assay
Meaning of assay: Behind almost every number on a laboratory report sits an assay, a method built to pick one substance out of a complicated mixture.
Definition of assay
An assay is a laboratory procedure that examines a sample to establish whether a particular substance is present and, in most cases, how much of it there is. The sample can be blood, urine, tissue, saliva or a culture, and the method is chosen to suit the chemistry of the substance and the precision the answer has to reach.
Qualitative, semiquantitative or quantitative?
Assays differ in how precise an answer they return, and the word covers all three kinds.
| Kind | What it reports | Example |
|---|---|---|
| Qualitative | Present or not present | A urine screen reporting a drug as detected |
| Semiquantitative | A graded band rather than a figure | Results read as trace, one plus, two plus |
| Quantitative | A number with units | A hormone concentration in a blood sample |
Every quantitative assay has a lower limit beneath which it cannot tell a very small amount from none at all, so a result described as undetectable means below that limit rather than absolutely absent.
What methods are used?
The chemistry of the target decides the approach. Immunoassays use antibodies that bind the substance being sought, and the enzyme-linked immunosorbent assay is the familiar example; most hormone and antibody measurements work this way. Enzymatic assays use a reaction the substance takes part in, and measure the color or light the reaction produces. Nucleic acid amplification copies a stretch of DNA or RNA until it can be detected, which is how many infections are identified. Chromatography paired with mass spectrometry separates a mixture and then weighs the fragments, giving high specificity for drugs and metabolites. Flow cytometry counts and sorts cells by the markers on their surface. Bioassays measure a biological effect rather than a chemical quantity.
How is a result read?
A figure on its own means little. It is compared against a reference range, which is derived from a population of apparently healthy people and therefore describes what is usual rather than what is ideal for one individual. Two other properties matter: sensitivity, the ability to catch what is there, and specificity, the ability to avoid flagging what is not. Assays can also be disturbed by substances in the sample, by a blood sample that has broken down in the tube, or by high concentrations of a supplement. Because laboratories may run different methods and publish different ranges, results for the same substance are not always directly comparable between them.
Used in a sentence
An assay was performed on the blood sample to measure the hormone's concentration.
Good to know
Not every assay is quantitative, since a qualitative assay reports only whether a substance was detected. Results for the same substance are not always interchangeable between laboratories, because methods and reference ranges differ.
Word origin
Where “assay” comes from
From Old French assai, meaning a trial or test, which traces back to Latin exagium, meaning a weighing.