Estimate free water deficit in hypernatraemia: weight × TBW factor × (Na/140 − 1).
Free water deficit guides the volume of hypotonic fluid replacement in hypernatraemia. The formula from the 2000 NEJM review by Adrogue and Madias: FWD (L) = TBW × (current Na / 140 − 1), with TBW (total body water) = weight × factor: 0.6 in men, 0.5 in women, 0.45 in elderly patients. Categories: <1 L mild, 1–3 L moderate, >3 L severe. Sodium should fall no faster than 10 mmol/L per 24 h to avoid osmotic demyelination. Spanish guidance from SEMI and SEEN recommends recalculating the deficit every 4–6 hours; faster correction is acceptable when the onset is acute (<48 h).
Adrogue HJ, Madias NE. N Engl J Med 2000;342(20):1493-1499
Formula version: adrogue-2000-v1