Short answer
Plasma is blood’s circulating fluid; serum is the liquid obtained after blood has clotted. Laboratory plasma is collected with an anticoagulant, while serum preparation allows clotting, which converts fibrinogen into fibrin. That preparation difference changes what remains in the liquid, so the specimen names are not interchangeable. 1
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At a glance
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| Question | Blood plasma | Blood serum |
|---|---|---|
| How is it obtained? | Prevent or delay clotting, then centrifuge | Allow clotting, then separate the liquid |
| What happens to fibrinogen? | It is not consumed by the intended clotting process | It becomes fibrin in the clot |
| What collection detail matters? | Anticoagulant, fill volume, and mixing | Adequate clot formation |
| What can affect analysis? | Additive effects and handling | Clotting and handling |
These distinctions concern preparation as well as composition. 1 2
What each thing is
In circulating blood, plasma surrounds red cells, white cells, and platelets. A laboratory plasma specimen is obtained by separating those components while suppressing clotting. Serum follows a different sequence: withdrawn blood clots, the clot retracts, and liquid can be recovered. Neither term means whole blood, which includes both the fluid and formed elements. 1
Key differences
Clotting is a compositional event, not merely a way to separate cells. Fibrinogen is converted into fibrin, leaving serum different from the starting plasma. Plasma preparation also introduces a variable: anticoagulants such as EDTA, heparin, or citrate can affect protein makeup and analytical uses. Thus, preventing clotting does not mean preserving an entirely unchanged specimen. 1 2
How to tell them apart
Use the documented collection pathway: anticoagulant-treated blood separated without intended clotting indicates plasma; liquid separated after clot formation indicates serum. The limit is that the label alone does not establish specimen quality. 1 2
Where they overlap
Both are blood-derived matrices used for chemical analysis, including trace-element investigations. Both also require careful handling: temperature and repeated freezing and thawing can affect specimen quality. Their shared origin explains why they may contain many of the same molecules, but it does not establish equal concentrations or equivalent analytical performance. 1 2
Edge cases
A specimen intended to be serum may still contain unwanted cellular material if clot formation is incomplete. The supplied serum SOP discussion also describes cellular components entering the liquid when cells in the clot break down after prolonged waiting. Such contamination is a preparation problem; it does not make the intended specimen a properly collected plasma sample. 2
Why the distinction exists
Laboratories distinguish specimen types because collection history helps define the material being measured. The trace-element source identifies collection, preparation, and contamination as important contributors to differing measurements. The SOP source likewise emphasizes documenting handling variables rather than assuming one plasma procedure suits every analytical purpose. 1 2
Common misconceptions
Serum is not simply plasma with blood cells removed: clot formation changes the fluid too. Nor does “plasma” specify a single additive or universally suitable protocol. The EDRN’s choice of EDTA was a choice for its own work, not evidence that every laboratory application should use it. 1 2
Examples
In a hypothetical collection, EDTA-treated blood is centrifuged to obtain plasma. In another, blood is allowed to form a clot before the liquid is separated, yielding serum. These cases differ by preparation, not simply by both ending with a liquid sample. 1 2