Nursing care
ROME method for ABGs: reading acid-base results, plus the tic-tac-toe check
Written and reviewed by Nicole Ramirez, BSN, RN, CMSRN · 4 min read · Updated November 2026
Short answer
ROME stands for Respiratory Opposite, Metabolic Equal. In a respiratory disorder, pH and PaCO2 move in opposite directions: pH falls as carbon dioxide rises. In a metabolic disorder, pH and bicarbonate move in the same direction. ROME tells you which system caused an abnormal pH, but not whether two disorders coexist.
What ROME stands for and why it works
Respiratory Opposite: carbon dioxide behaves as an acid, so when PaCO2 climbs the pH drops, and when PaCO2 falls the pH rises. An arrow up on one value and down on the other points to the lungs. Metabolic Equal: bicarbonate is a base, so when it falls the pH falls with it, and when it rises the pH rises too.
Use the reference ranges the question gives you. MSD Manuals defines acidemia as a pH below 7.35 and alkalemia as a pH above 7.45. MedlinePlus lists bicarbonate of 22 to 28 and notes that normal ranges vary slightly between laboratories, which is why a borderline value in a stem should be read against the stem's own numbers.
How to use the tic-tac-toe method for ABGs
Tic-tac-toe turns ROME into a grid. Draw three columns labeled acid, normal and base. Place pH: below the normal range goes under acid, above it under base. Place PaCO2: high goes under acid because carbon dioxide is an acid, low goes under base. Place bicarbonate: low goes under acid, high goes under base.
Then find the value that shares a column with the pH. That value is the cause: PaCO2 sharing the acid column with pH means respiratory acidosis, while bicarbonate sharing it means metabolic acidosis. Now look at the third value. In the normal column, the disorder is uncompensated. In the opposite column, the body is compensating.
How to tell if an ABG is compensated
Look at the pH and the non-causal value together. Uncompensated: pH abnormal and the other system normal. Partially compensated: pH still abnormal, but the other value has moved in the opposite direction. Fully compensated: pH back within the normal range, with PaCO2 and bicarbonate both abnormal in opposite directions.
In a fully compensated result, decide the cause by which side of 7.40 the pH sits on. A pH of 7.36 leans acid, so whichever value is acidic is the primary problem. MSD Manuals notes that compensation cannot return pH completely to normal and never overshoots, which is why the side of 7.40 still points back to where the problem started.
Where ROME misleads or is incomplete
ROME assumes one disorder at a time. When PaCO2 and bicarbonate both sit in the acid column with a low pH, nothing is compensating: the client has a mixed respiratory and metabolic acidosis. ROME will name one of them and quietly miss the other, and the same happens with a mixed alkalosis.
It also cannot judge whether compensation is the right size. MSD Manuals uses expected-compensation formulas, such as Winter's formula for metabolic acidosis, and a gap between expected and actual values reveals a second process. ROME ignores oxygenation too: a correctly labeled ABG with a low PaO2 still needs the nurse to act on hypoxemia first.
A worked exam scenario with ROME
Hypothetical scenario, not a real exam item: a client with COPD received opioids after surgery and is now drowsy and hard to rouse. The ABG shows pH 7.29, PaCO2 58 mm Hg and bicarbonate 32 mEq/L. Which interpretation fits: metabolic acidosis, uncompensated respiratory acidosis, or partially compensated respiratory acidosis?
The pH is low, so acid. PaCO2 is high and moves opposite to pH, so ROME says respiratory. Bicarbonate is high, in the base column, so the kidneys are compensating but the pH is still abnormal: partially compensated respiratory acidosis. Uncompensated is the tempting choice because the pH is clearly acidic, but it ignores the raised bicarbonate. The nursing priority is the hypoventilation: stimulate the client, support breathing and report at once.
Sources and further reading
MSD Manual Professional Version: Acid-Base Disorders. Acidemia below pH 7.35 and alkalemia above 7.45, stepwise interpretation, expected compensation formulas including Winter's formula, and compensation not fully normalizing pH or overshooting.
MedlinePlus: Blood gases. Normal arterial blood gas values including bicarbonate 22 to 28 and the note that ranges vary slightly between labs.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this page covers.
Common questions
What does ROME stand for in ABGs?
Respiratory Opposite, Metabolic Equal. In respiratory disorders pH and PaCO2 move in opposite directions; in metabolic disorders pH and bicarbonate move in the same direction. It identifies the primary system behind an abnormal pH.
How do you use the tic-tac-toe method for ABGs?
Draw columns for acid, normal and base. Place pH, PaCO2 and bicarbonate in them, remembering that high carbon dioxide counts as acid and high bicarbonate counts as base. The value sharing the pH column is the cause, and the third value shows compensation.
How do you tell if an ABG is compensated?
If the non-causal value is normal, the result is uncompensated. If it has moved opposite to the cause while pH is still abnormal, it is partially compensated. If pH is back within range with both values abnormal, it is fully compensated.
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