How to Check for Metabolic Acidosis
Obtain arterial or venous blood gases along with serum electrolytes to measure pH, bicarbonate (HCO3-), and calculate the anion gap—these are the essential tests to diagnose metabolic acidosis. 1, 2
Initial Laboratory Evaluation
The diagnostic workup requires specific laboratory tests performed simultaneously:
Core Tests Required
- Arterial blood gas (ABG) or venous blood gas (VBG) for pH measurement
- Serum electrolytes (sodium, chloride, bicarbonate)
- Blood glucose
- Blood urea nitrogen (BUN) and creatinine
- Serum ketones (if diabetic ketoacidosis suspected)
- Urine ketones by dipstick 2
Diagnostic Criteria for Metabolic Acidosis
Metabolic acidosis is defined by: 3
- pH < 7.35 (or [H+] > 45 nmol/L)
- Low serum bicarbonate (< 15-18 mEq/L depending on severity)
- Calculate the anion gap using: (Na+) - (Cl- + HCO3-) 1
Systematic Approach to Evaluation
Step 1: Confirm Metabolic Acidosis
Check if pH < 7.35 with low bicarbonate (< 22 mEq/L). This distinguishes it from respiratory acidosis where CO2 is elevated 3.
Step 2: Calculate Anion Gap
Anion gap = (Na+) - (Cl- + HCO3-) 1, 2
- High anion gap (>12 mEq/L): Suggests accumulation of unmeasured anions
- Normal anion gap (≤12 mEq/L): Suggests bicarbonate loss or impaired acid excretion
Step 3: Determine Etiology Based on Anion Gap
For HIGH anion gap metabolic acidosis, consider: 1, 2
- Diabetic ketoacidosis (glucose >250 mg/dl, positive ketones)
- Lactic acidosis (measure blood lactate)
- Renal failure (elevated BUN/creatinine)
- Toxic ingestions: salicylates, methanol, ethylene glycol (check osmolal gap)
- Starvation ketosis (glucose usually <250 mg/dl, bicarbonate typically >18 mEq/L)
For NORMAL anion gap metabolic acidosis, evaluate: 4, 5
- Serum potassium level (hyperkalemic vs. hypokalemic)
- Urine anion gap or urine pH
- History of diarrhea, renal tubular acidosis, or medications
Specific Context: Diabetic Ketoacidosis (DKA)
If DKA is suspected, the complete diagnostic criteria are: 1, 6
| Parameter | Mild DKA | Moderate DKA | Severe DKA |
|---|---|---|---|
| Glucose | >250 mg/dl | >250 mg/dl | >250 mg/dl |
| Arterial pH | 7.25-7.30 | 7.00-7.24 | <7.00 |
| Bicarbonate | 15-18 mEq/L | 10-15 mEq/L | <10 mEq/L |
| Anion gap | >10 | >12 | >12 |
| Ketones | Positive | Positive | Positive |
Important Caveats
β-hydroxybutyrate (β-OHB) measurement is preferred over nitroprusside methods for ketone detection, as nitroprusside only measures acetoacetate and acetone, not β-OHB (the predominant ketone in DKA). During treatment, β-OHB converts to acetoacetate, which can falsely suggest worsening ketosis if using nitroprusside methods 1, 6.
Monitoring During Treatment
For ongoing assessment of metabolic acidosis: 1, 2, 6
- Draw blood every 2-4 hours for electrolytes, glucose, BUN, creatinine, osmolality
- Monitor venous pH and anion gap rather than repeating arterial blood gases
- Track bicarbonate levels toward normal range (18-22 mEq/L)
Clinical Threshold for Treatment
Consider pharmacological treatment when serum bicarbonate falls below 18 mmol/L in adults, though this remains an area of clinical judgment based on recent guideline practice points 7, 8. The goal is to increase bicarbonate toward but not exceeding the normal range while monitoring blood pressure, potassium, and fluid status 7.
The evidence supporting aggressive treatment of mild metabolic acidosis (bicarbonate 18-22 mEq/L) remains limited, with recent trials failing to demonstrate clear benefit for CKD progression 8, 9. However, severe acidosis (bicarbonate <18 mEq/L) warrants intervention to prevent adverse cardiovascular, respiratory, and neurological consequences 10, 11.