How to interpret ABGs without panicking
Arterial blood gas questions become more manageable with a consistent screening sequence. The four-step method below helps classify common primary disturbances in exam questions. Real patients can have mixed disorders, so ABG values must also be interpreted with the history, assessment, oxygen delivery, trends and local reference ranges.
Common adult reference ranges
These are general arterial ranges. The reporting laboratory's ranges take priority, and values can vary with age and clinical circumstances.
| Value | Typical range |
|---|---|
| pH | 7.35–7.45 |
| PaCO2 | 35–45 mmHg |
| HCO3 | 22–26 mEq/L |
| PaO2 | 75–100 mmHg |
The four steps
- Look at the pH. Below 7.35 is acidemia; above 7.45 is alkalemia. A pH within range does not by itself prove full compensation; it can also occur with a mixed disorder.
- Look at the CO2. This is the respiratory value. It moves opposite to pH in a respiratory problem: CO2 up, pH down.
- Look at the HCO3. This is the metabolic value. It moves with pH in a metabolic problem: bicarbonate down, pH down.
- Identify the likely primary process. Determine which value explains the pH direction, then ask whether the other value has shifted in the expected compensatory direction. A response outside the expected range can signal a second primary disorder.
The introductory shortcut: in a straightforward single-disorder question, pH and CO2 moving in opposite directions points toward a respiratory process; pH and HCO3 moving in the same direction points toward a metabolic process. This is a screening aid, not a substitute for expected-compensation calculations or clinical context.
Compensation
The body defends pH. When a primary acid-base disturbance occurs, the respiratory or renal system may shift in the opposing direction.
| Teaching label | Screening pattern |
|---|---|
| No evident compensation | pH is abnormal and the expected compensatory variable remains within range. |
| Partial compensation | pH remains abnormal and the other system has shifted in the expected direction. |
| pH within reference range with both values abnormal | This can reflect a compensated primary disorder or a mixed disorder. Compare with expected compensation and the clinical picture rather than labeling it from pH alone. |
For simplified exam items, comparing a normal-range pH with 7.40 can help identify the likely primary direction: 7.37 may suggest an acidotic primary process, while 7.43 may suggest an alkalotic primary process. Both values are still within the normal pH range. Treat this comparison as an introductory clue, not proof. Expected-compensation rules, trends and the clinical presentation are needed to identify mixed disorders.
Causes worth recognizing
| Disturbance | Common exam scenarios |
|---|---|
| Respiratory acidosis | Hypoventilation — COPD, opioid oversedation or overdose, severe obesity, or brain injury. |
| Respiratory alkalosis | Hyperventilation — panic or anxiety, pulmonary embolism, pneumonia, or salicylate toxicity. |
| Metabolic acidosis | Diabetic ketoacidosis, renal failure, severe or prolonged diarrhea, lactic acidosis, or shock. |
| Metabolic alkalosis | Prolonged vomiting, diuretic-related losses, or hypovolemia. |
Two pairings recur often enough to be worth holding ready: vomiting produces metabolic alkalosis because acid is lost, while diarrhea produces metabolic acidosis because bicarbonate is lost.
Do not forget the oxygen
PaO2 sits outside the acid-base classification and is easy to skip. A PaO2 of 54 mmHg on room air is below the usual adult range, but oxygenation must be interpreted with the inspired oxygen concentration, age, trend and full clinical presentation. Read every reported value before deciding what the client needs; a normal PaO2 on supplemental oxygen does not by itself exclude respiratory failure.
Common questions
How do I interpret an ABG quickly?
Check the pH for acidemia or alkalemia, compare PaCO2 and HCO3 to identify the likely primary process, assess whether the other system moved in the expected compensatory direction, and then evaluate PaO2 in the oxygen-delivery and clinical context.
How do I tell if an ABG is compensated?
A shift in the other system in the expected direction suggests compensation. If both PaCO2 and HCO3 are abnormal with a normal-range pH, do not assume full compensation automatically; compare the values with expected compensation and assess for a mixed disorder.
Does vomiting cause acidosis or alkalosis?
Metabolic alkalosis, because gastric acid is lost. Prolonged diarrhea does the opposite, causing metabolic acidosis through loss of bicarbonate.
Sources and further reading
- MedlinePlus — Arterial Blood Gas (ABG) Test — U.S. National Library of Medicine; evidence locator: Measurements, uses, and results sections; source updated ; accessed
- Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility — StatPearls Publishing via NCBI Bookshelf; evidence locator: Pathophysiology; Results, Reporting, and Critical Findings; Clinical Significance; source updated ; accessed
- Physiology, Acid Base Balance — StatPearls Publishing via NCBI Bookshelf; evidence locator: Introduction; Related Testing; Pathophysiology; source updated ; accessed
- NCSBN — 2026 RN Test Plan — National Council of State Boards of Nursing; evidence locator: 2026 RN Test Plan; effective April 1, 2026–March 31, 2029; source updated ; accessed
Practice this for real. PulseRN drills ABG interpretation inside full clinical scenarios, not just isolated numbers.