ABG Interpretation Steps: A Simple Guide for Nurses

The basic ABG interpretation steps are: check the pH, find the primary problem by comparing PaCO₂ and HCO₃⁻, decide whether the body is compensating, and then look at oxygenation. Work through the same order every time and most arterial blood gas questions, on the ward or in the NCLEX and Philippine board exams, become quick to solve.

What is an arterial blood gas (ABG)?

An ABG is a blood test taken from an artery, usually the radial artery at the wrist, though the brachial and femoral arteries can also be used. It shows how well the lungs move oxygen in and carbon dioxide out, and whether the blood is too acidic or too alkaline.

Abnormal results can come from lung disease, kidney disease, metabolic problems such as diabetic ketoacidosis, medicines, or injuries that affect breathing. Venous blood gases are sometimes used instead, but venous values differ: venous PO₂ is much lower and venous pH slightly lower than arterial.

ABG normal values

Value	Typical adult range	What it tells you
pH	7.35–7.45	Acid–base status: below 7.35 = acidemia, above 7.45 = alkalemia
PaCO2	35–45 mm Hg	Respiratory component (ventilation)
HCO3−	22–26 mEq/L	Metabolic component (kidneys, buffers)
PaO2	75–100 mm Hg	Oxygenation
SaO2	95–100%	Oxygen saturation of haemoglobin
Base excess	−4 to +2	Metabolic component (extra check)

Oxygen values are lower in people living at higher altitude, so interpret PaO₂ with the patient’s setting and oxygen therapy in mind.

ABG interpretation steps

Step 1: Look at the pH

Below 7.35 is acidemia; above 7.45 is alkalemia. If the pH is within the normal range, use 7.40 as the midpoint: 7.36–7.39 leans acidic and 7.41–7.44 leans alkaline. This tells you which way the primary problem points.

Step 2: Find the primary disorder

Check which value matches the pH. If PaCO₂ is high with a low pH, or low with a high pH, the cause is respiratory. If HCO₃⁻ moves in the same direction as the pH (low with acidosis, high with alkalosis), the cause is metabolic.

Step 3: Check for compensation

Now look at the value that did not cause the problem. If it is normal, the disorder is uncompensated. If it has moved in the opposite direction to the primary problem and the pH is still abnormal, it is partially compensated. If both values are abnormal but the pH is back in the normal range, it is fully compensated.

Step 4: Assess oxygenation

Look at PaO₂ and SaO₂ for hypoxemia, taking into account how much oxygen the patient is receiving.

Step 5: For metabolic acidosis, go further

Calculate the anion gap: Na⁺ − (Cl⁻ + HCO₃⁻). A recent review gives 8–16 as normal, with higher values pointing to added acids such as ketones or lactate; reference values differ between laboratories. To check whether breathing is compensating as expected, use Winter’s formula: expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2. A measured PaCO₂ outside that range suggests a second, mixed disorder.

The four acid–base disorders at a glance

Disorder	pH	Primary change	Compensation	Common causes
Respiratory acidosis	↓	PaCO2 ↑	HCO3− ↑	Hypoventilation (slow or shallow breathing), respiratory failure
Respiratory alkalosis	↑	PaCO2 ↓	HCO3− ↓	Hyperventilation, for example from anxiety
Metabolic acidosis	↓	HCO3− ↓	PaCO2 ↓	DKA, septic shock, kidney failure, toxins, bicarbonate loss from the gut
Metabolic alkalosis	↑	HCO3− ↑	PaCO2 ↑	Prolonged vomiting, diuretics, hypokalemia, hypovolemia

Metabolic (kidney) compensation takes days to become effective, so a respiratory problem with strong metabolic compensation is usually not brand new. Electrolytes matter here too: low potassium and metabolic alkalosis often appear together, so review our electrolytes guide for nurses alongside this one.

