ABG interpretation: a complete guide for nursing students

Arterial blood gas (ABG) interpretation is one of the most important clinical skills a nurse can develop. An ABG result tells you whether your patient’s oxygenation, ventilation, and acid-base balance are within safe limits — and when they are not, it tells you exactly which system is failing. Whether you are managing a patient on a ventilator, treating DKA in the emergency department, or responding to acute respiratory distress, ABG interpretation drives your next nursing action.

This guide walks through ABG interpretation using a systematic 5-step method with the ROME mnemonic, covers all four primary acid-base disorders with their causes and nursing priorities, and includes compensation formulas you will need for clinical practice and the NCLEX.

What is an arterial blood gas?

An ABG is a blood test drawn from an artery — most commonly the radial artery at the wrist. Unlike a venous blood draw, arterial sampling measures how well the lungs are delivering oxygen and removing carbon dioxide. The result provides five key values: pH, PaCO2 (partial pressure of carbon dioxide), PaO2 (partial pressure of oxygen), HCO3 (bicarbonate), and SaO2 (arterial oxygen saturation).

Nurses order or assist with ABGs in situations that include acute respiratory failure, altered mental status with suspected acid-base disturbance, monitoring patients on mechanical ventilation, evaluating response to oxygen therapy in COPD, and assessing metabolic emergencies like DKA. You can also cross-reference ABG results with your nursing lab values cheat sheet for a broader picture of the patient’s metabolic status.

Nursing consideration for specimen collection: ABG samples must be placed on ice and transported to the lab immediately. Air bubbles in the syringe falsely elevate PaO2 and lower PaCO2. After collection, hold firm pressure on the arterial site for at least 5 minutes (longer if the patient is on anticoagulants) and document the patient’s FiO2 at the time of draw.

The 5-step method for ABG interpretation

A systematic approach prevents errors. Follow these five steps every time you interpret an ABG, and you will correctly identify the acid-base disorder in almost every clinical scenario.

Step 1: Evaluate the pH

The pH tells you whether the blood is acidotic or alkalotic — and that single value establishes the direction of the entire interpretation.

• pH < 7.35 → acidosis (the blood is too acidic)
• pH 7.35–7.45 → normal range
• pH > 7.45 → alkalosis (the blood is too alkaline)

A pH below 6.8 or above 7.8 is generally incompatible with life. When a patient’s pH is within the normal range but other values are abnormal, compensation is occurring — the body is correcting one system’s problem through another.

Step 2: Evaluate the PaCO2 (respiratory component)

PaCO2 reflects how well the lungs are ventilating. Carbon dioxide is an acid, so changes in CO2 directly affect pH.

• PaCO2 > 45 mmHg → respiratory acidosis (CO2 retention — the lungs are underventilating)
• PaCO2 35–45 mmHg → normal
• PaCO2 < 35 mmHg → respiratory alkalosis (CO2 blown off — the patient is hyperventilating)

Step 3: Evaluate the HCO3 (metabolic component)

Bicarbonate (HCO3) is a base regulated by the kidneys. Changes in HCO3 reflect the metabolic side of the acid-base equation.

• HCO3 < 22 mEq/L → metabolic acidosis (bicarbonate is consumed or lost)
• HCO3 22–26 mEq/L → normal
• HCO3 > 26 mEq/L → metabolic alkalosis (excess bicarbonate)

Step 4: Match the primary disorder using the ROME mnemonic

ROME stands for Respiratory Opposite, Metabolic Equal. This mnemonic tells you which abnormal value matches the pH change and is therefore the primary cause.

Respiratory Opposite: In respiratory disorders, the pH and PaCO2 move in opposite directions.

• pH is down (acidosis) + PaCO2 is up → respiratory acidosis
• pH is up (alkalosis) + PaCO2 is down → respiratory alkalosis

Metabolic Equal: In metabolic disorders, the pH and HCO3 move in the same direction.

• pH is down (acidosis) + HCO3 is down → metabolic acidosis
• pH is up (alkalosis) + HCO3 is up → metabolic alkalosis

If both PaCO2 and HCO3 are abnormal, look at which one matches the pH direction using ROME. That component is the primary disorder; the other is compensating.

Step 5: Assess for compensation

Once you have identified the primary disorder, check whether the other system is compensating. The body never overcorrects — compensation moves the pH toward normal but does not push it past 7.40 to the other side.

Example interpretation: A patient presents with pH 7.30, PaCO2 55 mmHg, HCO3 27 mEq/L.

