Electrolyte-Free Water Clearance (EFWC)

Estimates the kidney's net excretion (positive) or retention (negative) of electrolyte-free water to help explain and anticipate changes in serum sodium, counting only urine sodium and potassium and deliberately ignoring urea and other non-electrolyte osmoles, so it is not total osmolar clearance and is an assessment aid, not a substitute for serial serum-sodium monitoring or a free-water-deficit / sodium-correction-rate calculation.