, glycine, oxalomalic acid, formic acid Calcium oxalate crystals in urine Lactic acidosis Retained acid – lactic acid Methanol poisoning Osmolar gap –  high Retained acid – formic acid No crystals in urine Paraldehyde toxicity Salicylate toxicity Retained acids – salicylate, organic Starvation Retained acid – beta-hydroxybutyric acid

  Normal anion gap acidosis (inorganic acidosis)

  Gastrointestinal fluid loss Severe diarrhea – results from loss of Na, K, HCO3- Pancreatitis – loss of HCO3- production Intestinal fistula – loss of Na, K, HCO3- Drug-induced hyperkalemia Potassium-sparing diuretics Angiotensin-converting enzyme (ACE) inhibitors Cyclosporine Trimethoprim 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+

Pathophysiology

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

Indications for Laboratory Testing

• 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 the plus sign to expand the table of additional tests.

Test Name and Number	Comments
Bicarbonate (HCO3), Urine 0020245 Method: Enzymatic	Aid in diagnosis of metabolic acidosis
Electrolytes, Urine 0020498 Method: Quantitative Ion-Selective Electrode	Aid in differentiating renal from nonrenal causes of non-anion-gap metabolic acidosis Test includes sodium, potassium, chloride, creatinine
Pyruvic Acid 0080310 Method: Quantitative Enzymatic	Differential diagnosis of lactic acidosis
Calcium, Serum or Plasma 0020027 Method: Quantitative Spectrophotometry
Carbon Dioxide, Serum or Plasma 0020004 Method: Quantitative Enzymatic
Chloride, Serum or Plasma 0020003 Method: Quantitative Ion-Selective Electrode
Creatinine, Serum or Plasma 0020025 Method: Quantitative Enzymatic
Methanol 0090165 Method: Quantitative Gas Chromatography
Potassium, Plasma or Serum 0020002 Method: Quantitative Ion-Selective Electrode
Sodium, Plasma or Serum 0020001 Method: Quantitative Ion-Selective Electrode
Urea Nitrogen, Serum or Plasma 0020023 Method: Quantitative Spectrophotometry
Ethanol, Serum or Plasma - Medical 0090120 Method: Quantitative Gas Chromatography/Enzymatic
Arsenic, Urine with Reflex to Fractionated 0025000 Method: Quantitative High Performance Liquid Chromatography/Quantitative Inductively Coupled Plasma-Mass Spectrometry	Urine is preferred specimen for arsenic testing Differentiate between toxic inorganic, organic, and methylated forms of arsenic
Glucose, Plasma or Serum 0020024 Method: Quantitative Enzymatic