What does serum mean?
Serum most commonly means the watery part of blood that remains after the blood has clotted. It contains many of the proteins, antibodies, salts, and other substances involved in immune responses and metabolism, which is why it is widely used for laboratory testing. Doctors may measure serum glucose, cholesterol, electrolytes, hormones, or enzyme levels, and serum samples are also collected for research. In an older pharmaceutical sense, serum can mean a preparation made from blood or from the blood of immunized animals. The word originally referred to whey, the watery liquid left when milk is curdled, and its medical sense developed from the relationship between blood serum and the separated liquid component of blood. Although related to plasma, serum is specifically what remains after clotting and therefore differs from plasma in its composition.
nounThe watery portion of blood that remains after the blood has clotted, containing proteins, antibodies, electrolytes, and other substances. In medical and pharmaceutical contexts, it can also refer to a liquid preparation made from blood or from immune animals.
- The watery component of blood separated from the formed elements after clotting, also called blood serum.
- A liquid preparation derived from animal blood or from immunized animals, used for treatment, diagnosis, or research.
- In older and some technical contexts, the watery liquid remaining from milk in the process of making cheese, traditionally called whey.
"The laboratory measured the patient's serum glucose and electrolyte levels."
"A blood sample was centrifuged so that the serum could be separated for testing."
"The doctor recommended a course of immune serum for the patient's condition."
The standard plural is serums, although individual blood specimens are often called serum samples.
"The laboratory received serum samples from several participants."
From the Latin word for whey, serum became the name for the body's protein-rich liquid fraction—and then for some of medicine's most important diagnostic samples.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does serum come from?
The English word entered through French sérum, from Latin serum, an old term associated with whey or a watery fluid. The medical sense developed when the word was applied to the liquid portion of blood after clotting. Other terms built on the same root include serous, serology, serologist, and serosity; the combining form sero- also comes from this Latin source.
Related word forms
How serum is actually used
In modern medical usage, serum usually refers to blood serum and is common in phrases such as serum sample and serum levels. The older sense meaning whey is uncommon outside historical, agricultural, or food-science contexts. Serum should not be confused with plasma, which is obtained from blood before it has clotted.
Easily confused with serum
Serum is the liquid remaining after blood has clotted, whereas plasma is the liquid portion of unclotted blood and contains clotting proteins that serum lacks.