Enzyme-Linked Immunosorbent Assay

Enzyme-linked immunosorbent assay, ELISA (from Latin immunitas — liberation, deliverance from something, and fermentum — leaven) is a basic method of laboratory serological diagnosis based on the specific antigen–antibody binding reaction.

Unlike PCR, which detects the genetic material of the virus, ELISA is most commonly used to detect the immunological trace (antibodies) left by the virus in the patient’s body.

Etiology and pathophysiology

The method is based on the lock-and-key principle of molecular recognition. The diagnostic process consists of the following steps:

1. the manufacturer precoats the bottom of the wells of a special plastic microplate with purified antigens of the virus of interest (for example, measles virus);
2. the patient’s blood serum is added to the wells (if the person has or has previously had measles, their blood contains antibodies that bind firmly to the antigens at the bottom of the well);
3. to visualize this process, secondary antibodies chemically conjugated to an enzyme are added to the well; they bind to the complex that has already formed;
4. at the final stage, a chromogenic substrate is added.

The enzyme cleaves the substrate, changing the color of the liquid in the well. The greater the color intensity (optical density), the higher the concentration (titer) of antibodies in the patient’s blood.

Clinical significance

ELISA is the principal tool for determining the stage of the infectious process. It allows the physician not only to confirm the patient’s exposure to the infection, but also to differentiate antibody classes (IgM, IgG, IgA) and determine their avidity (binding strength).

ELISA is specifically used for:

• donor blood screening — for HIV, hepatitis, and syphilis;
• testing of pregnant women (TORCH screening) — to assess the risk of intrauterine fetal infection.

If ELISA yields a positive result for antibodies to a new infection, the diagnosis often requires confirmation using more complex methods (for example, immunoblotting or PCR).