From the Guidelines
The workup for metabolic acidosis should begin with a comprehensive history and physical examination, followed by laboratory tests including arterial blood gas, serum electrolytes, anion gap calculation, and urinalysis, as this approach helps identify the underlying cause of metabolic acidosis, which is crucial for appropriate management, as treatment must address the specific etiology rather than just correcting the acid-base disturbance. The initial testing should include a basic metabolic panel to assess bicarbonate levels, sodium, potassium, chloride, BUN, and creatinine 1. Calculate the anion gap (sodium - [chloride + bicarbonate]) to differentiate between high anion gap metabolic acidosis (HAGMA) and normal anion gap metabolic acidosis (NAGMA) 1. For HAGMA, additional tests should include serum lactate, ketones, toxicology screening, salicylate levels, and osmolal gap 1. For NAGMA, evaluate urinary anion gap, urine pH, and serum potassium 1. Specific tests based on clinical suspicion may include plasma renin and aldosterone levels for renal tubular acidosis evaluation, stool studies for diarrhea-related acidosis, or medication review for drug-induced causes 1. Imaging studies like abdominal CT may be indicated for suspected renal disease or obstruction. This systematic approach helps identify the underlying cause of metabolic acidosis, which is crucial for appropriate management, as treatment must address the specific etiology rather than just correcting the acid-base disturbance. Some key points to consider in the workup include:
- Measurement of blood lactate, serum salicylate, and blood methanol level can be helpful in situations where intoxication is suspected 1
- Ethylene glycol (antifreeze) is suggested by the presence of calcium oxalate and hippurate crystals in the urine 1
- Paraldehyde ingestion is indicated by its characteristic strong odor on the breath 1
- The nitroprusside method only measures acetoacetic acid and acetone, but -OHB, the strongest and most prevalent acid in DKA, is not measured by this method 1
- During therapy for DKA or HHS, blood should be drawn every 2–4 h for determination of serum electrolytes, glucose, blood urea nitrogen, creatinine, osmolality, and venous pH (for DKA) 1
From the Research
Metabolic Acidosis Workup
The workup for metabolic acidosis involves several steps to identify the underlying cause and assess the severity of the condition.
- Identify life-threatening issues in patients with metabolic acidosis, which may be present before therapy begins and/or anticipated after therapy commences 2.
- Determine the basis for metabolic acidosis by hunting for new anions, not only in plasma, but also in the urine 2.
- Measure the venous as well as the arterial PCO2 to assess the capacity to buffer H+ while minimizing H + binding to intracellular proteins 2.
- Assess the role of the kidney in a patient with metabolic acidosis by measuring the urine osmolal gap and the concentration of creatinine in the urine to provide an estimate of the rate of excretion of ammonium 2.
Diagnostic Approach
The diagnostic approach to metabolic acidosis involves calculating the serum anion gap, which aids in classifying the disorders into categories of normal (hyperchloremic) anion gap or elevated anion gap 3.
- The serum anion gap is calculated as [Na(+)] - ([HCO(3)(-)] + [Cl(-)]) 3.
- The anion gap can provide vital clues that direct the clinician toward the correct diagnosis, especially in cases of high anion gap metabolic acidosis (HAGMA) 4.
- The anion gap should be corrected for hypoalbuminemia and severe hyperglycemia, and plasma osmolality and osmolal gap calculations can be used to guide appropriate clinical decision making 4.
Common Causes and Treatment
Metabolic acidosis can result from various causes, including the overproduction of organic acids, bicarbonate wasting, and impaired renal acidification 3, 5.
- The treatment of metabolic acidosis depends on the underlying cause and may involve the administration of base, such as sodium bicarbonate 3, 6, 5.
- The use of base to treat acute metabolic acidosis is controversial due to a lack of definitive benefit and potential complications, while the administration of base for the treatment of chronic metabolic acidosis is associated with improved cellular function and few complications 3.