Blood collection is a routine procedure in healthcare, yet the preparation of a sample for laboratory testing is a precise process that directly affects diagnostic accuracy. Whole blood is composed of cellular material, which includes red blood cells, white blood cells, and platelets, and a liquid component that suspends these cells. For many laboratory tests, the cellular components must be separated from the fluid portion to allow for chemical analysis. The difference lies in whether the resulting liquid is plasma or serum, and a phlebotomist must ensure the correct sample type is collected for the specific test ordered, as the distinction is fundamental for accurate results.
Fundamental Distinction: Plasma Versus Serum
The most significant difference between plasma and serum is the presence of fibrinogen and other coagulation factors. Plasma is the liquid component of blood that retains all of its clotting factors, meaning the blood has been prevented from clotting naturally. This sample type is necessary for tests that analyze the coagulation cascade, such as prothrombin time (PT) or activated partial thromboplastin time (aPTT). Plasma makes up approximately 55% of the total blood volume and is composed of about 92% water, with the remaining percentage consisting of proteins, glucose, electrolytes, and hormones.
Serum is the liquid that remains after blood has been allowed to clot naturally. During this clotting process, fibrinogen is converted into the insoluble protein fibrin, which forms the meshwork of the clot. The clotting factors are consumed and entrapped within this solid clot, so the remaining fluid, the serum, is essentially plasma minus the fibrinogen and other clotting proteins. Serum is the preferred sample for many chemistry, serology, and immunology tests because the absence of clotting factors can prevent interference with certain assays.
The Procedural Key: Collection Tube Additives
For a phlebotomist, the primary method for distinguishing between a future plasma or serum sample is the color-coded cap of the collection tube, which indicates the presence or absence of chemical additives. The choice of tube is made based on the laboratory test ordered. Tubes designed to yield plasma contain an anticoagulant, a substance that chemically prevents the blood from clotting by interfering with the coagulation cascade.
Plasma-yielding tubes include the lavender-topped tube (EDTA) and the green-topped tube (heparin). EDTA works by binding to calcium ions, a mineral necessary for clotting, while heparin inhibits thrombin. The light blue-topped tube contains sodium citrate and is specifically used for coagulation testing, as the anticoagulant ratio is carefully calibrated. By chemically blocking the clotting process, these additives ensure the fluid portion retains all its original clotting factors, resulting in a plasma sample upon separation.
Conversely, tubes intended to yield serum either contain no additive (plain red-topped tube) or contain a clot activator. Gold or tiger-topped tubes (SSTs) contain a clot activator, often micronized silica particles, and a polymer gel. The silica accelerates the natural clotting process, ensuring the sample yields serum quickly, while the gel acts as a physical barrier during centrifugation. The absence of an anticoagulant allows the blood to clot completely, consuming the fibrinogen and leaving behind the serum. The tube’s color and its internal chemistry are the phlebotomist’s first line of distinction.
Visual Identification After Processing
While the tube color guides the phlebotomist during collection, the final distinction is confirmed visually after the sample is processed in the laboratory using a centrifuge. Centrifugation spins the blood at high speed, separating the components by density.
In a tube that contained an anticoagulant, the plasma sample separates into three distinct layers. The heaviest layer, consisting of red blood cells, settles at the bottom, followed by a thin, whitish layer called the buffy coat, which contains the white blood cells and platelets. The top layer is the plasma, a clear, typically straw-colored fluid that sits above the buffy coat. The defining visual characteristic of a plasma sample is the complete absence of a solid fibrin clot at the bottom of the tube.
In contrast, a serum sample, collected in a tube with a clot activator or no additive, will have a solid, compact clot of red cells and fibrin at the bottom. The liquid portion above this clot is the serum, which is also a clear, straw-colored fluid. The presence of the solid clot is the definitive visual marker for a serum sample after centrifugation, confirming that the fibrinogen was consumed in the clotting process. While both serum and plasma are generally clear and yellowish, their appearance can be altered by conditions like lipemia (cloudiness from high fat content) or hemolysis (redness from ruptured red cells), but the presence or absence of the solid clot remains the key visual difference.