*Purpose: Renal allograft survival indirectly correlates with acute rejection episodes. Subacute rejection is also associated with decreased graft survival. Protocol biopsies (bxs) have been implemented in many top centers to identify subacute rejection and has been reported in 29-39% of 3 mo protocol bxs. Our study aimed to evaluate the rate of rejection detected in protocol bxs in our single center pediatric renal transplant (tx) program.

*Methods: Retrospective chart review of 38 pediatric patients (pts) who received a renal allograft between April 2014 and January 2018 and followed through July 2018. Induction immunosupressive therapy consisted of simulect on days 0 and 4 (n=25) and thymoglobulin 5mg/kg total dose on days 0-3 (n=13). Thymoglobulin was used in pts considered high risk for rejection. A bolus dose of intravenous methylprednisolone (10 mg/kg) was given before vascular reperfusion (max dose 500 mg), with tapering to 0.5 mg/kg/d oral prednisone by day 5 with subsequent tapering to 0.1 mg/kg/d over the next 6 months (max dose 7.5 mg). Mycophenolate mofetil (600-900 mg/m²/d) was given in 2 divided doses. Tacrolimus was initiated in all pts with doses adjusted to maintain an initial whole blood trough level between 10-12 ng/mL for first 30 days, 8-10 ng/mL 31-60 days, 7-10 ng/mL 61-120 days and 5-7 ng/mL thereafter. Protocol percutaneous ultrasound guided allograft bxs were performed between 3-6 months after tx. Pathology slides were evaluated by trained pathologists and rejection was determined using the standardized Banff criteria.

*Results: Eleven pts received living donor renal allografts and 20 pts were female. Renal failure was due to congenital anomalies in 24 pts, glomerular disease in 13 pts, and unknown in 1 pt. Of the 38 pts, 5 pts underwent tx bx before the 3 months due to elevated creatinine levels or BK viremia. Of these 5 pts, 1 pt had acute cellular rejection, 3 pts had tacrolimus toxicity, and 1 pt had BK nephropathy. The remaining 33 pts underwent protocol bxs 12-23 weeks after tx. Of these 33 pts, 6 pts had elevated creatinine at time of protocol bx and would have been bx anyway. These bxs revealed 1 pt with acute rejection and 2 pts with tacrolimus toxicity. The remaining 27 pts were stable at time of protocol bx. In all of these pts, no evidence for rejection was observed, 4 pts had tacrolimus toxicity, and 2 pts had recurrent IgA.

*Conclusions: Our protocol bxs revealed a subclinical rejection rate of 0%, a significantly lower rate than the 29%-39% reported in other studies. Therefore, these findings do not substantiate the continued use of 3 month protocol bxs at our institution. Our study instead suggests that a low index of clinical suspicion for rejection and therefore bx are adequate for detection of acute rejection episodes. In addition, the total rejection rate in all allograft recipients at 6 months post tx was determined to be 0% in living donor and 7% in deceased donor recipients, with an overall rejection rate of 5%. This is a significant improvement from national rates. Our low rejection rates are likely multifactorial.

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