Overview
This lecture explains the delta ratio and its use in diagnosing complex acid-base disorders, including mixed and triple metabolic disturbances using arterial blood gases (ABGs).
Relationship Between Pathologic Acids and Bicarbonate
- The body maintains electrical neutrality during elevated anion gap acidosis by removing bicarbonate to buffer added pathologic acids.
- There is an expected 1:1 ratio between increased anion gap and decreased bicarbonate, though in reality, some acids are buffered by bone and cells.
Calculating Delta Ratio and Delta Gap
- The delta ratio compares the increase in anion gap to the decrease in bicarbonate.
- Formula: Delta ratio = (Measured anion gap – Normal anion gap) ÷ (Normal bicarb – Measured bicarb).
- Typical normal values: anion gap = 12 mEq/L, bicarb = 24 mEq/L.
- Delta ratio varies by etiology: Lactic acidosis (1.0–2.0), ketoacidosis (0.8–1.2).
- Delta gap = (Measured anion gap – Normal anion gap) – (Normal bicarb – Measured bicarb).
- If only elevated gap acidosis, delta gap should be 0 ± 6 mEq/L; delta anion gap + measured bicarb should be 24 ± 6.
Interpreting Delta Ratio in Mixed Disorders
- Delta ratio < expected: Both elevated gap acidosis and normal gap acidosis are present.
- Delta ratio within expected: Isolated elevated gap acidosis.
- Delta ratio > expected: Elevated gap acidosis and metabolic alkalosis.
- Delta ratio/Delta gap only reflect metabolic (not respiratory) disorders.
Limitations of Delta Ratio and Gap
- Extracellular, bone, and intracellular buffering invalidate the one-to-one assumption, especially in lactic acidosis.
- Delta gap can overdiagnose alkalosis and underdiagnose normal gap acidosis; delta ratio is preferred.
Clinical Examples
- Example 1: Delta ratio 0.8 (lactic acidosis + normal gap acidosis).
- Example 2: Delta ratio 1.5 (ketoacidosis + metabolic alkalosis).
- Example 3: Delta ratio 5 (triple disorder: respiratory acidosis, elevated gap metabolic acidosis, metabolic alkalosis).
Key Terms & Definitions
- Anion Gap — Difference between measured cations and anions in serum; reflects unmeasured anions.
- Delta Ratio — (Measured anion gap – Normal anion gap) ÷ (Normal bicarb – Measured bicarb); helps identify mixed metabolic disorders.
- Delta Gap — (Measured anion gap – Normal anion gap) – (Normal bicarb – Measured bicarb); alternative (less accurate) method for finding mixed disorders.
- Elevated Gap Metabolic Acidosis — Acidosis with increased anion gap due to unmeasured acids.
- Metabolic Alkalosis — Excess bicarbonate or loss of acid, resulting in high blood pH.
Action Items / Next Steps
- Practice calculating delta ratio and delta gap from ABG and chemistry panels.
- Review the expected delta ratio ranges for different metabolic acidosis etiologies.
- Prepare for the next lecture on diagnosing mixed acid-base disorders when pH is normal.