Interpreting an arterial blood gas (ABG) comes down to a systematic approach: check the pH first, then figure out whether the lungs or the kidneys are driving the problem, and finally assess oxygenation. Once you learn the pattern, you can work through any ABG in under a minute. The key is knowing the normal ranges and following the same steps every time.
Normal ABG Values
Before interpreting anything, you need to know what normal looks like. These are the reference ranges for adults:
- pH: 7.35 to 7.45
- PaCO2: 35 to 45 mmHg
- HCO3 (bicarbonate): 22 to 26 mEq/L
- PaO2: 80 to 100 mmHg
- SaO2 (oxygen saturation): 95% to 100%
Two of these values relate to acid-base balance from different systems. PaCO2 reflects the respiratory side, controlled by the lungs. Bicarbonate (HCO3) reflects the metabolic side, controlled primarily by the kidneys. The pH tells you the net result of both systems working together.
Step 1: Look at the pH
The pH tells you the overall direction of the problem. A pH below 7.35 means acidemia, where the blood is too acidic. A pH above 7.45 means alkalemia, where the blood is too alkaline. Lock this in your mind first, because it won’t change no matter what the other numbers say.
Sometimes the pH falls within the normal range of 7.35 to 7.45 but sits toward one end. A pH of 7.36 is technically normal but leans acidotic, which can signal a compensated acid-base disorder. If the pH is normal but PaCO2 or bicarbonate is abnormal, there’s still a disturbance present. The body has just managed to compensate for it.
Step 2: Respiratory or Metabolic?
Now look at the PaCO2 and bicarbonate to determine whether the primary problem is coming from the lungs (respiratory) or the kidneys and metabolism (metabolic). A simple mnemonic called ROME helps here: Respiratory Opposite, Metabolic Equal.
In respiratory disorders, the pH and PaCO2 move in opposite directions. If the pH is low (acidosis) and PaCO2 is high, the lungs aren’t blowing off enough carbon dioxide. That’s respiratory acidosis. If the pH is high (alkalosis) and PaCO2 is low, the lungs are ventilating too much. That’s respiratory alkalosis.
In metabolic disorders, the pH and PaCO2 move in the same direction. If the pH is low and PaCO2 is also low, the lungs are trying to compensate for a metabolic acidosis by breathing faster and blowing off CO2. The primary problem is metabolic because the low CO2 doesn’t explain the low pH. Similarly, if the pH is high and PaCO2 is also high, the lungs are slowing down to retain CO2 in response to a metabolic alkalosis.
Here’s a quick reference:
- Respiratory acidosis: pH down, PaCO2 up
- Respiratory alkalosis: pH up, PaCO2 down
- Metabolic acidosis: pH down, PaCO2 down, HCO3 down
- Metabolic alkalosis: pH up, PaCO2 up, HCO3 up
Step 3: Check for Compensation
The body doesn’t tolerate pH changes passively. If the lungs cause the problem, the kidneys respond, and vice versa. This is compensation. The key question is whether compensation is absent, partial, or full.
If only one value (PaCO2 or HCO3) is abnormal and the pH is clearly outside normal range, the disturbance is uncompensated. The other system hasn’t kicked in yet. If both PaCO2 and HCO3 are abnormal but the pH is still outside the normal range, compensation is partial. If both are abnormal but the pH has returned to the normal range, compensation is full. An important principle: full compensation rarely brings the pH all the way back to a perfect 7.40. It typically lands near the edge of normal.
Compensation Formulas
For metabolic acidosis, you can check whether the lungs are compensating the expected amount using Winter’s formula: expected PaCO2 equals (1.5 times the bicarbonate) plus 8, give or take 2. Full respiratory compensation for metabolic acidosis takes about 12 to 24 hours to develop. If the actual PaCO2 is higher than what the formula predicts, a concurrent respiratory acidosis is also present. If it’s lower, there’s an additional respiratory alkalosis on top of the metabolic acidosis.
For respiratory disorders, compensation happens through the kidneys adjusting bicarbonate levels. In acute respiratory acidosis (developing over hours), bicarbonate rises by about 1 mEq/L for every 10 mmHg increase in PaCO2. In chronic respiratory acidosis (developing over 3 to 5 days), the kidneys have had time to work harder, and bicarbonate rises by about 3.5 mEq/L for every 10 mmHg increase in PaCO2. This distinction between acute and chronic compensation is one way to estimate how long a respiratory problem has been going on.
When the observed compensation doesn’t match what you’d expect from these formulas, more than one acid-base disorder is likely present at the same time.
Step 4: Calculate the Anion Gap
This step applies when you’ve identified a metabolic acidosis. The anion gap helps you figure out why the acidosis is happening. The formula is: sodium minus (chloride plus bicarbonate). A normal anion gap is roughly 12 mEq/L, though it can be lower in people with low albumin levels (subtract about 2.5 from the normal value for each 1 g/dL drop in albumin).
