What does elisa test for

An ELISA assay is typically performed in a multi-well plate or wells , which provides the solid surface to immobilize the antigen. Immobilization of the analytes facilitates the separation of the antigen from the rest of the components in the sample. This characteristic makes ELISA one of the easiest assays to perform on multiple samples simultaneously.

Figure 2. In a direct ELISA, the antigen is immobilized to the surface of the multi-well plate and detected with an antibody specific for the antigen The antibody is directly conjugated to HRP or other detection molecules. Indirect ELISA is a technique that uses a two-step process for detection, whereby a primary antibody specific for the antigen binds to the target, and a labeled secondary antibody against the host species of the primary antibody binds to the primary antibody for detection.

The method can also be used to detect specific antibodies in a serum sample by substituting the serum for the primary antibody. This format requires two antibodies specific for different epitopes of the antigen. These two antibodies are normally referred to as matched antibody pairs. One of the antibodies is coated on the surface of the multi-well plate and used as a capture antibody to facilitate the immobilization of the antigen.

The other antibody is conjugated and facilitates the detection of the antigen. Each of the previous formats can be adapted to the competitive format. The sample antigen competes with a reference antigen for binding to a specific amount of labeled antibody.

The reference antigen is pre-coated on a multi-well plate and sample is pre-incubated with labeled antibody and added to the wells. Depending on the amount of antigen in the sample, more or less free antibodies will be available to bind the reference antigen. This means the more antigen there is in the sample, the less reference antigen will be detected and the weaker the signal.

The labeled antigen and the sample antigen unlabeled compete for binding to the primary antibody. The concentration of antigen in a sample can then be calculated using the optical density OD.

Figure 2. A typical standard curve. To work out the concentration of antigen in a sample, a standard curve using a solution of known concentration needs to be prepared.

Register Log in. Bitesize category Experimental Techniques. Related Articles Laser Confocal Microscopy. Multiplex analysis of cytokines. Proliferation assay. Flow Cytometry. In the lab, a technician will add the sample to a petri dish containing the specific antigen related to the condition for which you are being tested. If your blood contains antibodies to the antigen, the two will bind together.

The technician will check this by adding an enzyme to the petri dish and observing how your blood and the antigen react. You may have the condition if the contents of the dish change color. How much change the enzyme causes allows the technician to determine the presence and amount of antibody.

The blood draw lasts only a few moments and is mildly uncomfortable. Tell your healthcare provider if you have a fear of needles or become lightheaded or faint at the sight of blood or needles.

Learn more: What causes hemorrhage? How the test results are reported varies based on the laboratory that conducts the analysis. Your doctor should discuss your results and what they mean. False positives and false negatives can occur. Because of this, you may be asked to repeat the ELISA again in a few weeks, or your doctor may order more sensitive tests to confirm or refute the results. Although the test itself is relatively simple, waiting for the results or being screened for conditions such as HIV can cause a lot of anxiety.