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, he specifically focused on the question between volume contraction and ATN. In 1978, a team from Colorado endorsed the FENa in the paper Urinary diagnostic indices in acute renal failure: a prospective study.
This test can help greatly because volume contraction increases the risk of ATN. We need to aggressively replete volume, and often the physical examination does not adequately help us determine volume status.
The biggest mistake that I see is using the FENa in the wrong settings. Espinel designed the test for a specific purpose. I like ordering the components of the FENa when I suspect volume contraction and fear ATN. It does not help us in patients who already have significant (stage 3b or worse) chronic kidney disease. After obtaining a urinalysis, and urine sodium and creatinine, we can try empiric treatment. We may never need to use the results, but if we do, we have them.