2.3. Blood samples and measurement of plasma serotonin

For preparation of platelet‐poor plasma samples for determination of serotonin levels, we adhered strictly to the guidelines and procedures as stipulated by Brand et al. for serotonin measurements.22 For this study, venous blood was collected from controls at entry into the present study (controls were not followed up) and for CRC patients the day prior to surgery. The blood samples were centrifuged at 200×g for 10 minutes at room temperature to obtain platelet‐rich plasma and then at 4500×g for 10 minutes at 4°C to obtain platelet‐free plasma. The plasma was then removed and stored at −80°C until assaying. Plasma serotonin levels were measured in duplicate using a commercial radioimmunoassay kit (¹²⁵I‐Serotonin RIA; form DRG International, Inc., USA) according to the manufacturer's instructions. The final value was determined by the average of two measurements. The person conducting the assays was blinded with respect to sample identity and clinical information.