A high anion gap means there are extra unmeasured acids in the blood. The classic causes include diabetic ketoacidosis, kidney failure, lactic acidosis from poor tissue oxygen delivery or sepsis, and poisoning from substances like methanol or ethylene glycol. A normal anion gap acidosis (sometimes called a non-gap acidosis) means the body is losing bicarbonate or retaining too much chloride. Common causes include severe diarrhea and certain kidney disorders.
The Delta-Delta Ratio
If the anion gap is elevated, there’s one more check worth doing. Compare how much the anion gap increased (above 12) to how much the bicarbonate decreased (below 24). This ratio, called the delta-delta, should fall between 1.0 and 2.0 in a straightforward anion gap metabolic acidosis. If the ratio is less than 1.0, a simultaneous non-gap metabolic acidosis is also present. If it’s greater than 2.0, a concurrent metabolic alkalosis is hiding alongside the anion gap acidosis.
Step 5: Assess Oxygenation
After working through the acid-base portion, look at the PaO2 and oxygen saturation. These values tell you how well the lungs are getting oxygen into the blood, which is a separate question from how well they’re removing CO2.
PaO2 levels break down as follows:
- Normal: 80 to 100 mmHg
- Mild hypoxemia: 60 to 79 mmHg
- Moderate hypoxemia: 40 to 59 mmHg
- Severe hypoxemia: below 40 mmHg
When the PaO2 is low but PaCO2 is normal or low, the lungs have a problem with gas exchange itself, such as pneumonia, a blood clot in the lung, or fluid buildup. When both PaO2 is low and PaCO2 is high, the person isn’t ventilating enough. This could be from anything that suppresses breathing, like opioid use, neuromuscular disease, or severe COPD. In some cases, both problems coexist.
Common Causes of Each Disorder
Knowing the four primary disturbances is useful, but recognizing what typically causes each one makes interpretation more practical.
Respiratory Acidosis
Respiratory acidosis happens when the lungs can’t remove CO2 fast enough. The most common reasons include COPD, severe asthma, drug-induced respiratory depression (opioids, sedatives), obesity-hypoventilation syndrome, and neuromuscular conditions like Guillain-Barré syndrome or myasthenia gravis that weaken the muscles of breathing. Anything from a chest wall injury to a large pleural effusion compressing the lung can also drive up CO2.
Respiratory Alkalosis
Respiratory alkalosis results from breathing too fast or too deeply, which washes out CO2. Anxiety and panic attacks are the most familiar triggers, but it also occurs with pain, fever, high altitude, early asthma or pneumonia (before the person tires out), and over-aggressive mechanical ventilation in hospital settings. Low oxygen levels from any cause can drive faster breathing and push CO2 down.
Metabolic Acidosis
Metabolic acidosis means the body either has too much acid or has lost too much bicarbonate. High anion gap causes include diabetic ketoacidosis, lactic acidosis from sepsis or shock, kidney failure, and toxic ingestions. Normal anion gap causes include diarrhea (which drains bicarbonate from the gut), certain kidney tubular disorders, and large-volume saline infusions that raise chloride levels relative to bicarbonate.
Metabolic Alkalosis
Metabolic alkalosis is most commonly caused by vomiting or nasogastric suctioning, which removes acid from the stomach. Diuretics (particularly loop and thiazide types) can cause it by driving chloride and potassium loss. Low potassium itself pushes the kidneys to retain bicarbonate. Large-volume infusions of fluids containing bicarbonate precursors, like lactated Ringer’s or Plasma-Lyte, can also tip the balance toward alkalosis.
Putting It All Together
Here’s a quick example. Suppose the ABG shows: pH 7.28, PaCO2 24, HCO3 11, PaO2 98. Start with pH: 7.28 is below 7.35, so this is an acidosis. Next, PaCO2 is low at 24, which would normally push pH up, not down. That means CO2 isn’t causing the acidosis; the lungs are actually compensating. The low bicarbonate of 11 confirms a metabolic acidosis as the primary problem.
Check Winter’s formula: expected PaCO2 = (1.5 times 11) + 8 = 24.5, plus or minus 2. The actual PaCO2 of 24 falls right in that range, so respiratory compensation is appropriate with no additional respiratory disorder. The anion gap would be the next step, and you’d need the sodium and chloride values to calculate it. PaO2 is 98, which is normal, so oxygenation isn’t a concern here.
Every ABG follows this same sequence. With practice, the process becomes almost automatic: pH direction, respiratory or metabolic cause, compensation check, anion gap if metabolic acidosis, and oxygenation status.