N-Terminal Pro–B-Type Natriuretic Peptide (NT-proBNP) Immunoreactivity Is Reduced After 6 Years of Storage at −70 °C

Abstract

Abstract Background Clinical trial samples may be stored frozen for prolonged periods before analysis, which can reduce the immunoreactivity of numerous analytes, particularly peptides. We sought to determine the effect of 6 years of frozen storage on serum N-terminal pro–B-type natriuretic peptide (NT-proBNP). Methods NT-proBNP was measured from serum samples taken from 99 different patients enrolled in the CanPREDDICT study after <1 year of storage at −70 °C using the Roche first-generation NT-proBNP assay on an e411 instrument. Separate aliquots of the same samples were analyzed after an additional 6 years of storage at −70 °C using the Roche second-generation assay on an e601 instrument. Results The median NT-proBNP immunoreactivity for the first measurement was 572 pg/mL (interquartile range [IQR] 205–1606, range 49–12820), while after an additional 6 years of storage at −70 °C, this value decreased to 526 pg/mL (IQR 181–1338, range 18–12880), resulting in a median percent difference of −7% (IQR −10.6% to −3.4%, P < 0.001). Conclusions We report findings consistent with trends seen in previous work but have investigated the effect of a much longer storage period. Larger percent decreases in NT-proBNP reaching statistical significance are seen, although the median difference is still <10%.