Enzyme-Linked Immunosorbent Assay (ELISA)

Definition - What does Enzyme-Linked Immunosorbent Assay (ELISA) mean?

An enzyme-linked immunosorbent assay, also called ELISA or EIA, is a test that detects and measures the levels of molecules, such as hormones, drugs, bacteria or viruses in the blood or urine. The test measures the levels of these molecules by detecting the presence of proteins, called antibodies, which the body produces if the substance measured is present in the blood.

FertilitySmarts explains Enzyme-Linked Immunosorbent Assay (ELISA)

Apart from identifying various infectious agents like HIV, ELISA is also used to measure the concentrations of hormones, including reproductive hormones. These include follicle-stimulating hormone (FSH), luteinizing hormone (LH), human chorionic gonadotropin (hCG), estradiol (or estrogen), progesterone, and testosterone.

To understand ELISA, it is worthwhile to simplify two terms: Antigen and antibody. An antigen is a substance that is foreign to the body and could harm the body. This antigen induces the production of proteins, called antibodies that serve to defend a person's body.

To perform an ELISA, the blood sample is added to a petri dish or plate containing the specific tested antigen. An antigen could be a hormone or any infectious agent that induces the production of antibodies. If an antibody for the particular antigen or hormone is present in the tested blood sample, it will bind to the antigen. This will lead to the formation of an antigen-antibody sandwich. The antibody is labeled with an enzyme. A substrate is then added that is cleaved by the labeled enzyme attached to the antibody. This alters the color or fluorescence of the plate, which is then read by an apparatus, called spectrophotometer. The stronger the color change, the higher are the levels of the antibody bound to the tested antigen.

The pregnancy kits designed for use at home use the sandwich ELISA assay, which tells if a woman is pregnant by detecting the presence of hCG in urine.

Physicians or fertility specialists also use ELISA to measure the day-3 inhibin or FSH levels, which provides an estimation of a woman's ovarian reserve or the capability of the ovaries to grow more follicles.

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