Blood analysis plays a crucial role in understanding human health and diagnosing medical conditions. Whole blood is often separated into its constituent parts for detailed examination. Plasma and serum are two liquid portions derived from blood, each with distinct compositions suitable for different diagnostic purposes.
Understanding Plasma and Serum: Basic Differences
Plasma represents the liquid component of blood, making up about 55% of its total volume. It consists primarily of water (around 90%), along with dissolved proteins, salts, hormones, nutrients, and waste products. Plasma contains clotting factors, including fibrinogen, a protein essential for blood clot formation.
Serum, in contrast, is the liquid portion of blood that remains after the blood has clotted. It is essentially plasma from which the clotting factors, notably fibrinogen, have been removed. During the clotting process, fibrinogen is converted into fibrin, which forms the meshwork of a clot. Serum contains many of the same proteins, electrolytes, antibodies, and hormones as plasma, but it lacks components directly involved in coagulation.
The Collection Process: How Each is Obtained
To obtain plasma, blood is drawn into a tube that contains an anticoagulant. Common anticoagulants include ethylenediaminetetraacetic acid (EDTA), heparin, and sodium citrate. These work by binding to calcium ions or inhibiting key enzymes in the clotting cascade. After the blood is collected with an anticoagulant, the tube is centrifuged, rapidly spinning the sample to separate components by density. The cellular components, such as red and white blood cells and platelets, settle at the bottom, leaving plasma as the top layer.
Conversely, to obtain serum, blood is collected in a tube that does not contain an anticoagulant, or one that may contain a clot activator. The blood is then allowed to clot naturally, a process that typically takes 15 to 60 minutes at room temperature. Clot activators can be added to accelerate this natural clotting process. Once a solid clot has formed, the tube is centrifuged, which separates the clot (containing fibrin and blood cells) from the liquid serum. The serum is then drawn off for analysis.
Key Reasons for Plasma Preference in Medical Applications
Plasma is often preferred in medical applications due to several advantages. Its speed of results is a key factor, particularly in emergency situations. Plasma collection does not require waiting for blood to clot, allowing samples to be centrifuged and analyzed almost immediately, which can shorten the turnaround time for critical diagnostic tests. This rapid processing is especially beneficial for patients in critical care or emergency departments where swift medical decisions are necessary.
Plasma preserves certain analytes in their native state. Some substances in the blood can be consumed or altered during the clotting process that occurs when preparing serum. For example, glucose levels can decrease in a serum sample over time as blood cells continue to metabolize it, making plasma preferred for glucose determination. By preventing clotting, plasma ensures analyte stability and accurate measurements.
Plasma is essential for performing coagulation studies, which assess a patient’s blood clotting ability. Tests like Prothrombin Time (PT) and Activated Partial Thromboplastin Time (APTT) measure clotting factor function. Since plasma retains all clotting factors, it is the appropriate sample type for evaluating bleeding disorders or monitoring anticoagulant therapies.
Plasma yields a larger sample volume from the same amount of whole blood compared to serum, as some volume is lost within the clot during serum preparation. This higher yield is useful when blood volume is limited. Using plasma also prevents interferences from incomplete clotting or fibrin strands in serum samples, leading to cleaner and more reliable test results.