ABG Interpretation: A Three-Step Method That Works Every Time

The four values and their ranges

You need four numbers. pH runs 7.35 to 7.45. Partial pressure of carbon dioxide runs 35 to 45 mmHg. Bicarbonate runs 22 to 26 mEq/L. Partial pressure of oxygen runs 80 to 100 mmHg, with oxygen saturation 95 to 100 percent.

Carbon dioxide is the respiratory value; it is regulated by the lungs and changes within minutes. Bicarbonate is the metabolic value; it is regulated by the kidneys and changes over hours to days. That difference in speed explains most of what compensation looks like.

Oxygen and saturation tell you about oxygenation rather than acid–base balance. They are assessed separately and can be normal in a badly deranged gas, or abnormal in a perfectly balanced one.

The best academic structure does not decorate the clinical reasoning—it makes that reasoning visible.

Step one: is the pH acidotic or alkalotic

Look only at the pH first. Below 7.35 is acidosis. Above 7.45 is alkalosis. Between the two is normal, which either means a normal gas or a fully compensated one — a distinction step three resolves.

Resist the urge to look at the other values yet. Students who read all four numbers at once tend to pattern-match and misclassify. The method works because it is sequential.

One note on vocabulary: acidosis and alkalosis describe the process; acidaemia and alkalaemia describe the blood. The exam uses the former and does not test the distinction.

Step two: which value moved in the opposite direction to the pH

Now look at carbon dioxide and bicarbonate, and ask which one moved opposite to the pH. That value is the cause.

If the pH is low and carbon dioxide is high, they moved in opposite directions, so the cause is respiratory. If the pH is low and bicarbonate is also low, they moved in the same direction, so the cause is metabolic.

The reason this works is chemistry: carbon dioxide is an acid, so more of it lowers pH — opposite movement. Bicarbonate is a base, so less of it lowers pH — same movement. Remembering that carbon dioxide opposes and bicarbonate agrees is the whole of step two.

Step three: is there compensation

Look at the value that was not the cause. If it is still normal, there is no compensation — the disturbance is recent and the other system has not responded yet. This is described as uncompensated.

If it has moved out of range in the same direction as the cause, compensation is under way. If the pH has returned inside 7.35 to 7.45 while both carbon dioxide and bicarbonate remain abnormal, compensation is complete — fully compensated.

With a fully compensated gas, decide which disturbance came first by asking which side of 7.40 the pH sits on. Below 7.40 the underlying process was an acidosis; above 7.40 it was an alkalosis. The body compensates toward normal but does not overshoot.

Four worked examples

pH 7.28, carbon dioxide 52, bicarbonate 24. pH is low, so acidosis. Carbon dioxide is high and moved opposite, so respiratory. Bicarbonate is normal, so uncompensated. Respiratory acidosis, uncompensated — think hypoventilation from opioid oversedation, chronic obstructive pulmonary disease or atelectasis.

pH 7.50, carbon dioxide 30, bicarbonate 24. pH is high, so alkalosis. Carbon dioxide is low and moved opposite, so respiratory. Bicarbonate normal, so uncompensated. Respiratory alkalosis — think hyperventilation from anxiety, pain, or early sepsis.

pH 7.30, carbon dioxide 38, bicarbonate 17. pH is low, so acidosis. Bicarbonate is low and moved with the pH, so metabolic. Carbon dioxide is normal, so uncompensated. Metabolic acidosis — think diabetic ketoacidosis, diarrhoea or renal failure.

pH 7.37, carbon dioxide 50, bicarbonate 30. pH is normal but below 7.40, so the underlying process was acidosis. Carbon dioxide is high, so respiratory. Bicarbonate is high too, so the kidneys have compensated. Respiratory acidosis, fully compensated — typical of chronic obstructive pulmonary disease.

Matching the gas to a cause

Respiratory acidosis means retained carbon dioxide, so think anything that reduces ventilation: opioids and sedatives, chronic obstructive pulmonary disease, chest wall injury, neuromuscular disease, atelectasis.

Respiratory alkalosis means excess carbon dioxide blown off: anxiety, pain, fever, early sepsis, pulmonary embolism, mechanical over-ventilation.

Metabolic acidosis means acid gained or base lost: diabetic ketoacidosis, lactic acidosis from shock, renal failure, severe diarrhoea. Metabolic alkalosis means acid lost or base gained: prolonged vomiting, nasogastric suction, excessive antacids, potassium-wasting diuretics.

How the NCLEX frames these

The exam rarely asks you simply to classify a gas. It gives you a patient and a gas and asks what to do, or gives you a patient and asks which gas you would expect. Both directions require the same three steps.

A very common framing describes a patient — post-operative on morphine, drowsy, respiratory rate 8 — and asks which acid–base disturbance to anticipate. Work forward: hypoventilation retains carbon dioxide, so respiratory acidosis.

Practise both directions. Students who only ever classify given values are caught out when asked to predict, and prediction is the more clinically meaningful skill.