What is the difference between serum and plasma?

Serum is the liquid portion of blood after clotting (SST tubes). Plasma is the liquid portion with anticoagulants added (green, lavender, blue top tubes).

Serum: Blood collected in an SST tube is allowed to clot, then centrifuged. The clear yellow liquid on top (serum) contains no clotting factors. Used for most chemistry panels, serology, and immunology tests.

Plasma: Blood collected in an anticoagulated tube (EDTA, heparin, citrate) is centrifuged without clotting. Plasma retains clotting factors. Used for coagulation studies (PT, INR, PTT), plasma-based metabolic panels, and molecular testing.

The type of tube used determines whether the lab receives serum or plasma.