Renal Failure Index (RFI) Calculator For Acute Tubular Necrosis

Renal Failure Index

The RFI determines whether the ARF is caused by a prerenal or renal cause and at the same time, can be used to monitor creatinine levels.

There are three variables that are used by the Renal Failure Index calculator for acute tubular necrosis:

■ Urine sodium: measured in mEq/L. The normal range is between 25 and 150 mEq/dL.

■ Serum Creatinine: where creatinine is the breakdown product in the metabolism of the muscles and certain tissues, filtered by the glomerulus. A normal range is considered to be between 0.6 and 1.3 mg/dL.

■ Urine Creatinine: takes account of the creatinine levels that escape the filtration process and are excreted through urine. The normal range for urine creatinine is between 40 and 120 mg/dL.

Creatinine levels above the normal may indicate, amongst others, a high protein diet, use of anabolic steroids or impaired renal function.

RFI interpretation

The formula used to retrieve the RFI is:

RFI = Urine sodium / (Urine creatinine / Serum creatinine)

RFI >1 indicates renal causes which can be accompanied by palpable bladder, hydronephrotic kidneys, enlarged prostate or renal calculi.

RFI ≤1 indicates prerenal causes which can be accompanied by volume depletion, existing congestive heart failure, severe liver disease or edema.

The index is therefore differentiating between prerenal azotemia and oliguric acute renal failure.

ARF and ATN

Acute Renal Failure is abbreviated ARF whilst Acute Tubular Necrosis is abbreviated ATN.

ARF, as a condition affecting hospitalized patients, is characterized by azotemia, associated or not with oliguria, and that progresses very quickly, in a matter of days. ARF carries high morbidity and mortality rates.

RIFLE marks the evolution of this renal condition:

■ Risk of renal dysfunction;

■ Injury to the kidney;

■ Failure of kidney function;

■ Loss of kidney function;

■ End-stage kidney disease.

The above can be assigned through changes in serum creatinine or urine output.

Acute Tubular Necrosis presents with modifications of the tubular epithelial cells and is the most common cause of AKI (Acute Kidney Injury).

Low blood pressure and/or use of nephrotoxic drugs are likely triggers of ATN. ATN is therefore classed as toxic or ischemic, depending on the agent affecting the epithelial cells, be it toxic or oxygen depletion. Urinalysis showing epithelial cells in urine is the main diagnosis modality.

Some of the most commonly met risk factors for kidney disease are:

■ Older age, family history of CKD;

■ Urinary tract infections (UTI);

■ Diabetes mellitus;

■ Systemic infections;

■ Hypertension;

■ Decreased renal mass;

■ Nephrolithiasis;

■ Autoimmune disease.

Original source

Miller TR, Anderson RJ, Linas SL, Henrich WL, Berns AS, Gabow PA, Schrier RW. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978; 89(1):47-50.

Other references

1. Bellomo R, Ronco C, Kellum JA, Mehta RL, Palevsky P; Acute Dialysis Quality Initiative workgroup. Acute renal failure - definition, outcome measures, animal models, fluid therapy and information technology needs: the Second International Consensus Conference of the Acute Dialysis Quality Initiative (ADQI) Group. Crit Care. 2004; 8(4):R204-12.

2. Durakovic Z, Durakovic A, Durakovic S. The lack of clinical value of laboratory parameters in predicting outcome in acute renal failure. Ren Fail. 1989-1990; 11(4):213-9.