Type the three numbers and watch each step of the interpretation happen, with the comparison that decides it shown every time.
The call
Waiting for all three values
Step 1: read the pH. Below 7.35 is acidemia. Above 7.45 is alkalemia. Inside the range, the pH itself is normal, which does not yet mean the gas is normal.
Step 2: read both systems separately. PaCO2 is the lung number and it is an acid, so a PaCO2 above 45 means respiratory acid is piling up and below 35 means too much acid has been blown off. HCO3 is the kidney number and it is a base, so an HCO3 below 22 means base has been lost and above 26 means base is in excess. Label each one on its own before you try to name anything.
Step 3: find the system that matches the pH. The primary disorder is whichever system is abnormal in the direction that would produce the pH you saw in step 1. Acidemia plus a high PaCO2 is respiratory acidosis. Acidemia plus a low HCO3 is metabolic acidosis. Alkalemia plus a low PaCO2 is respiratory alkalosis. Alkalemia plus a high HCO3 is metabolic alkalosis.
Step 4: check compensation. If the other system is still inside its normal range, the body has not started to help yet, and the gas is uncompensated. If the other system has moved out of range in the opposite direction but the pH is still abnormal, the body is working on it and the gas is partially compensated. If the pH has come back inside 7.35 to 7.45 while both PaCO2 and HCO3 sit out of range, the body has finished the job and the gas is fully compensated.
Once a gas is fully compensated the pH is back in range, so step 1 can no longer tell you which side the trouble started on. The usual teaching fix is to split the normal range at its midpoint: a pH of 7.35 to 7.39 is treated as the acidotic half and 7.41 to 7.45 as the alkalotic half, and the system sitting on that side is called the primary disorder. That is a teaching convention for reading a single gas on paper. It is a reasonable rule of thumb, it is not a lab value, and a pH of exactly 7.40 gives it nothing to work with, which is why this tool says so instead of guessing.
ROME (Respiratory Opposite, Metabolic Equal) is a real and useful memory aid. It tells you that in a respiratory problem the PaCO2 moves opposite the pH, and in a metabolic problem the HCO3 moves with the pH. That is exactly step 3 and nothing more.
The trouble starts at step 4. ROME says nothing about compensation, nothing about a gas where both values are abnormal, and nothing about a fully compensated gas where the pH is normal and there is no direction left to pair with. That is the point where students most often report losing the thread, and it is also the point where swapping to a second mnemonic like the tic-tac-toe grid tends to make things worse, because the two methods label the same numbers differently. The comparison steps above cover the whole gas, compensation included, so there is nothing to switch to halfway through.
Sometimes both systems are abnormal in the same direction at once, for example a low pH with a high PaCO2 and a low HCO3. Neither one is compensating for the other; both are pushing the pH the same way. That is a mixed disorder, it needs the patient's history and often an anion gap to sort out, and a three-number walkthrough cannot do it. When the numbers you type look like that, this tool names the pattern and stops there rather than inventing a single answer.
| Value | What it is | Normal range |
|---|---|---|
| pH | Acidity or alkalinity of arterial blood | 7.35 to 7.45 |
| PaCO2 | Carbon dioxide, an acid, managed by the lungs | 35 to 45 mmHg |
| HCO3 | Bicarbonate, a base, managed by the kidneys | 22 to 26 mEq/L |
Source: Open RN (Open Resources for Nursing), Nursing Fundamentals 2e, chapter 15.5 Acid-Base Balance, WisTech Open. Licensed CC BY 4.0. wtcs.pressbooks.pub/nursingfundamentals/chapter/15-5-acid-base-balance. Retrieved 2026-08-14. Reference ranges differ slightly between labs and facilities; use the ones your facility prints on the report.
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