MLT GlobeSerum vs Plasma: What Is the Difference and Which Test Needs Which?
MLT Globe · 2026-10-02
Serum and plasma both come from blood after the cells are removed, but they are not the same. Knowing the difference helps you choose the right tube and avoid rejected samples.
The key difference
- Plasma is the liquid part of blood that has been prevented from clotting with an anticoagulant. It still contains fibrinogen and other clotting factors.
- Serum is the liquid left after blood has clotted. Fibrinogen and several clotting factors have been used up in the clot.
In short: serum = plasma – fibrinogen and consumed clotting factors.
Tubes
| Sample | Typical tube | Wait before centrifuging |
|---|---|---|
| Serum | Plain (red) or gel/clot-activator (gold) tube | Allow to clot fully, about 30 minutes |
| Plasma | Heparin (green), EDTA (lavender), citrate (light blue), fluoride (grey) | Can be centrifuged straight away |
Which tests use which?
- Serum: many routine chemistry tests, serology and serum protein electrophoresis.
- Heparin plasma: fast chemistry results, for example in emergency departments.
- Citrate plasma: coagulation tests such as PT and APTT.
- EDTA whole blood or plasma: CBC, HbA1c and many molecular tests.
- Fluoride plasma: glucose, because fluoride stops glycolysis.
Small differences in results
Potassium is slightly higher in serum than in plasma because platelets release potassium during clotting – much more so when the platelet count is very high. Total protein is higher in plasma because it still contains fibrinogen.
See every tube colour and its additive on the order of draw page.
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