Long-term stability of endogenous B-type natriuretic peptide during storage at − 20 °C for later measurement with Biosite Triage assay

Objective: To evaluate the long-term stability of B-type natriuretic peptide (BNP) in frozen samples at − 20 °C. Design and methods: During a health and nutrition survey of a representative sample of the adult population of Porto, Portugal, blood samples were collected in EDTA tubes. Aliquots of plasma were frozen at − 20 °C until measurement. Subjects with cardiac structural or functional abnormalities or renal dysfunction were excluded. BNP was assessed using a commercial immunofluorescence assay (Triage BNP Test®) as a continuous variable (n = 340) and in four subgroups selected according to different storage periods: ≤ 1 month (n = 35), 6–12 months (n = 80), 20–24 months (n = 18) and ≥ 36 months (n = 21). Results: Age- and sex-adjusted BNP values, analyzed as a continuous variable, decreased significantly over time of storage. When we stratified in four intervals of storage time, the median BNP value (25th–75th percentile, p value) was: 22.6 pg/mL (8.9–76.2, reference group) for ≤ 1 month, 19.8 pg/mL (9.5–48.4, p = 0.055) for 6–12 months, 15.3 pg/mL (7.5–35.0, p = 0.037) for 20–24 months, and 2.5 pg/mL (2.5–10.5, p < 0.001) for ≥ 36 months. Conclusions: BNP is stable at − 20 °C for 1 year, without protease inhibitors.