American Board of Internal Medicine (ABIM) Certification Practice Exam

A Δ-Δ ratio of less than 1 may indicate which of the following conditions?

1. Metabolic alkalosis

2. Concurrent normal anion gap metabolic acidosis

3. Respiratory acidosis

4. Respiratory alkalosis

The correct answer is: Concurrent normal anion gap metabolic acidosis

A Δ-Δ ratio, or delta-delta ratio, is used in evaluating acid-base disorders, particularly in the context of metabolic acidosis. This ratio helps assess the presence of concurrent acid-base disturbances by comparing the changes in anion gap to changes in bicarbonate levels. When the Δ-Δ ratio is less than 1, it typically indicates that there is a normal anion gap metabolic acidosis occurring alongside a primary metabolic acidosis. In this scenario, the decrease in bicarbonate associated with the primary metabolic acidosis can be matched by an increase in chloride levels, leading to a non-anion gap metabolic acidosis. This often happens in conditions such as diarrhea, renal tubular acidosis, or other causes of normal anion gap acidosis where bicarbonate losses occur without a corresponding increase in unmeasured anions. Understanding the context of this ratio is crucial in diagnosing the patient's acid-base status accurately and guiding further management. This is why a Δ-Δ ratio of less than 1 is indicative of concurrent normal anion gap metabolic acidosis.