What lab indicators are used to diagnose acidosis?

Laboratory Indicators of Acidosis

The primary laboratory indicators used to diagnose acidosis are arterial blood pH <7.35, serum bicarbonate <15-18 mEq/L, and venous pH <7.3, with additional assessment of the anion gap to determine the underlying mechanism. 1

Core Diagnostic Parameters

Blood Gas Analysis

• Arterial pH <7.35 is the definitive marker of acidemia, though venous pH (typically 0.03 units lower than arterial) can be used for monitoring and is often sufficient in clinical practice 1
• PaCO2 levels distinguish respiratory from metabolic causes: elevated PaCO2 >46 mmHg indicates respiratory acidosis, while normal or low PaCO2 suggests metabolic acidosis with respiratory compensation 2
• Arterial blood gas analysis provides assessment of oxygenation (pO2), respiratory function (pCO2), and acid-base balance (pH), and should be obtained in patients with severe respiratory distress 1

Serum Bicarbonate

• Serum bicarbonate <15 mEq/L is a diagnostic criterion for diabetic ketoacidosis (DKA) 1
• Bicarbonate <18 mEq/L indicates unresolved metabolic acidosis in DKA management 1
• Monthly monitoring of serum bicarbonate is recommended in maintenance dialysis patients, with target levels ≥22 mmol/L 1

Anion Gap Calculation

• Elevated anion gap (>12 mEq/L) indicates accumulation of unmeasured anions such as lactate, ketoacids, or uremic toxins 1, 3
• The anion gap should be corrected for serum albumin levels, as hypoalbuminemia artificially lowers the calculated gap 3
• Normal anion gap acidosis suggests bicarbonate loss (gastrointestinal or renal) or ingestion of acidifying chloride salts 4

Additional Laboratory Markers

Electrolytes and Metabolic Parameters

• Serum lactate elevation indicates lactic acidosis and tissue hypoperfusion; severe acidosis with high lactate is associated with poor prognosis 1
• Serum potassium, sodium, chloride, and calcium should be measured, as specific disturbances (hypocalcemia, hyponatremia, hypokalemia) can accompany certain acidotic states 1
• Blood urea nitrogen and creatinine identify renal failure as a cause of metabolic acidosis 1

Monitoring Parameters

• Total carbon dioxide (TCO2) serves as a surrogate marker for plasma bicarbonate and can be used to monitor acidosis, though it is less specific than direct pH measurement 1
• Blood should be drawn every 2-4 hours during treatment of DKA or hyperosmolar hyperglycemic state for electrolytes, glucose, and venous pH 1

Clinical Context Considerations

Severe Acidosis Thresholds

• Arterial pH <7.1 and base deficit >10 defines severe acidosis that may warrant bicarbonate therapy in cardiac arrest or specific overdose situations 1
• In caustic ingestion injuries, severe acidosis combined with elevated lactate predicts transmural necrosis and poor outcomes 1

Compensation Assessment

• In metabolic acidosis, expected respiratory compensation is hyperventilation with decreased PaCO2 4
• In chronic respiratory acidosis, compensatory elevation of serum bicarbonate occurs through renal retention 2
• The relationship between changes in bicarbonate and anion gap (delta-delta) helps identify mixed acid-base disorders 3

Common Pitfall: Relying solely on arterial blood gases without calculating the anion gap can miss the underlying mechanism of metabolic acidosis. Always calculate the anion gap and correct it for albumin to properly categorize the acidosis and guide treatment 3.