Intrarenal resistive index partially reflects renal graft outcome

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1. Higher intrarenal resistive index was associated with higher recipient mortality in renal allografts. 

2. Resistive index was not associated with histologic features or need for dialysis, and appeared to reflect characteristics of the recipient more than those of the graft. 

Evidence Rating Level: 2 (Good)

Study Rundown: Despite recent progress, tests with high sensitivity and specificity in predicting renal allograft outcome remain elusive. A study published in June of 2013 (see Relevant Reading) reported that the predictive power of molecular profiling can be moderately improved with a novel urinary molecular signature of CD3ε chain, interferon-inducible protein 10 (IP-10), and 18S rRNA. However, molecular profiling and protocol-driven biopsies are not widely used.

Intrarenal resistive index is one of many candidate procedures to predict allograft outcome, favored for its non-invasive nature and the wide availability of ultrasonography. The rationale is that vascular resistance may correlate with the health of the graft vascular supply. This study demonstrated that high intrarenal resistive index was moderately associated with higher recipient mortality, but was not associated with need for dialysis or histological changes at protocol-specified biopsies. The authors concluded that intrarenal resistive index reflected characteristics of the recipient more than those of the graft. As a result, the procedure appeared to have moderate predictive power in certain aspects of the outcome but fell short of the expectations for a “gold-standard” test.

This single-center, prospective study included 346 patients in Leuven, Belgium, followed for a minimum of 4.5 years. It is well-designed and adequately powered overall. One caveat is that 19% of patients missed the 3-month ultrasound exam and that 37% missed the 2-year exam.

Click to read the study in NEJM

Click to read an accompanying editorial in NEJM

Relevant Reading: Urinary-cell mRNA profile and acute cellular rejection in kidney allografts

In-Depth [prospective study]: This study assessed the predictive power of intrarenal resistive index in renal allograft outcome. The primary outcome was the composite of reduction of 50% or more in eGFR from the value at the time of ultrasonography, end-stage renal failure requiring reinstitution of dialysis, or recipient death with a functioning graft.

Multivariate Cox proportional-hazards analysis showed that a resistive index of 0.80 or higher was  associated with an increase in the composite primary end point at 12 and 24 months (P=0.02 and P=0.04, respectively) with a statistically insignificant difference at 3 months (P=0.12). The combined secondary end point of need for dialysis or recipient death was significantly higher among patients with a resistive index of 0.80 or higher at 3 and 12 months after transplantation, with borderline significance at 24 months (P=0.07). The need for dialysis alone, however, did not differ significantly at any time point.

Protocol-specified biopsies at 0, 3, 12, and 24 months did not show significant or consistent histological findings for patients that had a resistive index of 0.80 or higher. For biopsies performed due to graft dysfunction, there was a significant difference in the resistive index between acute antibody-mediated rejection and normal findings ( 0.87±0.12 vs. 0.78±0.14; P=0.05), as well as between patients with acute tubular necrosis and patients with normal histology (0.86±0.09 vs. 0.78±0.14; P=0.007).

By Xiaozhou Liu and Adrienne Cheung

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