Differences Between Serum and Plasma Tubes Explained
, by Andrew Odgers, 9 min reading time
Sample Types
Differences Between Serum and Plasma Tubes Explained
Serum and plasma are both liquid components of blood separated from cells, but they are produced differently and have different compositions. Understanding the difference between serum and plasma tubes and why it matters for specific tests is essential pre-analytical knowledge for phlebotomists and laboratory staff.
Serum and plasma: the key difference
How each sample type is produced
What is serum?
Serum is the liquid that remains after blood has been allowed to clot and the clot has been removed by centrifugation. It is produced in tubes with no anticoagulant: plain red tubes and gold SST tubes. As blood clots, fibrinogen is converted to fibrin and consumed in the clot, along with platelets and coagulation factors. Serum therefore contains everything that was in the plasma except fibrinogen and the clotting factors consumed during clotting. Serum is the standard sample for most routine biochemistry, immunology, and serology testing.
What is plasma?
Plasma is the liquid component of blood that has been anticoagulated to prevent clotting. It is produced in tubes containing an anticoagulant: lithium heparin for most biochemistry plasma, EDTA for haematology applications, and sodium citrate for coagulation. Because clotting is prevented, plasma retains fibrinogen and coagulation factors. Plasma can be produced more quickly than serum because there is no need to wait for clotting to complete before centrifugation.
Why it matters for laboratory tests
Many routine biochemistry tests can be run on either serum or plasma with comparable results. However, some analytes behave differently in serum and plasma. Potassium is higher in serum than plasma because platelet activation during clotting releases potassium into the serum. Coagulation tests must be performed on plasma specifically because they depend on the intact coagulation factors that are consumed during serum production. Some specific assays specify serum to avoid interference from anticoagulant additives present in plasma tubes.
Tube selection for serum and plasma
Which tube to use for common test requests
Serum tubes: gold SST and plain red
The gold SST with gel separator is the standard serum tube for most routine biochemistry, immunology, and serology. The clot activator speeds clotting and the gel creates a physical seal between serum and clot after centrifugation. The plain red tube without additive is used where the gel may interfere with specific assays, including some drug monitoring tests and specialist immunology. Both tubes require time to clot before centrifugation; typically 30 minutes at room temperature for SST and longer for plain red.
Plasma tubes: green lithium heparin
Lithium heparin tubes produce plasma rapidly, without the wait for clotting. They are the standard plasma tube for routine biochemistry when results are needed urgently and for tests that specify plasma. Lithium heparin must not be used for haematology tests (use EDTA) or coagulation tests (use citrate). The additive is lithium heparin, so results from this tube must not be used for lithium assays.
Coagulation plasma: light blue sodium citrate
Coagulation tests require plasma from citrate tubes specifically. The citrate anticoagulates by calcium chelation at a fixed 1:9 ratio. The precise ratio is essential for accurate coagulation test results. No other plasma tube type is acceptable for coagulation testing. Fill the tube to exactly the marked volume line.
Serum and plasma tubes in stock
SST, lithium heparin, and citrate tubes with next-day delivery
Charles Medical supplies serum separator, lithium heparin, and sodium citrate tubes across all standard volumes. Next-day UK delivery.
For tube colour identification, see Understanding Blood Tube Colours and What Each One Means.
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Back to the Blood Collection Tubes Knowledge Hub
This article is part of our complete blood collection tube knowledge base, covering tube types, colours, additives, order of draw, pre-analytical errors, disposal, and everything phlebotomists and laboratory staff need to know.
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Related guides in this hub
Understanding Blood Tube Colours and What Each One Means identifies which colours correspond to serum and plasma tubes. Why the Right Tube Matters for Each Blood Test maps specific tests to the correct sample type. And The Correct Order of Draw covers how serum and plasma tube positions interact in the sequence.
Frequently asked
Serum and plasma questions answered
Can I use a plasma tube instead of a serum tube for biochemistry?
For many routine biochemistry tests, lithium heparin plasma produces comparable results to serum and can be used when faster turnaround is needed. However, some specific assays specify serum to avoid anticoagulant interference, and the laboratory must accept the plasma tube for the requested tests. Confirm with your laboratory which tests they accept on lithium heparin plasma.
Why is potassium higher in serum than plasma?
Platelet activation during the clotting process releases potassium from platelets into the serum. This produces a systematically higher potassium result in serum compared to plasma from the same blood sample. Laboratories are aware of this difference and their reference ranges account for the sample type used. Using a plasma tube for potassium in a laboratory that uses serum reference ranges may produce a misleadingly low result.
What is the minimum clotting time before centrifuging an SST tube?
30 minutes at room temperature is the standard recommended clotting time for SST tubes. Centrifuging before clotting is complete produces fibrin strands in the serum that can block analyser probes and produce erroneous results. In patients on anticoagulant therapy, the clotting time may be longer and 45 to 60 minutes may be needed.
Can a heparin plasma tube be used for a lithium level?
No. Lithium heparin tubes contain lithium as the counter-ion for the heparin additive. Using a lithium heparin tube for a plasma lithium measurement produces a falsely elevated result. Use a plain EDTA or plain tube for lithium levels, following the laboratory's stated requirements for this assay.