Renal Failure Index (RFI)

What This Calculator Measures

Calculates the renal failure index (RFI), a classic urinary index using urine sodium relative to the urine:plasma creatinine ratio to help distinguish prerenal azotemia from acute tubular necrosis.

How to Use

1. Obtain spot urine sodium, urine creatinine, and plasma creatinine from the same time point.
2. Enter all three values.
3. Review the RFI and its suggested category.
4. Combine with FENa, FEUrea, urine microscopy, and clinical volume assessment.

Interpretation Guide

An RFI below 1 supports prerenal azotemia, 1–2 is indeterminate, and above 2 favors intrinsic renal injury such as ATN. The test shares the limitations of FENa: diuretics and saline resuscitation raise urine sodium and can produce false intrinsic-like values.

Clinical Significance

The renal failure index is a classic bedside urinary index introduced in the 1960s–70s. It is essentially FENa without the plasma sodium term and remains a useful teaching and clinical adjunct in the AKI differential.

When to Use

• Acute kidney injury differential when urine sodium is available
• Complementing FENa and FEUrea in the prerenal-versus-intrinsic distinction
• Bedside teaching of classic urinary indices

When Not to Use

• In patients receiving diuretics without accounting for their effect
• As a standalone diagnostic test
• When urine output is absent or creatinine assays are non-standardized

Limitations

• Shares the limitations of FENa, including diuretic and saline-resuscitation effects.
• The <1/>1 convention derives from classic literature and is not a validated diagnostic threshold alone.
• Requires simultaneous samples.

Worked Example

A 60-year-old man with volume depletion and rising creatinine has a urine sodium of 40 mmol/L, plasma creatinine 1.2 mg/dL, and urine creatinine 80 mg/dL.

RFI = (40 × 1.2) ÷ 80 = 0.6. This value below 1 supports prerenal azotemia.

Evidence

Source

Original derivation (peer-reviewed)

Reference

Miller TR, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50.

Reviewed By

MedCalcHub Clinical Team

Version

1.0

Updated

2026-08

1. Miller TR, et al. Ann Intern Med. 1978;89(1):47-50.
2. Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.

References

1. Miller TR, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50. (Original Description)
2. Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. (Original Description)

Frequently Asked Questions