ABG Interpretation Made Simple: ROME and the 4-Step Method

An arterial blood gas tells you whether the blood is too acidic or too alkaline, whether the cause is respiratory or metabolic, and whether oxygenation is adequate. Work it in four steps every single time: read the pH, find which value (PaCO2 or HCO3) matches the pH disturbance, decide how far compensation has gone, then check oxygenation. ROME is the shortcut that keeps the directions straight: Respiratory Opposite, Metabolic Equal. Get a clean system and ABGs stop being intimidating.

Normal ABG values: the numbers to memorize first

You cannot interpret a gas you can’t compare against. Lock in these five values before anything else. On the NCLEX, the pH, PaCO2, and HCO3 do all the acid-base work; PaO2 and SaO2 tell you about oxygenation, which is a separate question.

Two anchors make the rest of the method work. CO2 is an acid gas: when it climbs, the blood gets more acidic and pH falls. HCO3 (bicarbonate) is a base: when it climbs, the blood gets more alkaline and pH rises. Hold onto that and the directions never confuse you.

What is ROME, and how does it work?

ROME is the memory hook for which way each value moves relative to the pH.

Put it to work. If the pH is low (acidosis) and the PaCO2 is high, they moved opposite each other, so the problem is respiratory: respiratory acidosis. If the pH is low and the HCO3 is also low, they moved together, so it’s metabolic: metabolic acidosis. ROME tells you the direction logic; the four-step method below tells you exactly how to apply it.

The 4-step ABG method (use this every time)

This is the system. Do the steps in order, write down your finding at each step, and you will land the answer without guessing.

Step 1: Is the pH acidosis or alkalosis?

Look only at the pH. Below 7.35 is acidosis. Above 7.45 is alkalosis. If it sits inside 7.35-7.45, it is normal or fully compensated (you’ll sort that out in step 3). The midpoint is 7.40: a pH of 7.35-7.39 leans acidotic, and 7.41-7.45 leans alkalotic, which matters when the value is “normal” but compensated.

Step 2: Is it respiratory or metabolic?

Now look at PaCO2 and HCO3 and ask which one matches the pH disturbance from step 1.

• If the pH is acidotic, the cause is whichever value points acidic: a high PaCO2 (>45, respiratory acidosis) or a low HCO3 (<22, metabolic acidosis).
• If the pH is alkalotic, the cause is whichever value points alkaline: a low PaCO2 (<35, respiratory alkalosis) or a high HCO3 (>26, metabolic alkalosis).

The value that matches the pH disturbance is the primary problem. Remember CO2 is an acid (high CO2 = acidosis) and HCO3 is a base (high HCO3 = alkalosis).

Step 3: Is there compensation?

The body hates an abnormal pH and tries to pull it back toward 7.40. The lungs compensate fast (minutes) by changing the breathing rate; the kidneys compensate slowly (hours to days) by retaining or dumping bicarbonate. Look at the value that did not cause the problem and see whether it has shifted to help.

The trick with full compensation: when the pH is back in the normal range but both the PaCO2 and HCO3 are abnormal, use which side of 7.40 the pH sits on to name the primary problem. A pH of 7.37 with both values abnormal means the underlying disturbance was an acidosis.

Step 4: How is oxygenation?

This is a separate question from acid-base. Read the PaO2: normal is 80-100 mmHg. Below 80 is hypoxemia (60-79 mild, 40-59 moderate, below 40 severe). The SaO2 should be 95-100%; an SaO2 below 90% generally corresponds to a PaO2 under about 60 mmHg, which is the danger zone where you intervene with oxygen. A patient can have a perfectly normal acid-base picture and still be dangerously hypoxic, so never skip this step.

Acid-base balance and oxygenation lean heavily on the lungs, so it pays to study them alongside our respiratory NCLEX review, where ventilation and gas exchange are covered in depth.

What causes each acid-base disorder?

Naming the disorder is half the job; knowing the likely cause is how you pick the right intervention. Group the causes by whether they are a ventilation problem (respiratory) or a chemistry problem (metabolic).

One clinical pearl that organizes the GI causes: vomiting and NG suction remove acid from the stomach, so they push you alkalotic; diarrhea removes bicarbonate from the gut, so it pushes you acidotic. Same patient with GI losses, opposite acid-base direction depending on which end.

Notice that salicylate (aspirin) toxicity shows up on two lines — that is not a typo. It is classically a mixed acid-base disorder: early on, salicylates stimulate the respiratory center and the patient hyperventilates (respiratory alkalosis), and later the accumulating acid produces a metabolic acidosis. Both processes can be present at once, which is why it belongs under both headings.

Worked examples: one for each disorder

Here is the method in action. Walk every example top-down: pH first, then the matching value, then compensation, then oxygenation.

Respiratory acidosis (uncompensated)

pH 7.28, PaCO2 58, HCO3 24, PaO2 72.

1. pH 7.28 → below 7.35, so acidosis.
2. PaCO2 is 58 (high = acidic) and HCO3 is 24 (normal). The acidic value is the CO2, so this is respiratory. ROME check: pH down, CO2 up — opposite directions, respiratory confirmed.
3. HCO3 is still normal, so there’s no compensation yet: uncompensated.
4. PaO2 72 → mild hypoxemia.

Answer: uncompensated respiratory acidosis. Think COPD exacerbation or opioid-induced hypoventilation — the patient is retaining CO2.

Respiratory alkalosis (uncompensated)

pH 7.51, PaCO2 28, HCO3 24, PaO2 96.