Practice examples

1. pH 7.30, PaCO₂ 55, HCO₃⁻ 25. Acidemia; PaCO₂ is high and matches; HCO₃⁻ is normal. Answer: uncompensated respiratory acidosis.
2. pH 7.32, PaCO₂ 30, HCO₃⁻ 15. Acidemia; HCO₃⁻ is low and matches; PaCO₂ is low (compensating) but pH is still abnormal. Answer: partially compensated metabolic acidosis. Winter’s formula: 1.5 × 15 + 8 = 30.5 (range 28.5–32.5), so compensation is appropriate.
3. pH 7.43, PaCO₂ 50, HCO₃⁻ 32. pH normal but above 7.40; HCO₃⁻ is high and matches; PaCO₂ is high (compensating). Answer: fully compensated metabolic alkalosis.

Nursing assessment

• Respiratory rate, depth and effort; level of consciousness; skin colour; SpO₂.
• Current oxygen device and flow or FiO₂. MedlinePlus advises that oxygen settings stay constant for 20 minutes before an ABG is drawn.
• Anticoagulant or antiplatelet use, including aspirin, which raises bleeding risk at the puncture site.
• Circulation to the hand before a radial puncture (see below).
• Related labs: electrolytes, glucose, lactate and ketones when metabolic acidosis is suspected.

Nursing considerations when collecting an ABG

• Modified Allen test: occlude both the radial and ulnar arteries while the patient clenches the fist, then release the fist and the ulnar pressure. Colour returning within about 10–15 seconds suggests adequate collateral flow; if it does not, the radial site should not be used.
• Use a heparinised blood gas syringe, fill it fully, and remove air bubbles at once. Air in the sample can falsely lower PCO₂ and raise pH and PO₂.
• Mix gently, label with the oxygen therapy details, and send for analysis promptly, on ice if your facility requires it.
• Apply firm pressure to the site after the needle is removed and watch for hematoma, bleeding and changes in circulation to the hand. Follow your facility’s policy on duration.
• Report critical values and changes in the patient’s condition promptly.

Exam tips

• ROME: Respiratory = Opposite (pH and PaCO₂ move in opposite directions); Metabolic = Equal (pH and HCO₃⁻ move the same way).
• Trap: a normal pH with abnormal PaCO₂ and HCO₃⁻ is full compensation, not a normal result. Use 7.40 to decide which side it leans.
• Classic pairs: prolonged vomiting or diuretics → metabolic alkalosis; DKA or bicarbonate loss from the gut → metabolic acidosis; hypoventilation → respiratory acidosis; anxiety with hyperventilation → respiratory alkalosis.
• Many students find the “tic-tac-toe” method helpful: label each value acid, normal or base, and see which two line up.

Frequently asked questions

What is the fastest way to interpret an ABG?

Use the same three questions every time. Is the pH acidic or alkaline? Which value, PaCO₂ or HCO₃⁻, moves with that pH? Is the other value normal, moving to compensate, or bringing the pH back to normal? Then check PaO₂. With practice this takes under a minute.

Why is HCO3 metabolic and PaCO2 respiratory?

Carbon dioxide is removed by breathing, so PaCO₂ rises with hypoventilation and falls with hyperventilation. Bicarbonate is a base regulated mainly by the kidneys and lost or gained through metabolic processes such as diarrhoea, vomiting or kidney disease. That is why each value points to a different body system.

Is a normal pH always a normal ABG?

No. When compensation is complete, the pH can sit inside 7.35–7.45 even though PaCO₂ and HCO₃⁻ are both clearly abnormal. A mixed disorder can also cancel out at the pH level. Always look at all the values together with the patient’s condition and history.

Related on Nurses Notes

• Electrolytes made simple: sodium, potassium, calcium and magnesium for nurses
• Signs of dehydration in adults and what to do
• All nursing study guides

Sources

• Castro D, Zubair M. Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility. StatPearls, NCBI Bookshelf, updated 2026.
• MedlinePlus Medical Encyclopedia. Blood gases. U.S. National Library of Medicine, reviewed 2024.
• Núñez Sanagustín M, Osredkar J. Blood gas analysis: clinical applications, interpretation and future directions. Medicine International, 2025.

This article is for education only and does not replace advice from your own doctor, nurse or other qualified health professional. Always follow your facility’s policies and reference ranges.