Serum-separating tube

Serum-separating tubes, also known as serum separator tubes or SSTs, are used in medical clinical chemistry tests requiring blood serum.

SSTs are sometimes called "marble-top tubes" or "yellow topped tubes", referring to the stoppers which are either gold or red-gray. Trademarked versions include Covidien "Corvac" tubes.

They contain a special gel that separates blood cells from serum, as well as particles to cause blood to clot quickly. The blood sample may then be centrifuged, allowing the clear serum to be removed for testing.

These tubes should be used with care when measuring therapeutic endocrine levels because the drug/hormone may diffuse from the serum into the gel, causing a reduction in measured drug level. The gel in SST II tubes (which appears slightly less opaque) is supposed to have less effect on drug levels in serum.