What labs should be checked in a patient with severe metabolic acidosis?

Laboratory Evaluation for Severe Metabolic Acidosis

In a patient with severe metabolic acidosis, immediately obtain arterial blood gas, complete metabolic panel with calculated anion gap, serum lactate, serum ketones, plasma glucose, BUN/creatinine, complete blood count with differential, urinalysis with urine ketones, serum osmolality, and electrocardiogram. 1

Initial Essential Laboratory Tests

The diagnostic workup must be systematic and comprehensive to identify the underlying cause and guide treatment:

Core Acid-Base Parameters

• Arterial blood gas (ABG) to confirm pH <7.35, measure PaCO₂ to assess respiratory compensation, and determine PaO₂ 1, 2
• Serum bicarbonate from basic metabolic panel (typically <22 mmol/L in metabolic acidosis, with severe acidosis showing levels <15 mmol/L) 1, 3
• Serum electrolytes including sodium, potassium, and chloride to calculate the anion gap 1

Anion Gap Calculation and Classification

• Calculate anion gap using [Na⁺] - ([HCO₃⁻] + [Cl⁻]) to differentiate high anion gap (>12 mEq/L) from normal anion gap acidosis 2, 4
• This single calculation directs your entire diagnostic approach and determines whether you're dealing with organic acid accumulation versus bicarbonate loss 2, 3

Identifying the Underlying Cause

For High Anion Gap Acidosis:

• Serum lactate (>2 mmol/L suggests lactic acidosis from tissue hypoperfusion, sepsis, or shock) 2
• Serum ketones (β-hydroxybutyrate preferred) to diagnose diabetic ketoacidosis or alcoholic ketoacidosis 1, 2
• Plasma glucose (>250 mg/dL with ketones suggests DKA) 1
• Serum osmolality and calculate osmolal gap to detect toxic alcohol ingestion 1
• BUN and creatinine to assess for renal failure contributing to acid accumulation 1, 5

For Normal Anion Gap Acidosis:

• Urine pH helps differentiate renal tubular acidosis from GI bicarbonate losses 1
• Serum potassium aids in classification (hypokalemia suggests RTA types 1 or 2; hyperkalemia suggests RTA type 4) 1

Additional Critical Tests

Infection and Precipitating Factors

• Complete blood count with differential to identify infection or hematologic abnormalities 1
• Bacterial cultures (blood, urine, throat) if infection is suspected as a precipitating factor 1
• Urinalysis with urine ketones by dipstick 1

Cardiac and Other Assessments

• Electrocardiogram to detect arrhythmias, hyperkalemia changes, or myocardial infarction as a precipitating event 1
• Chest X-ray if clinically indicated to identify pneumonia or other pulmonary pathology 1
• HbA1c may be useful to determine if this represents poorly controlled diabetes versus an acute episode in a well-controlled patient 1

Special Considerations for Extreme Acidosis

• Serum phosphate as marked hyperphosphatemia can contribute to extremely elevated anion gaps 6
• Creatine kinase if rhabdomyolysis is suspected (can contribute to severe acidosis with extreme anion gap elevation) 6

Monitoring Parameters During Treatment

Once initial labs are obtained and treatment initiated:

• Repeat venous blood gas every 2-4 hours to monitor pH and anion gap resolution (arterial gases are usually unnecessary after initial diagnosis) 1, 2
• Serial serum potassium is crucial as correction of acidosis drives potassium intracellularly and can precipitate life-threatening hypokalemia 1
• Serial glucose monitoring in DKA patients 1
• Fluid input/output and hemodynamic monitoring to assess adequacy of resuscitation 1

Common Pitfalls to Avoid

Do not assume a single cause: Profoundly elevated anion gaps (>30-40 mEq/L) almost always indicate multifactorial pathogenesis, typically including renal insufficiency combined with organic acidosis 6

Do not overlook corrected sodium: Calculate corrected serum sodium for hyperglycemia (add 1.6 mEq to sodium for each 100 mg/dL glucose above 100 mg/dL) to guide fluid replacement 1

Do not miss mixed acid-base disorders: Calculate expected PaCO₂ compensation (PaCO₂ should decrease by approximately 1 mmHg for every 1 mmol/L fall in bicarbonate) 3. If PaCO₂ is higher or lower than expected, a concurrent respiratory disorder exists.

Do not forget albumin correction: Hypoalbuminemia lowers the measured anion gap, potentially masking a high anion gap acidosis 7