A titer is a measurement of the amount or concentration of a substance in a solution. It usually refers to the amount of antibodies found in a person's blood.
Blood titer measurements can help determine medical treatment. Antibody titers show if a person is immune to diseases such as measles, chickenpox, or hepatitis. They can also help measure harmful antibodies related to lupus.
A titer measurement is expressed as a ratio, such as 1:40.
Aoyagi K, Ashihara Y, Kasahara Y. Immunoassays and immunochemistry. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 44.