Renal Failure Index Calculator
Renal Failure Index Calculator
Calculate the renal failure index from urine and plasma values to help distinguish a pre-renal from an intrinsic pattern of acute kidney injury.
Renal Failure Index
Una + Ucr + Pcr → patternUrine sodium 45 mmol/L, urine creatinine 40 mg/dL, plasma creatinine 2.6 mg/dL
Renal failure index
- Una
- urine sodium, mmol/L
- Ucr
- urine creatinine, mg/dL
- Pcr
- plasma (serum) creatinine, mg/dL
- Ucr/Pcr
- the urine-to-plasma creatinine ratio, a measure of water reabsorption
Worked example
Urine sodium 45 mmol/L, urine creatinine 40 mg/dL, plasma creatinine 2.6 mg/dL
Ucr/Pcr = 40 ÷ 2.6 ≈ 15.4
RFI = 45 ÷ 15.4 ≈ 2.9
2.9 is at or above 2 → intrinsic renal pattern
RFI compared with FENa
| Index | Pre-renal | Indeterminate | Intrinsic renal |
|---|---|---|---|
| RFI | < 1 | 1 – 2 | > 2 |
| FENa | < 1% | 1 – 2% | > 2% |
What the renal failure index adds — and where it fails
The renal failure index is calculated as urine sodium divided by the ratio of urine creatinine to plasma creatinine. It was described before the fractional excretion of sodium became standard, and the two carry almost the same information: RFI is essentially FENa with the plasma sodium term left out, on the reasoning that plasma sodium varies little enough between patients to be dropped without much loss of discrimination.
Because the two indices are so closely related, they share the same limitations, and it is worth being candid about them rather than treating either as definitive. Both become unreliable after diuretics, since diuretics force natriuresis regardless of the underlying cause and can push a genuinely pre-renal picture into the intrinsic range. Both perform poorly in hepatorenal syndrome, where sodium retention persists despite intrinsic-looking renal physiology, and both behave inconsistently in early contrast-induced nephropathy and in some cases of pigment nephropathy. Sepsis and CKD-on-CKD presentations also blur the cut-offs.
In practice, FENa is the more widely used of the two, mainly by convention and familiarity rather than any real advantage in accuracy, and most clinicians will already be reaching for it. Neither RFI nor FENa should be relied on alone: they are one input into a diagnosis that should also draw on the history, the trajectory of the creatinine, the urine sediment, and the response to a fluid challenge where one is appropriate.
Frequently asked questions
How is RFI different from FENa?
RFI omits the plasma sodium term that FENa includes. The two carry almost the same information and are interpreted against very similar cut-offs — RFI below 1 and FENa below 1% both suggest a pre-renal pattern.
What RFI suggests acute tubular necrosis?
An RFI of 2 or above is consistent with an intrinsic renal pattern such as acute tubular necrosis, reflecting loss of the tubule's normal ability to reabsorb sodium.
Does RFI still work after diuretics?
No. Diuretics force sodium excretion regardless of the underlying cause and can push a genuinely pre-renal picture into the intrinsic range. Avoid using RFI, or FENa, within roughly 24 hours of diuretic administration.
What is the formula for the renal failure index?
RFI = urine sodium ÷ (urine creatinine ÷ plasma creatinine). All three values are needed: urine sodium, urine creatinine and plasma (serum) creatinine.
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References
- Miller TR et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47–50.
- Espinel CH, Gregory AW. Differential diagnosis of acute renal failure. Clin Nephrol. 1980;13(2):73–77.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.