Plasma GFAP Biomarker Analysis Protocol

Venous blood samples were originally drawn from all enrolled patients, and a part of the blood was analyzed at Tampere University Hospital (Tampere, Finland) for a previous study.^{22 (link)} The remaining serum and plasma samples were immediately frozen at −70°C for future use. Samples were sent to the Sahlgrenska University Hospital, Mölndal, Sweden (transfer in 20 kg of dry ice by a courier) for further analyses. On September 14–15, 2019, the samples were analyzed using the GFAP Discovery Kit (Quanterix, Billerica, MA) on an HD-1 Simoa instrument to determine plasma GFAP levels. The lower limit of detection and lower limit of quantification were 0.211 and 0.686 pg/mL, respectively. The laboratory technicians performing the analyses were blinded to the clinical data. The details of our GFAP analytics are described more comprehensively in a previous publication and in the Supplementary Material.^{25 (link)}