Serum vs. Plasma: A Deep Dive into Their Molecular Makeup and Implications for ELISA

Blood plasma and serum are essential components of blood and have distinct molecular compositions and uses in the medical and research fields. This article aims to elucidate the differences between serum and plasma and respond to the most frequently asked questions.

1. Serum is blood fluid without clotting factors; plasma contains these factors.
2. They're crucial in medical research, including ELISA, to measure various analytes.
3. Different collection methods are used for serum and plasma.

Blood consists of red and white cells, platelets, and a liquid portion, known as plasma. When blood is collected and coagulates, the solid components, such as fibrin and blood cells, are separated, leaving a clear yellowish fluid known as serum.

In contrast, plasma is the fluid that remains when an anticoagulant is used to prevent the blood from clotting. The primary difference between serum and plasma lies in the clotting factors. Plasma contains clotting factors, while serum is essentially plasma without these factors¹.

Both serum and plasma contain water, electrolytes, hormones, waste products, nutrients, gases, and proteins. However, plasma also contains fibrinogen and other coagulation factors that are used in clot formation. A half-life molecule, for instance, could exist differently in plasma and serum due to these differences. Its half-life could potentially be extended or reduced based on the presence or absence of these coagulating factors.

The collection process for serum involves allowing the blood to clot, usually taking about 30 minutes, then centrifuging the sample to separate the serum.

The collection process for plasma involves adding an anticoagulant, such as heparin or EDTA, before the sample is centrifuged. The upper layer of fluid, the plasma, is then collected².

The table below outlines the key steps in collecting serum.

The table below outlines the key steps in collecting plasma.

Plasma and serum zinc are two forms in which zinc can be found in the human body. Zinc is an essential mineral that plays a crucial role in various physiological processes. Plasma zinc refers to the concentration of zinc found in the liquid component of blood, which is obtained when anticoagulants are added to the blood sample. On the other hand, serum zinc refers to the concentration of zinc in the clear, yellowish fluid that remains after the blood has clotted and the cells have been removed.

Both plasma and serum zinc levels are commonly measured to assess a person's zinc status. However, there can be slight differences in their concentrations due to the clotting process involved in serum separation. Plasma zinc levels tend to be slightly higher than serum zinc levels because some zinc is released from the clot during the separation process.

Measuring plasma or serum zinc levels can provide valuable information about an individual's zinc status. Zinc deficiency can lead to various health issues, such as impaired immune function, growth retardation, and delayed wound healing. Conversely, excessive zinc levels can also have detrimental effects on health. Therefore, monitoring plasma or serum zinc levels is important in identifying deficiencies or excesses and determining the appropriate interventions, such as dietary modifications or zinc supplementation, to maintain optimal zinc balance in the body.

Different colored tops of phlebotomy tubes indicate the type of additive in the tube. For plasma tubes with light blue, green, or lavender tops are generally used, as these contain anticoagulants. For serum collection, red or gold (also known as tiger) top tubes are typically used, which lack anticoagulants.

Enzyme measurements can be done in either serum or plasma. However, some enzymes, like potassium-dependent enzymes, can be affected by the anticoagulant in the plasma. Therefore, serum is usually preferred for enzyme assays.

ELISA (Enzyme-Linked Immunosorbent Assay) is a widely used method for measuring analytes in serum and plasma. By utilizing specific antibodies that bind to the target analyte, ELISA enables the detection and quantification of proteins, hormones, and other molecules in these biofluids. Through a series of steps involving antibody binding, washing, enzyme labeling, and signal measurement, ELISA provides a sensitive and reliable technique to assess analyte concentrations in serum and plasma samples. This valuable tool aids in research, clinical, and diagnostic applications, offering insights into the levels of specific molecules and their implications for health and disease.

Lippi, G., & Plebani, M. (2012). Serum versus plasma: which is the better choice?. Clinical Chemistry and Laboratory Medicine, 50(9), 1523-1524. doi:10.1515/cclm-2012-0099

Van Der Meer, P. F., & Pietersz, R. N. (2016). Blood plasma and serum components. Wiley Encyclopedia of Blood Transfusion. doi:10.1002/9781119301141.wbebt032

Lowe, N. M., & Fekete, K. (2009). Methods of assessment