CCMA · Phlebotomy · 7 min read
Serum, Plasma, Centrifuging and Specimen Handling
- Differentiate between serum and plasma in terms of collection tube additives and coagulation mechanisms.
- Apply safe and accurate centrifugation techniques, including tube balancing, timing, and gel separator handling.
- Follow standard protocols for specimen aliquoting, light and temperature protection, and time-sensitive transport.
Understanding Blood Components: Serum Versus Plasma
In the outpatient clinic, a medical assistant frequently collects and prepares blood samples ordered by the supervising provider. Whole blood consists of formed elements suspended in liquid plasma. The formed elements comprise erythrocytes, leukocytes, and thrombocytes. When blood circulates inside the body or sits in a collection tube with an anticoagulant, these cellular elements remain surrounded by this fluid matrix.
The primary distinction between serum and plasma comes down to clotting. Plasma is the liquid portion of anticoagulated blood that contains clotting factors, including fibrinogen. When a tube contains an anticoagulant, the blood does not clot, so the resulting fluid retains these soluble proteins. Blood collected in tubes containing anticoagulants yields plasma after centrifugation. For instance, lavender-top tubes contain EDTA and yield whole blood or plasma. Light blue-top tubes contain sodium citrate and yield plasma for coagulation testing. Green-top tubes contain heparin and yield plasma for chemistry testing.
Serum, on the other hand, is the liquid portion of blood obtained after clotting has occurred and lacks fibrinogen. During the coagulation process, soluble fibrinogen is converted into insoluble fibrin strands that trap the formed elements into a solid clot. Blood collected in tubes with clot activators or without additives yields serum after centrifugation. Plastic red-top tubes contain a clot activator to facilitate serum production, whereas glass red-top tubes contain no additives and allow natural blood clotting. Serum separator tubes (SST) contain both a clot activator and a thixotropic gel barrier to separate the clot from the liquid.
| Specimen Type | Tube Types / Additives | Clotting Factor Status | Coagulation Process |
|---|---|---|---|
| Plasma | Light blue (sodium citrate), Green (heparin), Lavender (EDTA) | Contains clotting factors, including fibrinogen | Anticoagulants prevent clotting; cells remain unbonded to fibrin. |
| Serum | Red-top plastic (clot activator), Red-top glass (no additive), SST (clot activator + gel) | Lacks fibrinogen | Fibrinogen converts into insoluble fibrin strands, forming a solid clot. |
Common mistakes
- Placing a chilled specimen directly onto solid ice cubes instead of an ice-and-water slurry, which can cause local freezing and red blood cell hemolysis.
- Centrifuging a standard serum tube immediately after phlebotomy before allowing it to clot for the required 30 to 60 minutes at room temperature, leading to fibrin strand formation.
- Re-centrifuging a gel separator tube without laboratory authorization, which compromises the barrier and alters test results.
- Delaying separation of specimens that require serum or plasma beyond the laboratory’s specified limit can alter results, including decreased glucose and increased potassium, magnesium, or lactate dehydrogenase.
Key points
- Whole blood contains formed elements (erythrocytes, leukocytes, thrombocytes) suspended in plasma.
- Plasma contains clotting factors such as fibrinogen and is obtained from anticoagulated tubes, while serum lacks fibrinogen and is obtained from clotted blood.
- Routine serum tubes generally need 30 to 60 minutes to clot at room temperature before centrifugation; follow tube-specific clotting times and any test-specific temperature requirements.
- Centrifuges must always be balanced with tubes of compatible size and equal weight, and the lid must remain closed until the rotor stops completely without manual braking.
- Light-sensitive analytes (such as bilirubin, beta-carotene, vitamin A, and vitamin B6) require amber tubes, foil wrapping, or opaque bags.
- Chilled specimens require an ice-and-water slurry rather than bare ice cubes, whereas cold agglutinins and cryoglobulins must be maintained at approximately 37°C.
- Separate specimens that require serum or plasma within the laboratory’s specified time limit, commonly 2 hours; delayed separation can increase potassium, magnesium, and lactate dehydrogenase and decrease glucose through ongoing cellular metabolism.
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