Assessment of rejection risk following subtherapeutic calcineurin inhibitor levels after pediatric heart transplantation.

CNIs are the mainstay of immunosuppressive therapy after pediatric HTx. While regular laboratory surveillance is performed to ensure blood levels are within targeted range, the risk of acute rejection associated with subtherapeutic CNI levels has never been quantified. This is a retrospective single-center review of 8413 CNI trough levels in 138 pediatric HTx recipients who survived >1 year after HTx. Subtherapeutic CNI levels were defined as <50% of the lower limit of target range. The risk of acute, late (>12 months post-transplant) rejection following recipients' subtherapeutic CNI levels was assessed using time-varying multivariable Cox proportional hazards analysis. We found that 79 of 138 recipients (57%) had at least one subtherapeutic CNI level on routine surveillance laboratories during a mean follow-up of 5.5 ± 3.6 years. Following an episode of subtherapeutic levels, 17 recipients (22%) had biopsy-proven rejection within the next 3 months; the majority (9/17) within the first 2 weeks. After presenting with subtherapeutic CNI levels, recipients incurred a 6.1 times increased risk of acute rejection in the following 3 months (HR = 6.11 [2.41, 15.51], P = <.001). Age at HTx, HLA sensitization, or positive crossmatch were not associated with acute late rejection, but rejection in the first post-transplant year was (HR 2.61 [1.27, 5.35], P = .009). Thus, maintaining therapeutic CNI levels is the most important factor in preventing acute rejection in recipients who are >12 months after pediatric HTx. Recipients who present with subtherapeutic CNI levels on surveillance monitoring are 6.1 times more likely to develop rejection in the following 3 months.