1. pH 7.30 → acidosis
2. PaCO2 55 → elevated (respiratory acidosis direction)
3. HCO3 27 → slightly elevated (metabolic alkalosis direction — compensating)
4. ROME: pH down, PaCO2 up → opposite → respiratory is primary
5. HCO3 is elevated, meaning the kidneys are retaining bicarbonate to compensate. pH is still abnormal → partially compensated respiratory acidosis.

The four primary acid-base disorders

Respiratory acidosis

Respiratory acidosis occurs when the lungs fail to eliminate enough CO2, causing PaCO2 to rise above 45 mmHg and pH to fall below 7.35.

Common causes: COPD exacerbation, opioid or sedative overdose, neuromuscular diseases (myasthenia gravis, Guillain-Barr syndrome), severe asthma, airway obstruction, chest wall trauma, inadequate mechanical ventilation settings.

Clinical signs: Dyspnea, confusion, drowsiness progressing to somnolence, headache, tachycardia. In severe or chronic cases, asterixis (hand-flapping tremor) may be present.

Key nursing priorities: Position the patient upright to optimize ventilation. Administer oxygen cautiously in chronic CO2 retainers (target SpO2 88–92% per COPD guidelines). Prepare for possible noninvasive positive-pressure ventilation (BiPAP) or intubation. If opioid-induced, administer naloxone per order. Monitor level of consciousness closely using a tool like the Glasgow Coma Scale.

Respiratory alkalosis

Respiratory alkalosis occurs when the patient hyperventilates and blows off too much CO2, dropping PaCO2 below 35 mmHg and raising pH above 7.45.

Common causes: Anxiety and panic attacks, pain, fever, early sepsis, hypoxemia (the body increases respiratory rate to compensate for low oxygen), high altitude, salicylate (aspirin) overdose (early phase), pregnancy (progesterone stimulates ventilation), excessive mechanical ventilation.

Clinical signs: Lightheadedness, perioral and fingertip tingling, paresthesias, muscle cramps or tetany (alkalosis causes calcium to bind more tightly to albumin, reducing ionized calcium), carpopedal spasm.

Key nursing priorities: Treat the underlying cause. For anxiety-driven hyperventilation, coach slow breathing. For fever, administer antipyretics. For pain, address pain management. For mechanical ventilation, decrease rate or tidal volume as ordered. Monitor electrolytes — alkalosis shifts potassium intracellularly and can cause hypokalemia.

Metabolic acidosis

Metabolic acidosis occurs when acid accumulates in the blood or bicarbonate is lost, dropping HCO3 below 22 mEq/L and pH below 7.35. The body compensates through Kussmaul respirations — deep, rapid breathing that blows off CO2 to raise pH.

Common causes (grouped by anion gap):

High anion gap (MUDPILES): Methanol ingestion, Uremia (kidney failure), DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis (sepsis, shock, tissue hypoxia), Ethylene glycol, Salicylate overdose.

Normal anion gap (HARDUPS): Hyperalimentation, Acetazolamide/Addison disease, Renal tubular acidosis, Diarrhea (loss of bicarbonate through GI tract), Uretero-pelvic diversion, Pancreatic fistula, Saline infusion (dilutional).

The anion gap helps narrow the differential. Calculate it as: AG = Na+ − (Cl− + HCO3−). Normal anion gap is 8–12 mEq/L (or 10–14 mEq/L if potassium is not included).

Key nursing priorities: Identify and treat the cause. In DKA, the priority is insulin and fluid resuscitation (see the DKA reference). Monitor respiratory rate and depth — Kussmaul breathing is a compensatory mechanism, so do not suppress it with sedation unless the patient is being intubated. Monitor potassium closely — acidosis shifts potassium out of cells, so serum potassium may appear normal or elevated even when total body potassium is depleted. As acidosis corrects, potassium will drop.

Metabolic alkalosis

Metabolic alkalosis occurs when bicarbonate rises above 26 mEq/L and pH climbs above 7.45. The body compensates by slowing the respiratory rate to retain CO2.

Common causes: Prolonged vomiting or nasogastric suctioning (loss of hydrochloric acid), excessive diuretic use (loop and thiazide diuretics cause volume contraction and increased bicarbonate reabsorption), excessive antacid or sodium bicarbonate administration, hypokalemia (the kidneys excrete hydrogen ions instead of potassium, retaining bicarbonate), hypochloremia, Cushing syndrome.

