Metabolic Acidosis

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

Diagnosis

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 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 + [blood urea nitrogen (BUN)]/2.8)
        • Normal = -10 to +10 mOsm/kg
  • Based on clinical scenario and anion gap calculation, further testing may be appropriate
    • Glucose – evaluate for diabetes mellitus (DM)
    • BUN/creatinine – evaluate for renal failure
    • Lactate/pyruvate levels – evaluate for lactic acidosis
    • Beta-hydroxybutyrate acid – evaluate for DM, starvation
    • Ethanol levels – evaluate 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 – evaluate for 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 Background

Background

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)
    • Osmolar gap may be used to differentiate between different types within high anion gap acidosis

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)

ARUP Lab Tests

Aid 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

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

Aid in assessment of etiology of anion gap acidosis

Use to identify ethanol, methanol, isopropanol, or acetone ingestion

Aid in assessment of etiology of anion gap acidosis

Determine whether ethylene glycol poisoning exists

Related Tests

Aid in diagnosis of metabolic acidosis

Aid in differentiating renal from nonrenal causes of nonanion gap metabolic acidosis

Test includes sodium, potassium, chloride, creatinine

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

Screening test to evaluate kidney function

Screening test to evaluate kidney function

Assay interference (negative) may be observed when high concentrations of N-acetylcysteine (NAC) are present

Negative interference has also been reported with NAPQI (an acetaminophen metabolite) but only when concentrations are at or above those expected during acetaminophen overdose

Monitor exposure to methanol

Screening test to evaluate kidney function

Serum test to identify acute alcohol ingestion

Preferred test for assessment of acute or chronic arsenic exposure

Differentiate between toxic inorganic and methylated species as well as benign organic forms

Results are reported as total inorganic, total methylated, and organic arsenic

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

Preferred test for assessment of acute or chronic arsenic exposure

Able to differentiate between toxic inorganic and methylated species as well as benign organic forms

Results are reported as total inorganic, total methylated, and organic arsenic

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

Medical Experts

Contributor

Genzen

Jonathan R. Genzen, MD, PhD

Associate Professor of Clinical Pathology, University of Utah

Chief Operations Officer, Medical Director of Automated Core Laboratory and Farmington Health Center Clinical Laboratory, at ARUP Laboratories

Contributor
Contributor

Lehman

Christopher M. Lehman, MD
Professor of Clinical Pathology, University of Utah
Co-Medical Director, University Hospitals and Clinics Clinical Laboratory

References

Additional Resources
Resources from the ARUP Institute for Clinical and Experimental Pathology®