Proficiency testing project for brain natriuretic peptide (BNP) and the N-terminal part of the propeptide of BNP (NT-proBNP) immunoassays: the CardioOrmocheck study.

BACKGROUND We organized and conducted a proficiency testing study (CardioOrmocheck) to evaluate the differences in analytical performance of brain natriuretic peptide (BNP) and N-terminal proBNP (NT-proBNP) immunoassays. METHODS Approximately 90 Italian laboratories were involved in the 2005-2007 proficiency testing cycles, while 112 laboratories took part in the 2008 cycle (from January to May 2008). A total of 28 study samples were measured by participating laboratories for a total of 2354 determinations. RESULTS The mean total variability for BNP (50.6 %CV) was significantly higher than that for NT-proBNP (8.4 %CV). In addition, the mean variability due to differences between-methods (46.4 %CV) comprised the majority of the total variability for BNP. Between-method variability for BNP comprised, on average, 84% of total variability, while the within-method variability comprised an average of 20.2 %CV. On the contrary, for NT-proBNP the within-method variability (7.3 %CV) represented the majority of total variability (average 75%), while between-method variability was smaller (4.1 %CV). Imprecision around the cut-off value showed marked differences among methods, especially for BNP immunoassay methods. In addition, BNP methods were affected by large systematic differences, for example an average 2.7-fold difference between Access and ADVIA Centaur methods, while agreement between NT-proBNP methods was better (an average 1.2-fold difference between Dimension and ECLIA on the Elecsys methods). CONCLUSIONS This multicenter collaborative study demonstrates that there are significant differences in analytical characteristics and measured values among the most popular commercial methods for BNP and NT-proBNP. Clinicians should be very careful when comparing results obtained by laboratories that use different methods.