Systematic differences among the sites of sample collection
Population (e.g., racial mixtures, differences in sex and age, ethnicity)
Diurnal variations (i.e., time of collection)
Homeostasis (e.g., fed or fasting state)
Stress of donors
Comorbidity (diabetics versus hypertensive patients)
Collection container (red top vs. tiger separator or type of plasma)
Time from processing to freezing
Temperature and length of storage
Freeze thaw cycles
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