Every RT student learns ABG interpretation three times: once for the exam, once for clinicals, and once at 0300 when a real gas hits the printer and everyone looks at you. This is the six-step method that survives all three. Study reference, not medical advice — your program, your protocols, and the patient in front of you always win.
Know your normals cold
- pH: 7.35–7.45
- PaCO2: 35–45 mmHg
- HCO3: 22–26 mEq/L
- PaO2: 80–100 mmHg (room air)
- SaO2: 95–100%
Step 1 — pH: acidemia or alkalemia?
Below 7.35 = acidemia. Above 7.45 = alkalemia. Inside the range? Could be normal — or fully compensated. Don't stop yet.
Step 2 — PaCO2: the respiratory component
CO2 is an acid. High PaCO2 pushes pH down (respiratory acidosis); low PaCO2 pushes it up (respiratory alkalosis). If the PaCO2 moves opposite the pH, respiratory is your primary suspect.
Step 3 — HCO3: the metabolic component
Bicarb is a base. Low HCO3 with low pH = metabolic acidosis. High HCO3 with high pH = metabolic alkalosis. If HCO3 moves in the same direction as pH, metabolic is driving.
Step 4 — Name the primary disorder
Whichever component (CO2 or HCO3) explains the pH is primary. If both are abnormal in ways that each push pH the same direction, you may have a mixed disorder — flag it, don't force it into one box.
Step 5 — Check compensation
- Acute respiratory acidosis: pH falls ~0.08 for every 10 mmHg rise in PaCO2. Chronic: only ~0.03 per 10 (the kidneys have caught up).
- Metabolic acidosis — Winter's formula: expected PaCO2 = (1.5 × HCO3) + 8, ±2. If the measured PaCO2 is higher than expected, there's a hidden respiratory acidosis on top; lower, a respiratory alkalosis.
- Compensation moves pH toward normal, never past it. Overshoot = second primary disorder.
Step 6 — Oxygenation, separately
Acid-base and oxygenation are two different report lines. A perfectly compensated gas can still carry a PaO2 of 52. Check PaO2 against FiO2 (the classic sanity check: PaO2 should be roughly 4–5× the FiO2 percentage in healthy lungs), and don't let a pretty pH distract you from a shunt.
The one the instructor asks next
Anion gap = Na − (Cl + HCO3), normal ~8–12. A metabolic acidosis with an elevated gap sends you hunting (think MUDPILES); a normal-gap acidosis usually means bicarb loss — GI or renal.
Pull a clean gas before you interpret one — radial found, Allen's checked, one stick. That standard has a shirt: ABG SNIPER. More field references in FIELD NOTES.