FEUrea

In hypovolaemic states, the kidney maximally reabsorbs water and urea in the collecting duct under ADH stimulation, minimising urinary urea loss. In ATN, tubular cell damage impairs this reabsorption regardless of volume status, leading to high fractional excretion of urea in the urine.

FEUrea was proposed as a clinical tool in the early 2000s as a direct response to the recognised limitation of FENa in the modern era of widespread diuretic use. The landmark Carvounis study in 2002 finally provided the evidence base to make FEUrea a clinically actionable bedside calculation.