Metabolic Acidosis


Indications for Testing

  • Patient with altered mental status
  • 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.


  • 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
      • Osmolar gap – normal 
      • 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
    • 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+
    Low or negative anion gap (hyperchloremia)
    • Lithium toxicity
    • Monoclonal IgG gammopathy
    • Iodide or bromide intoxication – causes pseudohyperchloremia
    • Disorders of high calcium or magnesium levels


  • 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
Test Name and Number Recommended Use Limitations Follow Up
Basic Metabolic Panel 0020399
Method: Quantitative Ion-Selective Electrode/Quantitative Enzymatic/Quantitative Spectrophotometry

Aids in diagnosis of metabolic acidosis and calculation of anion gap and osmolar gap

Panel includes calcium, carbon dioxide, chloride, creatinine, glucose, potassium, sodium and urea nitrogen

Osmolality, Serum or Plasma 0020046
Method: Freezing Point

Aids in differential diagnosis of high-anion-gap metabolic acidosis; use in calculation of osmolar gap

Lactic Acid, Plasma 0020045
Method: Enzymatic

Aids in assessment of the etiology of anion gap acidosis

Beta-Hydroxybutyric Acid 0080045
Method: Quantitative Enzymatic

Aids in assessment of the etiology of anion gap acidosis

Alcohols 0090131
Method: Quantitative Gas Chromatography

Aids in assessment of the etiology of anion gap acidosis

Ethylene Glycol 0090110
Method: Quantitative Enzymatic

Aids in assessment of the etiology of anion gap acidosis

Determine whether ethylene glycol poisoning exists

Salicylate Assay 0090251
Method: Spectrophotometry

Aids in assessment of the etiology of anion gap acidosis

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Bicarbonate (HCO3), Urine 0020245
Method: Enzymatic

Aids in diagnosis of metabolic acidosis

Electrolytes, Urine 0020498
Method: Quantitative Ion-Selective Electrode

Aids 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

An isolated pyruvic acid concentration has little clinical value

Preferred test is lactate to pyruvate ratio, whole blood, which reports concentrations for lactate, pyruvate, and L:P ratio on the same specimen

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

Screening test to evaluate kidney function

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

Screening test to evaluate kidney function

Ethanol, Serum or Plasma - Medical 0090120
Method: Quantitative Gas Chromatography/Enzymatic

Serum test to identify acute alcohol ingestion

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

Differentiates between toxic inorganic, organic, and methylated forms of arsenic

Reflex pattern – if total arsenic concentration is between 35-2000 ug/L, then arsenic, fractionated, will be added to determine the proportion of organic, inorganic, and methylated forms

Glucose, Plasma or Serum 0020024
Method: Quantitative Enzymatic