Crazy numbers: lowest creatinine

The consult team is now taking care of a patient with what I think is the lowest creatinine I have ever seen: 0.29 mg/dL. I’m not a bench nephrologist but I think that is a pretty typical mouse creatinine.

Update: some commenters asked about the BUN: 6 mg/dL. FYI today the Cr is down to 0.28 and the BUN fell to 3!

The patient has SIADH and low creatinines are a usual finding. She also has a crazy low uric acid of 1.4. Not quite Uricase low but getting close. Her admission sodium was 108, her urine sodium today was a whopping 156 with a urine potassium of 34. So if you calculate her electrolyte free water clearance (the amount of her urine which is electrolyte free water):

You get a negative 826 mL. A negative electrolyte free water is not unusual in SIADH and distinguishes it from the hyponatremia seen with heart failure (or other conditions of decreased effective circulating volume).
In this case the negative clearance means that for every liter of urine this patient makes it is as if she drank 826 mL of fluid. The very act of urinating lowers the sodium further by diluting the plasma sodium.
This throws a wrench in the standard plan of adding the urinary output to the insensible losses and setting the fluid restriction to be 200-500 mL below that. This works in conditions like heart failure where the electrolyte free water is positive but with a negative free water clearance you need to account for the negative free water clearance by adding it to the water intake, not the renal losses.
See this presentation on electrolyte free water clearance for further details.