ABG showing pH 7.58, PCO2 55, HCO3- 18 indicates:

• A. Metabolic alkalosis, partially compensated
• B. Respiratory acidosis, uncompensated
• C. A dual problem of acidosis
• D. An error in measurement

Answer: (D)

Discrepancies among pH, PCO2, and HCO3− in an ABG signal an inconsistency that usually points to a measurement error. The pH of 7.58 shows alkalemia, which would be expected with a respiratory alkalosis or a metabolic alkalosis. But the PCO2 is elevated at 55, which tends to lower pH (causing acidosis), and the bicarbonate is low at 18, which also tends to lower pH (metabolic acidosis). This combination cannot be produced by a single physiologic disorder; the numbers are not chemically compatible. In practice, such discordant results suggest an erroneous measurement or sample issue, and the test should be repeated or rechecked (calibration, sample handling, or arterial vs venous mix).