An assay measures something in a sample. It might measure how much of a drug is in your blood, whether a virus is present, or how active an enzyme is. Assays are the workhorses of the lab. Every diagnostic test, every drug development program, every quality control check relies on them.

Assays come in many formats. A colorimetric assay changes color when a target is present. An immunoassay uses antibodies to capture and detect a protein. A cell-based assay measures how living cells respond to a treatment. A binding assay measures how tightly two molecules interact. The choice depends on what you are measuring and how sensitive the method needs to be.

Good assays share certain qualities. They are specific, meaning they detect the target and not something else. They are sensitive, meaning they detect low concentrations. They are reproducible, meaning the same sample gives the same result. They have a known range, meaning they can distinguish between different concentrations. Developing an assay takes months of optimization and validation.

The word comes from the French essai, meaning trial or test. The assay is the trial. The result is the data. Without reliable assays, biology would be guesswork.