A simple aid to the differential diagnosis of oliguria

A study was conducted in oliguric and acutely azotemic patients, measuring: (i) the fractional excretion of sodium (FENa) using creatinine clearance as a measure of glomerular filtration rate, and (ii) sodium clearance relative to urea clearance, designated as the sodium/urea clearance ratio (Na:urea CR). It was found that FENa discriminated between "tubular" and "non-tubular" disorders in 96% of patients. Further, Na:urea CR was as discriminating as FENa. Patients with Na:urea CR above 2.5% can be reliably diagnosed as having acute tubular necrosis or acute urinary tract obstruction; those with a value less than 2.5% will have acute glomerulonephritis or pre-renal azotemia. As urea and sodium measurements are so readily available, this test can now be applied in the assessment of the oliguric or acutely azotemic patient in any hospital practice.