serum
Meaning of serum: Serum is the clear fluid left after blood clots, containing proteins, hormones, and other dissolved substances but no cells or clotting factors.
Definition of serum
Serum is the clear, pale yellow fluid portion of blood that remains once blood has been allowed to clot and the clot, along with the blood cells, is removed. It contains water, proteins, hormones, electrolytes, and other dissolved substances, but it lacks the clotting proteins that were used up forming the clot.
Serum differs from plasma, which is the fluid portion of blood obtained when clotting is prevented, typically with an anticoagulant, so plasma still contains the clotting factors that serum does not.
Because serum carries so many circulating substances, it is widely used in laboratory testing to measure hormone levels, antibodies, electrolytes, and many other markers, and the specific substance being measured, rather than serum itself, determines what a given test result indicates about a person's health.
Used in a sentence
A serum sample was sent to the laboratory to measure hormone levels.
Good to know
Serum and plasma are often used loosely as though interchangeable, though serum lacks the clotting factors that plasma retains, a distinction that matters for certain specialized laboratory tests.
Word origin
Where “serum” comes from
Formed from Latin serum, meaning whey or watery fluid.