Figure 4

BDNF increases in plasma frozen at − 80 °C for 12 months. (A) Plasma collected in K2EDTA tubes and frozen at − 80 °C had significantly increased BDNF by 12 months following collection (p ≤ 0.0001, Kruskal–Wallis test with Dunn’s post hoc). The same was observed for (B) lithium-heparin tubes (p ≤ 0.0001, Kruskal–Wallis), (C) lithium-heparin gel separators (p ≤ 0.0001, Kruskal–Wallis test with Dunn’s post hoc), (D) sodium heparin collection tubes (p ≤ 0.0001, Kruskal–Wallis test with Dunn’s post hoc), and (E) glass sodium heparin (p ≤ 0.0001, Kruskal–Wallis test with Dunn’s post hoc), which was the least stable when plasma isolated by this tube was stored at − 80 °C. Na-Hep (glass) also had significantly different quantities of BDNF recovered at 3 vs. 6 months, and 6 vs. 12 months. BDNF: brain-derived neurotrophic factor. K2EDTA: potassium EDTA. Li-Hep: lithium heparin. Na-Hep: sodium heparin. *p ≤ 0.05, **p <  ≤ 0.01, ***p ≤ 0.001, ****p ≤ 0.0001. Dotted lines indicate the expected range of the means for each plasma separator (calculated to be ± 47.9%, based on inter-assay CV and the mean concentration of BDNF recovered from each plasma separator at Time 0). + : indicates the mean.