Serum-separating tube
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Serum-separating tubes also known as a serum separator tubes, Tyco Healthcare "Corvac" (trademark) tubes, or SSTs. They are used in medical clinical chemistry tests requiring blood serum, and contains a special gel that separates blood cells from serum, as well as particles to cause blood to clot quickly.
These tubes cannot be used for therapeutic drug levels because the drug will diffuse from the serum into the gel, causing a reduction in measured drug level. SST stoppers are either gold or red-gray (they are sometimes called "marble-top tubes" for this reason).
The gel in SST II tubes (which appears slightly less opaque) is supposed to have less effect on drug levels in serum.