A titer, also spelled titre, is a way of expressing concentration. It is a laboratory test that measures the presence and amount of antibodies in blood. A titer test employs serial dilution to obtain approximate quantitative information from an analytical procedure that inherently only evaluates as positive or negative. The term "titer" has three different meanings:

• In titration, the titer is the ratio of actual to nominal concentration of a titrant, which is used to compensate for possible degradation of the titrant solution.

• In textile engineering, titre is a synonym for linear density.

• In medicine, a titer is a measurement of the amount or concentration of antibodies found in a persons blood.

A titer lab report is generated by the lab that tested the blood sample and must include the test type, exact values, signature, and date. A titer test is commonly used to determine if an individual has immunity to a particular disease. If the test is positive (above a particular known value), the individual has immunity. If the test is negative (no immunity) or equivocal (not enough immunity), vaccination is needed. The antibody titer is also used to determine if an individual had an infection in the past or to decide which vaccines are needed.

In summary, a titer is a laboratory test that measures the presence and amount of antibodies in blood, and it is commonly used to determine if an individual has immunity to a particular disease.