No doubt this is extremely lame, and I ought not to be confessing it in public. But as a result of studying for the inservice exam, I have finally grasped the significance of FeNa (the fractional excretion of sodium in the urine, calculated as (urine sodium/creatinine)/(plasma sodium/creatinine) ), and urine sodium, and urine electrolytes, relative to acute renal failure. This is one of those topics that people have been asking me about for the last three years, and I always gave them a blank stare and some kind of mumble. (The answer to the equation is either more or less than 1%, so you have a 50% chance of guessing the right one in a yes/no question.) Then they would give me some rapid and forceful explanation about its extreme and vital importance, which I of course didn’t understand. Back at the beginning I used to look it up in huge textbooks, and by the time I got to the end of the five-page section on the subject, I’d forgotten the beginning, and didn’t get anything out of it.

Now, having had more patients than I can remember suddenly develop oliguria (low urine output), and having stood there staring at their ins-and-outs sheet, and trying to correlate it with the latest electrolyte panel, and being still at a loss to figure out whether they were dehydrated, and needed a lot more fluid, or were well hydrated, and had acute tubular necrosis for some other, possibly reversible, reason, I am very interested to discover that the FeNa will help me figure this out. They told me this many times in the last six months, but now I get it; and you will find me checking urine electrolytes without having to be reminded, because I finally want to know what they show.

I know; I should have figured it out a long time ago. But the relationship between the esoteric calculation and the patient’s problem never clicked for me before. Better late than never, hmm?