Urinary Biomarkers and Kidney Transplant Rejection: Fine-Tuning the Radar

Division of Nephrology, University of Arizona HealthSciences Center, Tucson, AZ Corresponding author: Titte R. Srinivas,srinivat@musc.eduReceived 31 May 2013, revised 12 June 2013 andaccepted for publication 04 July 2013Acute rejection in the transplanted kidney is diagnosedin the clinic using a combination of renal dysfunction,renal biopsy and alloantibody levels (1). Despite itsinvasive nature and high interobserver variability, therenal biopsy remains the gold standard for the diagnosisof acute rejection (2). Because of these limitations of therenal biopsy, the goal of discovering accurate andreproducible biomarkers in the urine and blood amongkidney transplant recipients has been an important line ofresearch (3–5).InthisissueofAJT,Hriciketal(6)reportontheﬁndingsofamulticenter observational study conducted under the aegisof the NIH-sponsored Collaborative Trials in Organ Trans-plantation study consortium (CTOT). In the CTOT-01 study,Hricik et al discovered that low urinary CXCL9 (a T cellchemoattractantinducedbyinterferongamma)at6monthsposttransplantationwasassociatedwithalowerprobabilityof development of acute rejection or a decline in renalallograft function over subsequent follow-up to 24 monthsin the 280 kidney transplant recipients studied (6). Theauthors concluded that low urinary CXCL9 protein could beused as a biomarker to identify transplant recipients at lowrisk for future immunologic events. The authors are to beapplaudedontheconductandreportingoftheirmulticenterprospective inquiry. However, the study raises severalquestions that bear reﬂection.The CTOT-01 study examined, serially, urinary mRNA ofseveral candidate cytokine and effector molecules as wellas proteins pertaining to, in the context of a 6-month perprotocol allograft biopsy, biopsies performed for cause andclinical follow-up.Using this approach, the positive predictive value (PPV) ofurinary CXCL9 protein for acute rejection was modest at67% whereas the negative predictive value (NPV) wasmuch higher at 92%. The performance of urinary CXCL9mRNAinthepredictionofacuterejectionwasinferior(PPV,61.5%; NPV, 83%). This discrepancy in urinary CXLC9proteinandmRNAwasattributedtothedifﬁcultiesinusingurine as a matrix for mRNA samples. Further, the authorspropose that low urinary CXCL9 at Month 6 posttransplan-tation would allow the identiﬁcation of subjects at low riskfor acute rejection or decline in GFR between 6 and24 months posttransplantation (NPV, 92.5–99.3%). TheCTOT-01studypopulationwasatlowimmunologicrisk;themean panel reactive antibody of the subjects was 12.9%.Most recipients received living donor transplants, mostreceived antibody induction and all ﬂow crossmatches attransplantwerenegative.Notably,asreportedbyHriciketal(6),intheCTOT-1population,rejection(gradesuspiciousorhigher)wasreportedin25/170(13.8%)of6-monthprotocolbiopsies and in 43/79 (54.43%) of for-cause biopsies byMonth 6.These ﬁndings are an important contribution to thedevelopment of biomarkers in transplantation and certainlyare a welcome addition to all clinicians as a piece in thepuzzle of clinical decision making. However, the studyalso teaches us another important lesson. In the modernera of transplantation, 1-year acute rejection rates in low-immunologic risk patients should approach 10%. In thissetting, if one were to consider using a biomarker as asubstitute for the invasive renal biopsy, what is desirableis a test not only with a high NPV but also with a high PPV.The ideal test would help detect disease early in its clinicalcoursewithoutanexcessoffalsepositives.Inthiscase,themarker must have high sensitivity and speciﬁcity. As thebaseline event rate (acute rejection) is expected to be lowwithmodernimmunosuppressioninalow-riskpopulation,ahigh negative predictive value for rejection while indisput-ably of value does not answer what one may consider theeven more difﬁcult question of who will develop disease(rejection/injury).Clinical decisions are in large part based on conditionalprobabilities given the known pretest probability (preva-lence) and the ability to distinguish when a test is either atrue positive or true negative. Given this one must invokeBayes theorem, simply stated mathematically as, PPV