ELISA

ELISA — enzyme-linked immunosorbent assay — is the workhorse non-radioactive immunoassay used to quantify hormones and many other proteins. The idea is to swap the dangerous radioactive label of older methods for a harmless enzyme that, when given its chemical substrate, produces a visible color change. The deeper the color, the more hormone is present.

In a typical sandwich ELISA, the wells of a plastic plate are coated with a capture antibody. Patient sample is added and the target hormone sticks; everything else is washed away. A second 'detection' antibody linked to an enzyme is added, binding the trapped hormone. Finally a substrate is added, the enzyme converts it into a colored product, and a plate reader measures the color intensity in each well to compute concentration against a standard curve.

ELISA is sensitive, scalable to hundreds of samples at once, and needs no radiation license, which is why it dominates both clinical labs and research. Like all antibody methods, it can be tripped up by interfering substances and by the so-called hook effect, where extremely high hormone levels paradoxically lower the signal — a trap worth remembering when a result seems too normal in an obviously sick patient.