Serum vs Plasma: What’s the Difference?

Serum and plasma are both liquid parts of blood used for laboratory testing, but they are not the same specimen. The key difference is what happens before centrifugation: plasma comes from blood that has been prevented from clotting, while serum is collected after the blood has been allowed to clot.

That distinction changes which proteins remain in the sample, which collection tube is used and which tests the laboratory can perform.

Serum vs plasma at a glance

What exactly is plasma?

Plasma is the liquid portion obtained when blood is collected in a tube that prevents coagulation and the blood cells are then separated, usually by centrifugation. Because the sample has not been allowed to form a clot, soluble clotting components such as fibrinogen remain in the plasma.

The anticoagulant is not an interchangeable ingredient. Different laboratory tests require different collection systems because additives can affect the reaction being measured. Citrate plasma, for example, is used for many coagulation studies because the collection and test system is designed around a controlled relationship between the blood and citrate. Other plasma-based assays may use heparin or EDTA according to the method validated by the laboratory.

This is why the phrase “a plasma sample” is not enough to choose a tube. The specific test request determines the acceptable plasma type.

What exactly is serum?

Serum is the liquid remaining after blood has been allowed to clot and the clot and cells have been separated. During clot formation, fibrinogen is converted as part of the coagulation process and becomes associated with the clot, so serum does not contain the same functional clotting system as anticoagulated plasma.

Serum is widely used for laboratory investigations because many biochemical, immunological and serological methods have been validated for it. A serum tube may contain a clot activator and, in some collection systems, a gel that helps form a barrier between the serum and cells after centrifugation.

The important point is that serum is not simply “better” or “cleaner” plasma. It is a different analytical matrix created by a different pre-analytical process.

Why does the difference matter in the laboratory?

The specimen matrix can affect the validity of a result. Laboratory analysers, reagents and reference intervals are developed and verified for specified sample types. If an assay is validated for serum, substituting a different plasma type without evidence that the method accepts it can introduce analytical problems.

The clearest example is coagulation testing. Tests that assess the coagulation system require an appropriate plasma specimen because the laboratory needs the relevant clotting components to remain available for the test reaction. Serum has already undergone clot formation, so it is unsuitable for that purpose.

For routine chemistry, the distinction is more method-dependent. Some assays accept both serum and certain types of plasma, while others specify one matrix. Laboratories therefore follow the analyser manufacturer’s instructions together with their own validated procedures rather than assuming that serum and plasma are interchangeable.

Does plasma give faster results than serum?

Plasma can sometimes reduce one pre-analytical delay because the laboratory does not need to wait for clot formation before centrifugation. Serum preparation includes a clotting step, so there is normally a period between collection and centrifugation to allow an adequate clot to form.

That does not mean every plasma test has a faster turnaround time. Overall turnaround depends on transport, centrifugation, analyser workflow, batching, urgency, repeat testing and result validation. The specimen type is only one part of the process.

For students, the useful principle is simple: plasma avoids a clotting wait, but the laboratory chooses the specimen based on analytical suitability, not speed alone.

Can serum and plasma results be different?

Yes. The clotting process changes the sample, and the collection additives used for plasma can also influence what is measured. Research comparing paired serum and plasma specimens has shown that concentrations of some proteins, metabolites and other analytes can differ between the two matrices.

This matters particularly when a laboratory establishes a method and its reference interval. A result from one specimen type should not automatically be assumed to be directly equivalent to a result from another specimen type unless the method supports that comparison.

It is another reason to use the specimen named on the test request and laboratory instructions rather than treating “blood” as a single universal sample.

Which tube should be used for serum or plasma?

There is no single colour rule that can safely be applied across every collection system worldwide. Tube-cap colours can vary by manufacturer and local policy, and similar-looking tubes can contain different additives.

For serum, laboratories use collection tubes designed to allow clot formation, sometimes with a clot activator and separator gel. Plasma requires a tube containing the anticoagulant specified for the test, such as citrate, heparin or EDTA.

The practical rule is to follow the local laboratory collection guide and the test-specific instructions. Never choose a tube only from a colour remembered from another hospital, country or manufacturer.

The key difference to remember

If you only remember one concept, use this:

– Plasma: blood is prevented from clotting before the liquid is separated.

– Serum: blood is allowed to clot before the liquid is separated.

That one pre-analytical difference explains why plasma retains fibrinogen and why serum and plasma cannot always be used for the same tests.

Related reading

For the collection step itself, see [LINK: blood-sample-collection-explained — how blood samples are collected]. For the next stage, see [LINK: what-happens-to-your-blood-sample-in-the-lab — what happens to your blood sample in the laboratory].