Distinguishes pre-renal from intrinsic AKI using creatinine ratio
Laboratory Inputs
Required (1–400)
Required (1–800)
Required (0.1–30)
—RFI
RFI
Interpretation
Mechanism
< 1
Pre-renal AKI
Intact tubular function; avid Na and water reabsorption
1–2
Indeterminate
Overlap; consider clinical context and other indices
> 2
Intrinsic AKI (ATN)
Tubular damage; impaired Na reabsorption
RFI = UNa / (UCr / SCr)
Equivalent to: RFI = (UNa × SCr) / UCr
Compare: FENa = (UNa × SCr) / (SNa × UCr) × 100
RFI is similar to FENa but does not require serum sodium
Comparison with FENa: RFI and FENa give similar clinical information. RFI <1 and FENa <1% both indicate pre-renal AKI. RFI has same exceptions as FENa: may be <1 in early contrast nephropathy, pigment nephropathy, and NSAID-induced AKI. Unreliable after diuretic use.
