ELISA, short for enzyme-linked immunosorbent assay, is a medical laboratory test used to detect specific proteins in a liquid sample. It is widely used to test for hormones, antibodies, disease markers, allergens, and proteins linked to infection. The method matters because it can identify very small amounts of a target molecule with high specificity.
In a medical device such as a microplate reader, many samples can be tested at the same time in a 96-well plate.
Understanding Medical Technology: ELISA Testing
A test well is more than a tiny container. Its plastic surface can hold molecules in place. In a common sandwich method, a capture antibody is first attached to the well.
The sample is added, and any target protein sticks to that antibody. A second antibody then binds to another part of the same target. This second antibody carries an enzyme, or it is recognized by an enzyme carrying partner.
Washing steps are crucial between each stage. They remove molecules that are not specifically bound. Without careful washing, leftover enzyme can create colour where no target protein is present.
The enzyme creates a measurable signal after a chemical substrate is added. The substrate may change colour when the enzyme acts on it. A darker colour usually means more target protein was trapped in the well.
The reader shines light through each sample and measures how much light is absorbed. Colour alone is not enough to report an exact amount. Laboratories run known concentrations beside unknown samples.
Their results form a standard curve. The absorbance of an unknown sample is compared with this curve to estimate its concentration. Results are most trustworthy when the unknown value lies within the range covered by the standards.
Good testing depends on controls. A blank well contains no target and shows how much signal comes from the plate, reagents, or liquid itself. A negative control should give little signal.
A positive control contains a known target and confirms that the antibodies and enzyme are working. Samples are often tested in duplicate or triplicate. If repeats disagree, the result may be affected by pipetting errors, air bubbles, uneven washing, or a damaged reagent.
Very high target concentrations can sometimes give unexpectedly low readings in certain assay designs. This is called the hook effect, and it is one reason unusual results may need dilution and retesting.
Students may meet this method in discussions of infection testing, allergy studies, pregnancy hormone measurements, food safety, and research on new medicines. The same basic logic appears whenever a laboratory needs to identify one molecule among many molecules in blood, saliva, food, or environmental water. When learning the method, separate the jobs of each part.
The antibody provides recognition. The wash provides selectivity. The enzyme turns binding into a visible signal.
The reader measures that signal. The standards turn a measurement into a concentration.
It is important to remember that a result can support a diagnosis, but it does not usually stand alone. Doctors consider symptoms, medical history, and other tests before making decisions.
Key Facts
- ELISA stands for enzyme-linked immunosorbent assay.
- Antibodies bind specific target proteins because their binding sites match the target antigen.
- Signal strength increases when more enzyme-labeled antibody is bound to the target protein.
- Absorbance is measured by a microplate reader using light passed through each well.
- Beer-Lambert law: A = εlc, where absorbance increases with concentration for many solutions.
- A standard curve relates known protein concentrations to absorbance values so unknown concentrations can be estimated.
Vocabulary
- Antibody
- A Y-shaped immune protein that binds to a specific molecule called an antigen.
- Antigen
- A molecule or part of a molecule that is recognized and bound by an antibody.
- Enzyme-linked antibody
- An antibody attached to an enzyme that produces a color change when the correct substrate is added.
- Substrate
- A chemical that reacts with an enzyme to make a detectable product such as a colored solution.
- Absorbance
- A measure of how much light a sample absorbs, often used to estimate the concentration of a colored product.
Common Mistakes to Avoid
- Skipping wash steps, which leaves unbound antibodies or enzymes in the well and creates a falsely high color signal.
- Assuming darker color always means a positive result, which is wrong because the color must be compared with controls and a standard curve.
- Mixing up antigen and antibody roles, which is wrong because the antigen is the target being detected while the antibody is the binding tool.
- Reading unknown samples outside the standard curve range, which is wrong because concentration estimates are unreliable beyond the calibrated values.
Practice Questions
- 1 A standard curve gives the relationship A = 0.40c + 0.05, where A is absorbance and c is protein concentration in ng/mL. If an unknown sample has A = 0.85, what is its protein concentration?
- 2 In a 96-well ELISA plate, 12 wells are used for standards and 8 wells are used for controls. How many wells remain for patient samples?
- 3 A well turns deep blue after substrate is added, while a negative control remains nearly clear. Explain what this suggests about the target protein and why controls are needed.