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How are plasma and serum different?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

Plasma and serum are both components of blood, but they differ primarily in their composition regarding clotting factors.

Plasma is the liquid portion of blood that remains after all blood cells—red blood cells, white blood cells, and platelets—have been removed. It contains water, electrolytes, hormones, and various proteins, including fibrinogen, which is essential for blood clotting.1
Serum is the fluid that remains after blood has been allowed to clot. During the coagulation process, fibrinogen is converted into fibrin, which forms a clot and traps cellular elements. Consequently, serum is essentially plasma that is devoid of fibrinogen and other clotting factors.213

Both fluids are vital in clinical diagnosis, as biochemical testing of their constituents helps monitor health and identify various medical conditions.1