A Commutable Cytomegalovirus Calibrator Is Required to Improve the Agreement of Viral Load Values between Laboratories

BACKGROUND: Viral load testing for cytomegalovirus (CMV) is an important diagnostic tool for the management of transplant recipients and immunocompromised individuals; however, inconsistency among laboratories in quantitative measurements of viral load limits interinstitutional comparisons. These inconsistencies stem from the lack of assays cleared by the US Food and Drug Administration, the absence of international standards, the wide variety of CNW-extraction and -detection methods, and differences in materials used for calibration. A critical component of standardization is the use of calibrators that are traceable and commutable. METHODS: Bland-Altman plots and prediction ellipses were used to test the commutability of 2 CMV calibrators for 2 different quantification methods. RESULTS: Tests with 2 methods showed 1 calibrator to be commutable and the other to be noncommutable. The results for the commutable calibrator were within the 95% prediction interval of the clinical samples in the Bland-Altman plot and within the 95% prediction ellipse for a simulated commutable calibrator, whereas the results for the noncommutable calibrator were not within these prediction intervals. When used to calibrate patient results, only the commutable calibrator, the OptiQuant (R) CMVtc Calibration Panel, significantly improved the comparability of viral loads for the 2 different measurement methods. CONCLUSIONS: This study demonstrates that an important goal in the effort to improve healthcare for patients with CMV-related disease is the establishment of traceable and commutable reference materials, including both calibrators and controls. (C) 2009 American Association for Clinical Chemistry