What is the white substance in bottom of a vial with blood it?

The white substance at the bottom of a vial of blood is a gel used to separate the serum from the red blood cells and plasma during centrifugation. During centrifugation, the gel becomes viscous and forms a barrier between the serum and the other components of the blood, allowing for easy extraction of the serum for testing. This gel is designed to not interfere with the blood sample, ensuring accurate and reliable test results.