FERE: Fractional excretion of random electrolytes

Magnesium:

• 142 controls: 1.8% (range 0.5-4%)
• 74 hypomagnesemic
• Extra-Renal origin 1.4% (range 0.5-2.7%)
• Renal origin 15% (range 4-48%)
• Authors conclusion: >4% per cent is indicative of inappropriate renal magnesium loss

Fractional excretion of magnesium in normal subjects and in patients with hypomagnesemia.

Potassium

• 312 normal subjects: 8% (range 4-16%)
• 84 hypokalaemic patients
• Extra-renal origin: 2.8% (range 1.5-6.4%)
• Renal origin: 15% (range 9.5-24%)
• Authors conclusion: >6.5% per cent is indicative of inappropriate renal potassium loss

Fractional excretion of potassium in normal subjects and in patients with hypokalaemia.

That's a really low fractional excretion of sodium

Patient with acute renal failure and constipation. Normal saline to the rescue.
That's a low FeNa, but not the lowest I have ever seen.

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lowest FeNa I have ever seen

I was consulted on a patient with autoimmune hepatitis and acute kidney injury. The patient has ascites and was admitted with a small bowel obstruction.

I'm not sure if it is actually the lowest but it is remarkably low:

serum Na 147 mmol/L
serum Creatinine 1.77 mg/dL

urine Na <10
urine Cr 148

I assumed a urine sodium of 5 mmol/L

FENa= 0.04%

This is 4 molecules of sodium excreted for every 10,000 filtered. Amazing.