Clinical signs: Often subtle. May include confusion, muscle twitching, hand tremor, nausea. Severe alkalosis can cause seizures and cardiac arrhythmias. Like respiratory alkalosis, metabolic alkalosis reduces ionized calcium, so Chvostek and Trousseau signs may be positive.

Key nursing priorities: Replace volume and chloride (normal saline is the first-line treatment for most cases). Replace potassium — hypokalemia both causes and sustains metabolic alkalosis. Discontinue or reduce the offending agent (diuretics, NG suction, antacids). Monitor cardiac rhythm — alkalosis predisposes to arrhythmias, especially when combined with hypokalemia.

Expected compensation formulas

When one system is in primary failure, the other system compensates predictably. These formulas help you determine whether compensation is appropriate or whether a second acid-base disorder is also present. If measured values fall outside the expected range, suspect a mixed disorder.

Why this matters: If your patient has a metabolic acidosis with HCO3 of 14 mEq/L, Winter’s formula predicts PaCO2 should be approximately (1.5 × 14) + 8 = 29 mmHg (range 27–31). If the measured PaCO2 is 40, the patient is not compensating adequately and likely has a concurrent respiratory acidosis — a mixed disorder requiring urgent attention.

Clinical applications

COPD and respiratory acidosis

Patients with chronic COPD often have a baseline respiratory acidosis with full renal compensation — their PaCO2 runs high (50–60 mmHg), but their HCO3 is proportionally elevated and pH sits near normal. During an acute exacerbation, PaCO2 climbs further and pH drops because the kidneys cannot compensate fast enough. Always compare the current ABG to the patient’s baseline when one is available.

DKA and metabolic acidosis

DKA produces a high anion gap metabolic acidosis. The hallmark ABG shows pH below 7.30, HCO3 often in single digits, and a low PaCO2 from vigorous Kussmaul respirations. As insulin therapy corrects the ketoacidosis, watch for a transition to normal anion gap (hyperchloremic) acidosis from aggressive normal saline resuscitation — the pH may stall even though ketones are clearing.

Sepsis

Early sepsis often presents with respiratory alkalosis (low PaCO2) as the patient hyperventilates in response to systemic inflammation. As sepsis progresses to septic shock with tissue hypoperfusion, lactic acid accumulates and the picture shifts to metabolic acidosis. A rising lactate with worsening metabolic acidosis signals clinical deterioration.

Mechanical ventilation

ABGs are the gold standard for adjusting ventilator settings. If PaCO2 is too high, increasing the respiratory rate or tidal volume will blow off more CO2. If PaCO2 is too low, the patient is being overventilated. PaO2 and SaO2 guide FiO2 and PEEP adjustments.

NCLEX tips and common pitfalls

The pH tells you the primary problem. If the pH is on the acidotic side (below 7.40), the primary disorder is whichever value creates acidosis, even when both systems are abnormal. Do not get distracted by the compensating value.

Compensation never overcorrects. If the pH is 7.47 and the patient has a chronically elevated PaCO2, the primary disorder is metabolic alkalosis with chronic respiratory acidosis — the respiratory system did not cause the alkalosis, the metabolic system did. The body’s compensatory mechanisms do not push the pH past 7.40 to the other side.

Watch for mixed disorders. When both PaCO2 and HCO3 are abnormal and moving in the same direction (both acid or both alkaline), you likely have a mixed disorder rather than compensation. Compensation always opposes the primary disorder.

The anion gap catches what HCO3 alone misses. A patient can have a normal HCO3 and still have a hidden metabolic acidosis if a concurrent metabolic alkalosis is masking it. Calculating the anion gap reveals the organic acid accumulation that HCO3 alone does not show. This is the concept behind the delta-delta calculation: compare the change in anion gap to the change in HCO3. If the anion gap has risen more than the HCO3 has fallen, a concurrent metabolic alkalosis is present.

PaO2 is separate from acid-base status. A patient can have perfect acid-base balance and still be severely hypoxemic. Always assess PaO2 and SaO2 independently — they tell you about oxygenation, while pH, PaCO2, and HCO3 tell you about ventilation and acid-base balance.

Related references

• COPD pathophysiology: a complete nursing reference guide
• DKA in nursing: pathophysiology, assessment, and management
• Nursing lab values cheat sheet
• Electrolyte imbalances in nursing
• EKG interpretation cheat sheet
• Glasgow Coma Scale