Serum vs Plasma: What's the Difference?

Serum and plasma are both the liquid part of blood, separated from the cells by centrifugation. The difference is whether the blood was allowed to clot first. Plasma comes from blood that was kept from clotting with an anticoagulant. Serum is what is left after blood has clotted.

What plasma is

Plasma makes up a little over half the volume of whole blood. It is mostly water, with proteins such as albumin, globulins and fibrinogen, plus clotting factors, electrolytes, glucose, hormones and waste products. To collect plasma, blood is drawn into a tube containing an anticoagulant, then centrifuged so the cells settle to the bottom and the plasma can be removed from the top.

What serum is

Serum is collected from blood drawn into a tube with no anticoagulant. The blood clots, which uses up fibrinogen and other clotting factors to form the clot. After centrifugation, the clear liquid above the clot is serum. In short, serum is plasma without fibrinogen and the other clotting factors consumed during clotting.

Serum vs plasma at a glance

Anticoagulants and tube types

Blood collection tubes are color-coded by additive. The most common conventions:

• Lavender or purple top (EDTA): binds calcium. Used for hematology on whole blood and for plasma where EDTA does not interfere.
• Light blue top (sodium citrate): reversible calcium binding. The standard for coagulation testing.
• Green top (heparin): blocks thrombin. Used for plasma chemistry.
• Red top (no additive) or gold/tiger top (clot activator and gel separator): for serum.

The anticoagulant matters for downstream work. EDTA and citrate chelate metal ions and can interfere with assays that depend on calcium or magnesium. Heparin can inhibit some enzymes, including the polymerase used in PCR.

How to prepare serum and plasma

Plasma:

1. Collect blood into an anticoagulant tube and invert gently several times to mix.
2. Centrifuge at about 1,000 to 2,000 × g for 10 minutes, ideally within an hour of collection.
3. Pipette the plasma off the top without disturbing the thin buffy coat of white cells and platelets.
4. For platelet-poor plasma, centrifuge the plasma a second time and take the upper portion.

Serum:

1. Collect blood into a tube without anticoagulant.
2. Let it clot upright at room temperature for about 30 minutes.
3. Centrifuge at about 1,000 to 2,000 × g for 10 minutes.
4. Pipette the serum off the top, avoiding the clot.

Always follow the assay or tube manufacturer's instructions, since recommended times and forces vary. Centrifuge settings should be given in × g, not RPM, because the same speed produces different forces in different rotors.

Which should you use?

Use whatever the assay was validated for. Many ELISA kits list both serum and plasma as accepted sample types but give different expected ranges for each, so the two should not be mixed within a study. For coagulation work, citrated plasma is required. For antibody detection and most clinical chemistry panels, serum is the usual choice.

Storage

Separate serum or plasma from cells promptly, aliquot it to avoid repeated freeze-thaw cycles, and store at -20 °C for short periods or -80 °C for longer-term storage. Label each aliquot with the sample type and anticoagulant.

Equipment for sample preparation

A swing-out benchtop centrifuge covers routine serum and plasma separation. The ELMI CM-8S reaches 4,500 RPM and 3,500 × g, well above the 1,000 to 2,000 × g these protocols call for; it is sold as the centrifuge only, so order a rotor sized for your tubes with it. Compare options in laboratory centrifuges and centrifuge tubes. For running samples, see the ELISA kits collection and sample diluent.