In blood, the serum (/ˈsɪərəm/) is the component that is neither a blood cell (serum does not contain white blood cells- leukocytes, or red blood cells- erythrocytes), nor a clotting factor; it is the blood plasma not including the fibrinogens. Serum includes all proteins not used in blood clotting and all the electrolytes, antibodies, antigens, hormones, and any exogenous substances (e.g., drugs and microorganisms).

The study of serum is serology. Serum is used in numerous diagnostic tests, as well as blood typing. Measurements of serum concentrations has proved useful in many fields including clinical trials of therapeutic vs toxic response.

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