The ABG interpreter analyses arterial blood gas values to identify acid-base disorders, including respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis, and fully or partially compensated states. ABG interpretation is a core critical care nursing skill tested in NCLEX and applied daily in ICU, ED, and respiratory wards. Enter the pH, PaCO₂, HCO₃⁻, and optional PaO₂ to get an instant interpretation with clinical context.
| Parameter | Normal Range |
|---|---|
| pH | 7.35 – 7.45 |
| PaCO₂ | 35 – 45 mmHg |
| HCO₃⁻ | 22 – 26 mEq/L |
| PaO₂ | 80 – 100 mmHg |
| SpO₂ | 95 – 100% |
Step 1 — pH: <7.35 = acidosis, >7.45 = alkalosis, 7.35–7.45 = normal.
Step 2 — PaCO₂: >45 = respiratory acidosis cause, <35 = respiratory alkalosis cause.
Step 3 — HCO₃⁻: <22 = metabolic acidosis cause, >26 = metabolic alkalosis cause.
Step 4 — Match: Which value matches the pH direction? That is the primary disorder. If both are abnormal in the same pH direction, partial compensation has occurred.
Almost every ABG resolves into one of four primary disturbances. Read the pH first to see which direction things have gone, then check whether CO₂ or bicarbonate explains it.
| Disturbance | pH | PaCO₂ | HCO₃⁻ | Common causes |
|---|---|---|---|---|
| Respiratory acidosis | ↓ low | ↑ high | normal / ↑ | Hypoventilation, COPD, sedation, airway obstruction |
| Respiratory alkalosis | ↑ high | ↓ low | normal / ↓ | Hyperventilation, anxiety, pain, sepsis, altitude |
| Metabolic acidosis | ↓ low | normal / ↓ | ↓ low | DKA, renal failure, lactic acidosis, diarrhoea |
| Metabolic alkalosis | ↑ high | normal / ↑ | ↑ high | Vomiting, NG suction, diuretics, hypokalaemia |
The memory aid most nurses end up using is ROME — Respiratory Opposite, Metabolic Equal. In a respiratory problem the pH and the CO₂ move in opposite directions; in a metabolic one the pH and the bicarbonate move together. It sounds like a gimmick until it saves you thirty seconds at 4am, at which point it stops sounding like one.
| ABG | Reading | Interpretation |
|---|---|---|
| pH 7.28, CO₂ 55, HCO₃ 24 | Low pH, high CO₂ | Respiratory acidosis, uncompensated |
| pH 7.32, CO₂ 60, HCO₃ 32 | Low pH, high CO₂, high HCO₃ | Respiratory acidosis, partly compensated |
| pH 7.50, CO₂ 28, HCO₃ 23 | High pH, low CO₂ | Respiratory alkalosis, uncompensated |
| pH 7.25, CO₂ 38, HCO₃ 15 | Low pH, low HCO₃ | Metabolic acidosis, uncompensated |
| pH 7.36, CO₂ 30, HCO₃ 17 | Normal pH, low CO₂, low HCO₃ | Metabolic acidosis, fully compensated |
| pH 7.52, CO₂ 42, HCO₃ 34 | High pH, high HCO₃ | Metabolic alkalosis, uncompensated |
Compensation is the part people find slippery. The body tries to pull the pH back toward normal by moving the other system — lungs compensate for metabolic problems within minutes to hours, kidneys compensate for respiratory ones over hours to days. If the pH has made it back inside 7.35–7.45 but the CO₂ and bicarbonate are both clearly abnormal, that's full compensation, and the direction the pH sits within the normal range tells you which disturbance came first: a pH of 7.36 with both values deranged points to an acidosis that's been compensated, not a healthy gas.
Step 1 — Check pH (acidosis <7.35, alkalosis >7.45). Step 2 — Check PaCO₂ (>45 = respiratory acidosis cause, <35 = respiratory alkalosis cause). Step 3 — Check HCO₃⁻ (<22 = metabolic acidosis cause, >26 = metabolic alkalosis cause). Step 4 — Match the abnormal value to the pH direction to find the primary disorder. If both are abnormal, compensation has begun.
Normal ABG values: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg, SpO₂ ≥95%.