The Enzyme-Linked Immunosorbent Assay (ELISA) determines concentrations of something within a sample, from interleukins (immune system chemicals), proteins, antibodies (such as for HIV or other diseases. ELISA is reliable and accurate, which is why investigators use it in research labs as well as for patient testing. Its sensitivity makes it a favoured common tool.
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ELISA is sensitive because it uses antibodies for detection. Unlike some drug molecules that target various cells and chemicals outside of its main target (dirty drugs), antibodies are like bloodhounds with an even greater sense of smell. They only home in on a specific target. If a person was trying to detect HIV antibodies in a blood sample, the ELISA detection antibodies would home in and glue themselves onto those specific antibodies. Similar molecular structures usually do not fool them.
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ELISA is also sensitive to contamination issues because if the samples are mixed up just a little (e.g., the investigator used the same pipett tip for different sample wells), the test will detect that contamination and give a false positive for what was supposed to be a negative sample. Contamination can also occur when other reagents associated with the test, such as the wash or coating buffers, accidentally get some sample mixed in their solutions.
Another reason ELISA can be sensitive is based on the incubation. Depending on the protocol, the incubation period allows time for the antibodies and sample to adhere to one another and stick to the wells. If the lab worker rushes this step too much or skips it, the amount of sample sticking to the wells may diminish, allowing unstuck sample to be discarded in the washing steps. In this situation, the technician may not get an accurate reading about the sample in question.