Which parameter is not directly measured by most automated blood gas analyzers and is typically calculated instead?

Which parameter is not directly measured by most automated blood gas analyzers and is typically calculated instead?

• A. pH
• B. PCO2
• C. PO2
• D. HCO3

Answer: (D)

The main concept is that bicarbonate is not directly measured by most automated blood gas analyzers; it’s calculated from the measured pH and PCO2 using the carbonic acid–bicarbonate buffering relationship. The device directly senses hydrogen ion activity (pH) and CO2 tension (PCO2), and from these values, applies Henderson-Hasselbalch: pH = pKa + log([HCO3-] / (0.03 × PCO2)). At body temperature, pKa for carbonic acid is about 6.1, so rearranging gives [HCO3-] = 0.03 × PCO2 × 10^(pH − 6.1). Thus HCO3- (in mEq/L) is derived, not measured, leading to its presentation as a calculated parameter. For example, pH 7.40 and PCO2 40 mmHg would yield a bicarbonate around 24 mEq/L. This is why bicarbonate is considered a calculated value rather than a directly measured one.

The main concept is that bicarbonate is not directly measured by most automated blood gas analyzers; it’s calculated from the measured pH and PCO2 using the carbonic acid–bicarbonate buffering relationship. The device directly senses hydrogen ion activity (pH) and CO2 tension (PCO2), and from these values, applies Henderson-Hasselbalch: pH = pKa + log([HCO3-] / (0.03 × PCO2)). At body temperature, pKa for carbonic acid is about 6.1, so rearranging gives [HCO3-] = 0.03 × PCO2 × 10^(pH − 6.1). Thus HCO3- (in mEq/L) is derived, not measured, leading to its presentation as a calculated parameter. For example, pH 7.40 and PCO2 40 mmHg would yield a bicarbonate around 24 mEq/L. This is why bicarbonate is considered a calculated value rather than a directly measured one.