A patient has pH 7.28, PaCO2 28 mmHg, and HCO3⁻ 12 mEq/L. Using Winter's formula for respiratory compensation in metabolic acidosis (expected PaCO2 = 1.5×[HCO3⁻] + 8 ± 2), what conclusion can be drawn?

• A PaCO2 is lower than expected — additional primary respiratory alkalosis is present
• B PaCO2 is higher than expected — additional primary respiratory acidosis is present
• C The anion gap must be calculated before any conclusion is possible
• D The respiratory compensation is appropriate — simple metabolic acidosis ✓

Explanation

Winter's formula: expected PaCO2 = 1.5×12 + 8 ± 2 = 18 + 8 ± 2 = 24–28 mmHg. The measured PaCO2 is 28 mmHg, which falls within the expected range of 24–28, indicating appropriate respiratory compensation with no additional respiratory disorder — this is a simple (pure) metabolic acidosis. If PaCO2 were lower than expected, a superimposed respiratory alkalosis would be present; if higher, a concurrent respiratory acidosis. Accurate application of Winter's formula is essential in NEET PG clinical biochemistry questions.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.