American Board of Internal Medicine (ABIM) Certification Practice Exam

What does Winter's Formula help to estimate in a clinical setting?

1. Serum Potassium

2. PCO2

3. Sodium Bicarbonate

4. Osmolar Gap

The correct answer is: PCO2

Winter's Formula is used in clinical settings to estimate the expected partial pressure of carbon dioxide (PCO2) in patients with metabolic acidosis. This formula provides a way to assess the adequacy of respiratory compensation when there is an acid-base disturbance. In metabolic acidosis, the body attempts to compensate for the decreased pH by increasing respiratory rates, which helps to blow off carbon dioxide and thus reduce the acid load. Winter's Formula predicts the appropriate level of PCO2 that should be achieved based on the bicarbonate (HCO3-) level. The formula can be stated as: PCO2 = (1.5 × HCO3-) + 8 ± 2. The calculated value can guide healthcare providers in determining whether the lungs are effectively compensating for the metabolic acidosis or if there is an additional respiratory disorder contributing to the acid-base imbalance. While other options like serum potassium, sodium bicarbonate, and osmolar gap are important markers in different clinical contexts, they do not relate to Winter’s Formula. Instead, they are used for other diagnostic considerations and do not provide information on the respiratory compensation for metabolic acidosis. Thus, using Winter’s Formula allows clinicians to maintain a clearer picture of the patient's acid-base status and