What is the purpose of the blue top 3.2% sodium citrate tube used in recollection?

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Sodium citrate in the blue top tube acts as an anticoagulant by binding calcium, a mineral essential for the blood clotting process. By chelating calcium, it prevents clot formation, keeping the blood in liquid form so plasma-based tests can be run and the cells remain in a stable suspension for accurate counting and morphology assessment. It does not fix cells, it does not raise calcium, and it does not acidify the sample. This anticoagulant effect is why the blue top tube is used when recollecting blood for coagulation studies and for preserving the sample in a state suitable for accurate measurements. Note that for routine complete blood counts, EDTA is typically preferred for best morphology preservation.