Renal Failure Index Calculator

Renal Failure Index Calculator for Nephrology. The calculator reports arithmetic without a prerenal/intrinsic label. A very low, borderline or high value does not by itself identify the cause of AKI or establish whether fluid administration is appropriate.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

RFI = urine sodium x plasma creatinine / urine creatinine. The creatinine units cancel; the remaining concentration dimension follows urine sodium. With sodium in mmol/L the result is numerically equivalent to mEq/L. It is neither a percentage nor FENa.

When to use this calculator

For historical study of urinary diagnostic indices and comparison with a clinician's paired laboratory measurements. The original prospective work addressed acute oliguria, not every presentation of kidney dysfunction.

Inputs used

  • Urine sodium (mmol/L)
  • Paired plasma creatinine (umol/L or mg/dL)
  • Urine creatinine in matching units

Clinical interpretation

The calculator reports arithmetic without a prerenal/intrinsic label. A very low, borderline or high value does not by itself identify the cause of AKI or establish whether fluid administration is appropriate.

Worked example

Urine sodium 20 mmol/L, plasma creatinine 176.8 umol/L and urine creatinine 8840 umol/L give 20 x 176.8 / 8840 = 0.4 mmol/L. Using 2 and 100 mg/dL for the creatinine pair produces the identical result.

Limitations and safety notes

Samples need a compatible time point and matched creatinine units. Diuretic exposure, changing kidney function and clinical context complicate inference. Urine creatinine must be positive; zero urine sodium is mathematically permitted. No claim is made that the historical thresholds reliably diagnose contemporary unselected patients.

Frequently asked questions

Why is this not a fractional excretion percentage?

FENa additionally uses plasma sodium and a factor of 100. RFI does not.

Does a result above 1 prove tubular injury?

No. This tool deliberately does not assign an etiological diagnosis at that threshold.

References

  • Miller TR et al. Urinary diagnostic indices in acute renal failure. A prospective study. Ann Intern Med 1978. https://doi.org/10.7326/0003-4819-89-1-47
  • Authors' institutional record and abstract. https://scholars.uthscsa.edu/en/publications/urinary-diagnostic-indices-in-acute-renal-failure-a-prospective-s/

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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