Diurnal variation in platelet aggregation with the PFA-100 platelet function analyser

Background: Myocardial infarction is commoner in the morning, and previous small studies suggesting diurnal variation in platelet aggregation have bee n limited to optical aggregometry with platelet-rich plasma and low shear. This phenomenon was studied using whole blood at high shear rates. Method: Fifteen healthy volunteers were venesected at 0800 hrs supine in be d immediately before rising, at 0830 hrs 30 min after rising, at 1200 hrs a nd 1700 hrs. Samples underwent the high shear method of PFA-100 using addit ional chemical agonists of collagen with ADP or collagen with epinephrine. PFA-100 results are reported as closure time of the experimental aperture i n seconds, a longer time indicating less platelet aggregation. Results: With both epinephrine and ADP, a non-significant shortening of clo sure time was seen on rising. Subsequently, with both agonists the closure time lengthened through the day. With ADP the difference was small (medians 0830 hrs: 85 s, 1700 hrs: 87.5 s) but statistically significant (p = 0.03) . With epinephrine it was much more marked (medians 0830 hrs: 114.3 s, 1700 hrs: 140.5 s) and highly significant (p = 0.002). Conclusions: These findings demonstrate a diurnal rhythm in platelet functi on using whole blood at high shear rates. This is likely to be more applica ble to the in vivo situation than previously reported optical aggregometry studies.