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Platelet activation and subsequent aggregation is a vital component of atherothrombosis resulting in acute myocardial infarction. Therefore, quantifying platelet aggregation is a valuable measure for elucidating the pathogenesis of acute coronary syndromes (ACS). Circulating platelet-monocyte conjugates (PMC) as determined by flow cytometry (FCM) are an important measure of in vivo platelet aggregation. However, the influence of sample handling on FCM measurement of PMC is not well-studied. The changes in FCM measurement of PMC with variation in sample handling techniques were evaluated. The stability of PMC concentrations over time with changes in fixation and immunolabeling intervals was assessed. The effect of Time-to-Fix and Time-to-Stain on FCM PMC measurements was investigated in five healthy volunteers. Time-to-Fix (i.e., interval between phlebotomy and sample fixation) was performed at 3, 30, and 60 min. Time-to-Stain (i.e., time of fixed sample storage to staining) was performed at 1, 24, and 48 h. Increasing Time-to-Stain from 1 to 24 or 48 h resulted in lower PMC measures (p < 0.0001). A statistically significant difference in PMC measurement with increasing Time-to-Fix was not observed (p < 0.41). Postponement of sample staining has deleterious effects on the measurement of PMC via FCM. Delays in immunolabeling of fixed samples compromised measurement of PMC by 30% over the first 24 h. Staining/FCM should be completed within an hour of collection.

Elevated measures of matrix metalloproteinases (MMPs) are associated with acute myocardial infarction (MI), but it is not known how long these changes persist post-MI or if these measures differ between atherothrombotic versus non-atherothrombotic MI. MMPs-2, 3, and 9 were measured in 80 subjects with acute MI (atherothrombotic and non-atherothrombotic MI) or stable coronary artery disease (CAD). Measurements were made at, the time of acute MI, and > 3-month following acute MI (quiescent phase). Outcome measures were compared between groups and between time of acute MI and quiescent post-MI follow-up using Wilcoxon's and repeated measures analysis of variance. Forty-nine subjects met the criteria for acute MI with clearly defined atherothrombotic (n = 22) and non-atherothrombotic (n = 12) subsets. Fifteen subjects met criteria for stable CAD. MMP-3 was higher in acute MI versus stable CAD subjects at the time of acute MI: (453 vs. 217 pg/mL, p = 0.010) but not at quiescent phase follow-up (p > 0.05). MMP-9 was higher in acute MI versus stable CAD subjects at the time of acute MI: (412 vs. 168 pg/mL, p = 0.002) but not at the quiescent phase follow-up (p > 0.05). MMP-9 was higher at the time of acute MI versus quiescent phase follow-up in acute MI (412 vs. 213 pg/mL, p = 0.001) and atherothrombotic MI specifically (458 vs. 212 pg/mL, p = 0.001). No difference in MMP-2, 3, or 9 was observed between atherothrombotic versus non-atherothrombotic MI subgroups. MMPs-3 and 9 are significantly elevated in acute MI verses stable CAD subjects at time of acute MI but not different at quiescent phase follow-up. MMP-9 is elevated at the time of acute MI and specifically in acute atherothrombotic MI at time of MI versus quiescent phase follow-up.