Technical Considerations and Confounders for Urine CXCL10 Chemokine Measurement

Background: The urine C-X-C motif chemokine 10 (CXCL10) is a promising screening biomarker for renal allograft rejection. The aim of the study was to investigate important technical and biological aspects as well as potential confounders when measuring urine CXCL10. Methods: We analyzed 595 urine samples from 117 patients, who participated in a randomized controlled trial investigating the clinical utility of urine CXCL10 monitoring for posttransplant management. Urine CXCL10 was measured by an immunoassay using electrochemiluminescence. Results: Intraassay coefficient of variation was 2.5%, and interassay coefficient of variation was 10%. Urine CXCL10 remained stable (ie, <10% degradation) for 8 hours at 25 degrees C or 37 degrees C and for 3 days at 4 degrees C. CXCL10 concentrations [pg/mL] strongly correlated with urine CXCL10/creatinine ratios [ng/mmol] (r(2) = 0.98; P < 0.0001). Leucocyturia and active BK-polyomavirus infection are associated with higher CXCL10 concentrations, while allograft function, serum CRP, patient age, proteinuria, urine pH, hematuria, squamous epithelia cell count, and bacteriuria did not correlate with urine CXCL10 concentrations. In 145 paired samples obtained within 1-2 weeks, 80% showed a CXCL10/creatinine ratio change of < +/-2 ng/mmol or +/-50%, respectively. Conclusions: Urine CXCL10 measurement on the used platform is accurate and robust. Leucocyturia and active BK-polyomavirus infection are major confounders, which can be easily detected but represent important diagnostic "blind spots" when using urine CXCL10 to screen for allograft rejection. The intraindividual biological variability of urine CXCL10 within 1-2 weeks is mostly below +/-50%, which is still much higher than the technical variability due to sample handling/processing (<20%).