Radiology: Renal resistive index of 0.70 indicates permanent renal damage

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A renal resistive index (RI) of 0.70–the commonly used threshold value–or higher is predictive of an unfavorable outcome in patients with chronic nephropathies, according to a study in Radiology.

According to Claudia Parolini, MD, of the department of nephrology at the Tor Vergata University in Rome, and colleagues, the RI has long been used to predict the progression of renal dysfunction.

Previous studies on the behavior of RI in different renal diseases has found the mean reference value for normal RI in adults was 0.60, with 0.70 as the upper limit of normal. Other researchers have created wide ranging threshold values for renal impairment, which leads to questions about the usefulness of RI as a measurement, the scientists noted.

As a result, the authors wrote that “a better-defined threshold value is necessary if RI is to be a useful measurement for prognosis and therapy,”

The authors evaluated 177 patients referred to their nephrology center between 1995 and 2002, 86 of whom were followed up from 2 to 11 years for RI and renal function annually.

The authors classified patients in four groups on the basis of normal or abnormal intitial renal function and RI, and observed that at the end of the follow-up period, “those with an initial RI of 0.70 or higher had the greatest reduction of renal function, independent of initial eGFR [epidermal growth factor receptor].”

While several previous studies had indicated that an RI higher than 0.60 was predicted of renal damage, the authors in this study determined that patients with who had an RI at least 0.70 and higher than 0.61 (high-normal-RI and normal-RI) had survival curves of 75 percent of the 11 years.

“According to the data of this study,” Parolini and colleagues reported, “only an RI of 0.70 or higher in patients with chronic nephropathies seems to indicate irreversible damage."

RI does not change significantly even within a long period of observation unless structural changes develop, the authors concluded. And while “an RI lower than 0.70 can be found in diseased kidneys but does not presage a rapid progression of disease, an RI higher than 0.70 is a strong predictor of progression to renal failure, equal or better than is eGFR and RI evaluation can help determine prognosis and guide therapy in patients with chronic nephropathies.”