1 ml of serum from all species (no primates or avian) in red top tube; shipped chilled overnight from Monday to Thursday.

Collect blood in a non-additive clot tube to allow clot formation. Centrifuging serum samples before a complete clot forms may result in the presence of fibrin. Ensure complete clot formation prior to centrifugation of samples. Usually within the next 4 hours to avoid loss of progesterone.