Overview

Serum is the pale yellow, cell‑free liquid that separates from coagulated blood. It contains many of the soluble constituents of blood but lacks the cellular elements and the major clotting protein fibrinogen that are present before clotting. In routine laboratory and research contexts the term distinguishes material derived from clotted samples from plasma obtained by preventing coagulation.

Composition and characteristics

Serum is a complex fluid that carries molecules reflecting physiology and disease. Key categories of constituents include:

  • Proteins such as albumin, immunoglobulins and other transport or regulatory proteins.
  • Electrolytes and small solutes that indicate metabolic and hydration status.
  • Immune components including antigens and antibodies; these are central to many diagnostic assays.
  • Hormones and other signaling molecules measured for endocrine evaluation.
  • Exogenous substances such as drugs and traces of microorganisms or their products when infection is present.

Preparation and laboratory handling

To obtain serum, a blood sample is allowed to clot and then processed by centrifugation to separate the liquid from the clot and cells. In contrast, collecting blood into anticoagulant tubes yields plasma, which still contains fibrinogen and some clotting factors. The absence of fibrinogen in serum is a defining distinction from plasma and affects which laboratory tests are appropriate.

Uses in medicine and research

Serum is widely used in clinical laboratories and biomedical research. Common applications include:

  1. Serological assays for antibodies and antigens in infectious and autoimmune disease testing.
  2. Biochemical measurements such as electrolytes, enzymes and hormone levels used in diagnosis and monitoring.
  3. Specialized testing like blood typing or seroepidemiology studies.
  4. Research and cell culture: serum supplements support growth of some cell types and is used in protocols for embryonic stem cell maintenance together with defined factors such as the cytokine leukemia inhibitory factor.

Clinical laboratories use serum in many diagnostic tests because of its stability and the absence of clotting proteins that can interfere with some assays.

Distinctions, safety, and practical notes

Unlike plasma, serum cannot be used to evaluate clotting factor activity because it is generated after clot formation. When handling serum, biosafety precautions are important since it can contain infectious agents; testing and storage practices are designed to reduce risk. For long‑term preservation, serum samples are typically refrigerated for short periods or frozen for archival storage. Researchers and clinicians choose serum or plasma depending on the analytes of interest and the requirements of specific tests.

For more background on blood processing and terminology consult resources on blood cells and clotting proteins such as fibrinogen.