Renal Failure Index Calculator

Overview

Calculates the renal failure index from urine sodium and paired urine and plasma creatinine.

Clinical Significance

The renal failure index is affected by diuretics, CKD, sepsis and timing. It should not be used alone to determine the cause of acute kidney injury.

When to Use

• Use when the stated clinical calculation is relevant to the patient and decision being considered.
• Use contemporaneous measurements obtained with appropriate technique.
• Confirm that the equation and units match local practice before acting on the result.

How It Is Calculated

• RFI = urine sodium × plasma creatinine ÷ urine creatinine.

Interpretation

• The renal failure index is affected by diuretics, CKD, sepsis and timing. It should not be used alone to determine the cause of acute kidney injury.
• Interpret trends and the full clinical picture rather than relying on a single calculated value.

Worked Example

Enter a valid set of the required measurements. MedicalC applies: RFI = urine sodium × plasma creatinine ÷ urine creatinine. The displayed result should then be checked against the clinical context and local guidance.

Patient Considerations

• Consider whether physiology, treatment or sampling conditions changed between measurements.
• Consider whether the patient differs from the population in which the equation is normally used.

Limitations

• The result is only as reliable as the entered measurements and assumptions behind the equation.
• The calculator does not replace clinical assessment, definitive measurement or specialist advice.
• Thresholds and interpretation may vary between populations, laboratories and clinical pathways.

Clinical Pearls

• Confirm units before calculation.
• Use values obtained at the same clinical time point.
• Document the component measurements with the calculated result.

Common Mistakes

• Mixing SI and conventional units.
• Using estimated or non-contemporaneous measurements without acknowledging uncertainty.
• Treating a calculated value as a diagnosis or treatment instruction.

Evidence Base

The implemented equation follows the established published or guideline-based relationship: RFI = urine sodium × plasma creatinine ÷ urine creatinine.

References

1. Miller TR, et al. Urinary diagnostic indices in acute renal failure. Ann Intern Med. 1978. — Primary publication, guideline or established physiologic source supporting the equation.