Winter's formula and how is it used?

• A. PaCO2 = 0.5 × HCO3− + 12
• B. In metabolic alkalosis, PaCO2 = 0.9 × [HCO3−] − 3
• C. For metabolic acidosis, expected PaCO2 = 1.5 × [HCO3−] + 8 ± 2; compare to actual PaCO2 to assess compensation and possible mixed disorder.
• D. PaCO2 equals HCO3− plus 10

Answer: (C)

Winter's formula is used to determine whether the respiratory system has appropriately compensated for a metabolic acidosis. It provides the expected PaCO2 in the setting of metabolic acidosis as 1.5 × [HCO3−] + 8, with a tolerance of about ±2 mm Hg.

To use it, take the bicarbonate value from the ABG, calculate 1.5 × [HCO3−] + 8, and compare that to the actual PaCO2. If the actual PaCO2 falls within the predicted range, the pattern is consistent with a simple metabolic acidosis with appropriate respiratory compensation. If the actual PaCO2 is higher than predicted, there may be an accompanying respiratory acidosis or inadequate compensation. If the actual PaCO2 is lower than predicted, there may be a concurrent respiratory alkalosis or overcompensation, suggesting a mixed disorder.

Note that this applies to simple metabolic acidosis. The other stated relationships are not the standard Winter’s formula.