Serum tubes are coated with micronised silica particles,
which activate clotting when tubes are gently inverted.
Serum tubes with gel contain a barrier gel that is present
in the bottom of the tube. The specific gravity of this material
lies between the blood clot and the serum. During centrifugation
the barrier gel moves upward to the serum - clot interface,
where it forms a stable barrier separating the serum from
fibrin and cells. Serum may be aspirated directly from the
collection tube, eliminating the need for transfer to another
container. This barrier allows for the stability of certain
parameters in the primary tube under the recommended storage
conditions for up to 48 hours.
Serum tubes with beads contain polystyrene beads, which are
present in the bottom of the tube. The specific gravity of
the beads lies between that of the blood clot and serum. During
centrifugation the beads move upwards to form a layer between
the serum and blood clot.