In the exam scenario with discordant ABG values (pH 7.58, PCO2 55, HCO3- 18), what is the most likely conclusion?

• A. Normal ABG
• B. Measurement error
• C. Hyperventilation
• D. Anion gap

Answer: (B)

The main idea is that arterial blood gas values should fit together through the acid–base relationship described by Henderson-Hasselbalch. A pH of 7.58 shows alkalemia, which would normally be produced by either a metabolic alkalosis (high HCO3−) or a respiratory alkalosis (low PCO2). In this sample, the numbers don’t align: the CO2 is high (55 mmHg), which would push pH toward acidosis, while the bicarbonate is low (18 mEq/L), which would also push toward acidosis. Putting those together, the expected pH would be near acidic, not highly alkaline. That mismatch indicates the values are not physiologically consistent, pointing to a measurement or sampling error rather than a true clinical state.

Common sources of such discordant ABG results include issues with sample handling or analysis, like delays, air contamination, or instrument calibration problems. Those artifacts can produce a pH that doesn’t match the measured CO2 and bicarbonate.

So the most likely conclusion is that this ABG result reflects measurement or sampling error rather than a real combined respiratory and metabolic disturbance. Normal ABG, hyperventilation, or an anion gap on its own don’t explain the inconsistent values observed.