1. pH 7.51 → above 7.45, so alkalosis.
2. PaCO2 is 28 (low = alkaline) and HCO3 is 24 (normal). The CO2 explains it, so this is respiratory. Run the ROME test: pH up while CO2 goes down — opposite directions, so respiratory holds.
3. HCO3 normal → uncompensated.
4. PaO2 96 → normal oxygenation.

Answer: uncompensated respiratory alkalosis. Classic anxiety-driven hyperventilation; the patient is blowing off CO2. Coaching slow breathing is the first-line nursing intervention.

Metabolic acidosis (uncompensated)

pH 7.30, PaCO2 40, HCO3 16, PaO2 90.

1. pH 7.30 → acidosis.
2. PaCO2 is 40 (normal), HCO3 is 16 (low = acidic). The bicarbonate explains it, so this is metabolic. Apply ROME here: pH and HCO3 both fall together — same direction, which confirms metabolic.
3. PaCO2 is still normal → uncompensated.
4. PaO2 90 → normal.

Answer: uncompensated metabolic acidosis. The usual culprits are DKA, lactic acidosis, or renal failure. If the lungs were compensating, you would expect the PaCO2 to drop (Kussmaul respirations blowing off CO2).

Metabolic alkalosis (uncompensated)

pH 7.50, PaCO2 42, HCO3 32, PaO2 94.

1. pH 7.50 → alkalosis.
2. PaCO2 is 42 (normal), HCO3 is 32 (high = alkaline). Bicarbonate explains it → metabolic. ROME once more: pH and HCO3 both rise — same direction, so metabolic.
3. PaCO2 normal → uncompensated.
4. PaO2 94 → normal.

Answer: uncompensated metabolic alkalosis. Picture prolonged vomiting or NG suction stripping gastric acid.

The compensated case (this is where students lose points)

pH 7.36, PaCO2 60, HCO3 34, PaO2 70.

1. pH 7.36 → technically inside 7.35-7.45, but it sits on the acidic side of 7.40. Flag it as “normal but leaning acidotic.”
2. Both values are abnormal: PaCO2 60 (high = acidic) and HCO3 34 (high = alkaline). Match to the pH: the pH leans acidic, and the acidic value is the high CO2, so the primary problem is respiratory acidosis. The high HCO3 is the kidneys compensating by retaining base.
3. The pH made it back into the normal range while both values are abnormal → fully compensated.
4. PaO2 70 → mild hypoxemia.

Answer: fully compensated respiratory acidosis. This is the classic chronic COPD picture: the patient lives with a high CO2, and over about three to five days the kidneys retain bicarbonate to drag the pH back into range — a compensated state the patient then lives in chronically. The pH-near-7.40 tiebreaker is the whole game here — whichever side of 7.40 the pH sits on names the primary disorder.

How do you decide which value is primary in a fully compensated gas?

When the pH is normal but both PaCO2 and HCO3 are off, students freeze. The rule is simple: the pH always sides with the primary problem. Split the normal range at 7.40.

A pH of exactly 7.40 with both values abnormal is rare on exams; if you see it, look for the value that is more dramatically out of range, but you’ll almost always be handed a pH that tips one way.

Linking electrolytes to acid-base

Potassium and acid-base move together more often than students expect. In acidosis, hydrogen ions shift into cells and potassium shifts out, so acidosis tends to raise serum potassium (hyperkalemia) — a major reason DKA patients can present with a high potassium even when total body potassium is depleted. Alkalosis does the reverse and tends to lower potassium. If you want the full picture on how these shifts play out clinically, pair this with our fluid and electrolytes review.

Sample NCLEX questions

Frequently Asked Questions

How do I know if a problem is respiratory or metabolic?

After you decide acidosis or alkalosis from the pH, look at PaCO2 and HCO3 and find which one points the same way as the pH. If the abnormal CO2 explains the pH, it’s respiratory; if the abnormal bicarbonate explains it, it’s metabolic. ROME keeps the directions straight: in respiratory problems pH and CO2 move opposite, and in metabolic problems pH and HCO3 move together.

What’s the difference between partially and fully compensated?

Compensation is the body’s attempt to normalize the pH using the other system (lungs for metabolic problems, kidneys for respiratory ones). If the pH is still abnormal but the second value has started to shift, it’s partially compensated. If the pH has made it back into the 7.35-7.45 range while both PaCO2 and HCO3 remain abnormal, it’s fully compensated.

Why is CO2 an acid and bicarbonate a base?

Carbon dioxide combines with water in the blood to form carbonic acid, so a rising CO2 makes the blood more acidic and a falling CO2 makes it more alkaline. Bicarbonate (HCO3) is the body’s main buffer base; more bicarbonate means a more alkaline blood, less means more acidic. That single relationship drives the entire interpretation.

Does a normal PaO2 mean the acid-base balance is normal too?

No. Oxygenation and acid-base are separate questions. PaO2 and SaO2 tell you whether the tissues are getting enough oxygen; pH, PaCO2, and HCO3 tell you the acid-base status. A client can be perfectly balanced on pH yet severely hypoxemic, which is why oxygenation is its own step at the end of the method.

What PaO2 should make me act?

Normal PaO2 is 80-100 mmHg. A PaO2 below 60 mmHg (which roughly corresponds to an SaO2 under 90%) is significant hypoxemia and generally warrants supplemental oxygen and reassessment. The lower the number, the more urgent: 40-59 is moderate and below 40 is severe hypoxemia.

Want to drill ABGs until the four steps are automatic? Practice free NCLEX questions with full rationales for every answer, right and wrong, no credit card required.