Effects of Delayed Laboratory Processing on Platelet Serotonin Levels

Tryptophan hydroxylase-1 located in the gastrointestinal tract converts tryptophan into serotonin. Available peripheral serotonin is then secreted into the vascular system and transported into the platelets for storage by the serotonin transporter gene (SERT; Cote, Fligny, Fromes, Mallet, & Vodjdani, 2004). In the periphery, the majority of the serotonin is stored inside the platelets. Stored platelet serotonin is released and bound to available serotonin receptors. The effects of this peripheral platelet serotonergic system produce marked physiological changes in the cardiovascular system. A number of these potential physiological changes are also associated with acute coronary syndrome (ACS), including (a) morphological platelet changes, (b) serotonin-mediated platelet aggregation, and (c) thrombus formation (Cote et al., 2004; Parissis et al., 2007; Steptoe & Brydon, 2008). Knowledge about the relationship between peripherally available stored platelet serotonin levels and ACS may help identify patients at risk of adverse cardiac events associated with serotonin-mediated platelet aggregation. Measurement of platelet serotonin levels, however, may pose difficulties for researchers, especially in the hospital setting. Established guidelines for the handling, processing, and storage of human blood samples for measurement of platelet serotonin levels are available, yet they are not definitive regarding time to processing (Immuno-Biological Laboratories [IBL], 2009). The standardized IBL Immuno-Biological Laboratory guidelines require “immediate” sample processing after blood sample collection followed by either conducting of an assay or frozen storage until the assay can be conducted. “Immediate” processing in this case is not defined, and is, therefore, open to interpretation. While immediate sample processing is easy to achieve in a laboratory site or with a stable population, it is more difficult to accomplish in a hospital setting where there may be less control, for example, when patients are hospitalized for ACS. It is not always feasible to transport a blood sample immediately from the catheterization laboratory or the emergency department to a research lab. As a result, processing times in patients admitted around the clock in a hospital setting for ACS may vary from sample to sample. While authors have reported problems with measurement of platelets themselves (e.g., unreliable laboratory results due to the effects of delayed time to processing after blood sample collection, exposure to anticoagulant during blood sample collection, storage temperature of the blood), we found no published papers on the effect of time to processing on platelet serotonin levels. It is, therefore, possible that time to processing influences the in vitro diagnostic quantitative determination of platelet serotonin. We conducted the present study as part of the preparation for a larger ACS trial in order to evaluate the impact of time to processing on the stability of blood samples collected for platelet serotonin analysis.