Long-term stability of procalcitonin in frozen samples and comparison of Kryptor® and VIDAS® automated immunoassays

Objectives: To assess long-term stability of PCT in deep-frozen samples and assess whether Kryptor procalcitonin cutoff ranges can be transferred on the bioMerieux VIDAS system. Design and methods: Samples from two intervention studies (ProRESP and ProCAP) were retested on both systems and compared with Passing-Bablok regression, Bland-Altman difference plot, and analysis of concordance over 4 medically relevant cutoff ranges. Results: Prolonged storage resulted in a significant decline of median procalcitonin values of 11.4% and 13.5% in ProRESP and ProCAP cohorts. Regression analysis showed that both methods correlated well (r=0.982, n=304) but with a deviation from the line of identity (y=1.234x -0.018). The Bland-Altman plot indicated an average relative bias of 17.1% (2 SD limits: -56.4% to 90.5%). Concordance over all 4 categories was 82.6%, without a significant difference in frequency distribution between methods. Conclusion: An only modest decline of measured PCT levels of around 10% after long-term deep-frozen storage was found. The VIDAS procalcitonin assay showed good correlation and concordance with the established Kryptor method. Both techniques can be used with the same nominal procalcitonin cutoff ranges in clinical routine. (C) 2009 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.