One Sheet Method to Acid Base
One-sheet method
- Write pH → PCO2 → expected PCO2 → HCO3− → anion gap every time.
- The fixed sequence:
- Locks the facts into memory
- Drives the analysis in a straight line
- Lets you “see” the disorder on one line
Core facts to remember
- pH labels the blood as alkalemic, neutral, or acidemic
- PCO2 shows the respiratory side
- HCO3− shows the metabolic side
- Expected PCO2 =
15 + HCO3−(works at any HCO3−) - Anion gap = Na+ − (Cl− + HCO3−); normal ≈ 6-18
Four-step check
- 1. pH → decide acid vs alkali
- 2. PCO2 vs expected → spot extra respiratory shift
- 3. HCO3− alone → net metabolic tilt
- 4A. Anion gap
- Gap > 18-20 → high-gap metabolic acidosis (HAGMA)
- Normal gap + low HCO3− → normal-gap metabolic acidosis (NAGMA)
- 4B. Delta-delta
- ΔGap ≈ ΔHCO3− → one metabolic problem
- ΔGap > ΔHCO3− → hidden metabolic alkalosis
- ΔGap < ΔHCO3− → extra normal-gap acidosis
Two add-ons
- Osmolal gap when the patient is obtunded/intoxicated
- Gap > 25 → toxic alcohols (methanol, ethylene glycol, propylene glycol)
- High osmolal gap but normal acid-base → isopropyl alcohol
- Urine anion gap for every NAGMA.
- Negative → gut HCO3− loss (diarrhea)
- Zero or positive → renal tubular acidosis
Case drills
- Delirious drinker – respiratory acidosis; osmolal gap ↑ → isopropyl alcohol
- Diarrhea & neglect – NAGMA → check urine gap
- Pill overdose – respiratory alkalosis + HAGMA (salicylate pattern)
- DKA with vomiting – HAGMA + metabolic alkalosis
- COPD pneumonia – respiratory acidosis, HAGMA, metabolic alkalosis
- Obesity hypoventilation – respiratory acidosis + metabolic alkalosis
Practice checklist
- Write the five numbers before thinking
- Calculate expected PCO2 and the anion gap every time
- Run delta-delta if the gap is high
- Add osmolal or urine gaps when needed
- Target ⩽ 60 s for a full read
Key numbers
- Normal pH 7.35-7.45
- Normal PCO2 35-45 mm Hg
- Normal HCO3− 22-28 mEq/L
- Normal anion gap 6-18 mEq/L