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Half of circulating calcium is bound to albumin. The Payne formula, derived in 1973 from 200 patients, often is used to “correct” hypocalcemia when hypoalbuminemia exists. In the largest study to date on calcium correction (>22,000 patients; mean age, 60), Canadian researchers aimed to validate this and 9 similar formulas against levels of ionized calcium. All patients had simultaneous total and ionized calcium measurements, with ionized calcium levels between 1.15 and 1.35 mmol/L considered normal; half had serum albumin measured.
Overall, total calcium levels correlated better with ionized calcium than did formula-corrected calcium levels. Formulas with stronger correlation than total calcium levels were either complex (e.g., requiring blo…