The story of the FeNa test

Previously published at Centor’s Corner (an MDCalc blog)   September, 1976: I was a 2nd year internal medicine resident at the Medical College of Virginia. My attending physician, Dr. Carlos Espinel, had just published a now-classic article: The FENa test. So that month, I had the wonderful opportunity to understand the rationale behind a test that I now have used for over 40 years. To understand the test, one must first understand the assumptions. Dr. Espinel defined clearly that one could use this test to help differentiate between volume contraction and acute tubular necrosis (ATN) in oliguric patients. He defined oliguria as <20 cc/hr (approximately 500 cc daily). He applied the test to patients in whom the diagnosis was clinically confusing. The idea is a simple one. Volume-contracted patients with otherwise normal kidneys should avidly retain sodium and water. ATN patients have a tubular dysfunction that prevents adequate sodium and water reabsorption. This test expanded on the less satisfactory tests—urine Na <20 mEq/mL or urine/plasma creatinine ratio—and combined them. The fractional excretion of any element or molecule defines the percentage of the filtered element that one excretes. Thus, a low FENa (<1%) means that the kidney is reabsorbing more than 99% of the filtered sodium. So, the test comes directly from an understanding of the underlying physiologies of volume contraction versus ATN. When Dr. Espinel wrote about the test,...
Source: DB's Medical Rants - Category: Internal Medicine Authors: Tags: Medical Rants Source Type: blogs