When a pH value is not given, which measurements are used to determine acidosis or alkalosis?

• A. pCO2 and HCO3
• B. pCO2 and pH
• C. HCO3 and pH
• D. pCO2 alone

Answer: (A)

The measurements pCO2 and HCO3- are what you use to sort acid-base disturbances when pH isn’t provided. The pH value is determined by the Henderson-Hasselbalch relationship, pH = 6.1 + log10(HCO3- / (0.03 × pCO2)). So, knowing both the respiratory component (pCO2) and the metabolic component (HCO3-) lets you infer the direction of the imbalance and whether the body has attempted to compensate, even if the exact pH isn’t given. This is why those two values are sufficient to determine whether the disorder is of respiratory or metabolic origin and whether there is compensation. In contrast, pCO2 alone cannot tell you if a metabolic issue is present, and pH or a combination including pH is needed to label acidemia versus alkalemia directly.