There is an insightful article in this month's C-JASN entitled "Misapplications of Commonly-Used Kidney Equations: Renal Physiology in Practice" by Nguyen et al which is kind of fun to read.
One of the clinical vignettes they present involves the use of the FENa calculation by a medical student to erroneously conclude that a CKD patient had ATN. I see the FENa quite commonly misused even by housestaff, so the discussion is pertinent.
To summarize the case: a woman with CKD and a baseline creatinine of 1.7mg/dL presents to the emergency room with a three day history of diarrhea. She had acute-on-chronic renal failure with a Cr of now 3.4 mg/dL, and despite the fact that her clinical exam was consistent with hypovolemia (orthostatic vital signs, skin tenting), she was erroneously diagnosed with ATN because her FENa was calculated to be 1.45%--above the magical cutoff of 1% we all learn as young internists.
Here's why it's incorrect in this instance. Recall that FENa is simply the ratio of the Na excreted divided by the Na filtered. For the same amount of daily dietary Na intake, A CKD patient will have a much higher FENa than a patient with normal kidneys, as in order to achieve a steady-state they will have to excrete the same amount of Na but using a much lower GFR. The authors calculate that this patient's resting FENa on the typical American diet would be about 2.4%. Thus, the measured FENa of 1.45% probably represents an appropriate Na retention response to volume depletion.
The FENa is most useful in the setting of oliguric acute kidney injury, and in the absence of metabolic alkalosis, significant CKD, or diuretic use. A value <1%>1% suggests ATN.