Cross-Reactivity of BNP, NT-proBNP, and proBNP in Commercial BNP and NT-proBNP Assays: Preliminary Observations from the IFCC Committee for Standardization of Markers of Cardiac Damage To the Editor: B-type natriuretic peptide (BNP)

B-type natriuretic peptide (BNP) is a 32 amino acid cardiacsynthesized hormone that reduces blood pressure and increases sodium excretion (1 ). Following proteolytic cleavage of proBNP, a 108-amino acid precursor, an Nterminal fragment (NT-proBNP) and BNP are released (2 ). Increased concentrations of BNP and NT-proBNP can be used clinically to monitor heart failure, but a lack of alignment between commercial BNP and NT-proBNP assays (3 ) can lead to confusion when clinicians or laboratorians compare results measured for the same analyte on different instruments. Some of this confusion arises from variable assay specificity regarding what peptides are being measured. We studied whether (a) BNP assays demonstrated crossreactivity with NT-proBNP or proBNP, and (b) whether NTproBNP assays demonstrated crossreactivity with BNP or proBNP, by using 5 commercial BNP and 3 commercial NT-proBNP assays with 2 BNP, 2 NT-proBNP, and 2 proBNP materials. The NPs studied were: Peptide Institute synthetic BNP (aa 77–108), Scios human recombinant BNP (aa 77–108), HyTest human recombinant NT-proBNP (aa 1–76), Roche modified (amidated for stabilization) synthetic NTproBNP, HyTest human recombinant proBNP (aa 1–108), and Scios glycosylated human recombinant proBNP. BNP assays evaluated were Abbott Architect, Abbott AxSYM, Bayer Centaur, Biosite Triage, and Beckman Access (Biosite assay packaged for use by Beckman). NT-proBNP assays (all based on Roche reagents) were Dade-Behring Dimension, OrthoClinical Diagnostics Vitros, and Roche Elecsys 2010. All assays, for which epitopes of the antibodies used have been previous described (3 ), were run according to the manufacturers’ guidelines. BNP, NT-proBNP, and proBNP materials were diluted with normal (low NP concentration) EDTAplasma pools and lithium-heparin plasma (Dade assay only) pools, collected from healthy donors after institutional review board approval was obtained, to achieve target concentrations of 250, 500, and 1000 ng/L. Baseline BNP and NT-proBNP were quantified first in the pools and then after the pools were spiked with NP peptides. All measurements were performed in duplicate. Baseline BNP or NT-proBNP concentrations were subtracted from each spiked pool measurement. Percent crossreactivity was calculated by dividing the measured concentration for the spiked pool into the expected peptide concentration, multiplying by 100, and then averaging across all 3 expected concentrations. Recoveries and cross-reactivity percentages between peptides and BNP and NT-proBNP assays are displayed in Table 1. The BNP assays were more specific for the BNP peptides, with recovery ranging from 79% to 199%, compared to 5% to 38% cross-reactivity to the proBNP peptides and 1% to 7% cross-reactivity to the NTproBNP peptides. Similarly, the NT-proBNP assays were more specific for the NT-proBNP peptides, showing 47% to 243% recovery, with substantial crossreactivity to proBNP peptides ( 1% to 249%), and no crossreactivity to the BNP peptides ( 1% across all assays). This study demonstrates that the BNP peptides used do not substantially cross-react with NTproBNP assays, and that the NTproBNP peptides do not substantially cross-react with BNP assays. We confirm that there is substantial cross-reactivity between proBNP peptides and commercially available BNP and NT-proBNP assays. Variations depended on the different sources and types of peptide tested in each assay. We observed minimal crossreactivity with the glycosylated Scios proBNP peptide, compared with substantial cross-reactivity to the nonglycosylated HyTest proBNP peptide with the NTproBNP assays. Glycosylation likely interfered with peptide antibody binding. The mechanisms responsible for different reactivities between the HyTest and Roche NTproBNP peptides using different NT-proBNP assays, which use the same antibodies but different assay architectures, cannot be explained presently. The modest differences in reactivities for the recombinant (Scios) and synthetic (Peptide Institute) BNP materials using different BNP assays also requires additional study; with different assay architectures for the same reagents (Biosite, Beckman) showing diverse recovery. Little is known about which NP forms are circulating physiologically. The clinical significance of measured cross-reactivities will Letters