Answer: The fractional excretion of lithium (FELi)
Humans have endogenous lithium present in their circulation. Renally filtered lithium gets reabsorbed in the proximal tubule. So, proximal reabsorption of lithium is increased in prerenal disease, which will decrease the lithium excretion. In kidney disease, this excretion is increased as proximal tubule fails to reabsorb lithium.
- Normal value of FELi is 20%
- FELi below 15 % is consistent with prerenal causes
- FEli above 25% signifies renal disease
The beauty of FELi is that it is not influenced by loop or thiazide (except chlorothiazide) diuretics. Unfortunately, not all labs have the capacity to perform FELi.
1. Boer WH, Koomans HA, Dorhout Mees EJ. Acute effects of thiazides, with and without carbonic anhydrase inhibiting activity, on lithium and free water clearance in man. Clin Sci (Lond) 1989; 76:539.
2. Steinhäuslin F, Burnier M, Magnin JL, et al. Fractional excretion of trace lithium and uric acid in acute renal failure. J Am Soc Nephrol 1994; 4:1429.