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