Acid-Base Balance and ABG Interpretation Practice Test 2026 - Free ABG Practice Questions and Study Guide

Key idea: use pH to identify alkalemia or acidemia, then look at HCO3 and PaCO2 to find the primary disorder and whether there is respiratory or metabolic compensation.

Here, pH is 7.56, clearly alkalemic. HCO3 is elevated at 30, so the primary process is metabolic alkalosis. In metabolic alkalosis, the expected respiratory response is to retain CO2 and raise PaCO2. The commonly used estimate for expected PaCO2 with a metabolic alkalosis is about 0.7 mmHg increase in PaCO2 for every 1 mEq/L increase in HCO3 above 24, giving around 44 mmHg for this case (40 + 0.7 × (30 − 24) ≈ 44). The observed PaCO2 is 36 mmHg, well below that expectation, so there is little to no respiratory compensation. This fits an uncompensated metabolic alkalosis.

The scenario cannot be a respiratory alkalosis, since HCO3 is high (not low) and the pH is alkaline. It isn’t a compensated metabolic alkalosis, because compensation would raise PaCO2 toward the higher end (around the mid-40s) and the pH would be closer to normal rather than 7.56. It also isn’t metabolic acidosis, which would require a low pH and low HCO3.