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

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.

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.