Donor Platelet Activation Varies by Time of Day

Platelets, are anucleate cells produced by megakaryocytes in the bone marrow that circulate in the bloodstream for about 10 days. Circulating platelets play a central role in the hemostatic response to vascular injury. They are also involved in maintaining vascular integrity and contributed to immune and inflammatory responses.

Platelet donors, like whole blood and plasma donors, are considered to be one of the healthiest groups of the general population. A number of factors may influence the in vivo platelet activation state in healthy individuals. In particular, platelet activation has been shown to vary according to the time of day, with the highest measured levels during the morning hours.

Hematologist at the Akershus University Hospital (Lørenskog, Norway) collected whole blood samples from 86 platelet donors presenting at their Blood Bank between April 2019 and February 2020. The mean age was 50.8 ± 9.7 years with 56% of donors were female. Samples were collected from the attached sampling bags (containing 42 mL whole blood collected immediately before plateletpheresis) using standard vacutainers. A blood sample was collected into a citrated tube for flow cytometric determination of platelet surface P-selectin expression.

For flow cytometry, 100 µL of citrated blood (diluted 1:20 with phosphate-buffered saline [PBS] containing 0.1% bovine serum albumin [BSA], pH 7.4) was incubated at room temperature with FITC-conjugated anti-CD61 (5 µL) and PE-conjugated anti-CD62P (10 µL) for 15 minutes. Background fluorescence was assessed by labeling diluted samples with 10 µL PE-conjugated isotype control antibody. After incubating, the samples were diluted to 4 mL with PBS/BSA and analyzed immediately on a FACS Canto II flow cytometer (BD Bioscience, San Jose, CA, USA). The percentage of activated platelets (i.e., CD61 and CD62P double-positive platelets) was obtained from a total of 10,000 recorded events with the CD61+ gate.

The investigators reported that the percentage of activated (i.e., P-selectin-expressing) platelets in pre-donation blood samples ranged from 0.2% to 7.5% (1.85 ± 1.57%; median, 1.3%; IQR, 0.8–2.3%). Twenty-seven donors (32%) had less than 1.0% activated platelets, and five donors (6%) had more than 5.0% activated platelets, indicating considerable donor variability in the levels of circulating activated platelets.

The percentage of activated platelets was significantly and inversely correlated with the collection time (i.e., the time of day blood samples were collected) and positively correlated to mean platelet volume (MPV). A weaker positive correlation was also observed with ABO blood group. Analysis of the collection time as a categorical variable showed a greater degree of activated platelets in samples collected between 08:00 and 10:00 than in samples collected during the hours after 10:00 to 14:00 (2.5 ± 1.8 versus 1.1 ± 0.74).

The authors concluded that their results showed that platelet activation was significantly and inversely associated with the collection time by univariate analysis and after adjustment for covariates. The study also showed that blood group A was associated with higher platelet activation. This work may have implications for optimizing the timing of platelet donation, if, for example, the temporal variation in platelet activation at the time of donation can impact the final heterogeneity of the stored platelet concentrates and, potentially, transfusion outcomes. The study was published on June 3 2022 in the Journal of Blood Medicine.

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