What is Immunoassays?

An immunoassay is a diagnostic test that uses the reaction of an antibody to an antigen to diagnose a disease. When a patient is infected with a pathogen, the pathogen can be detected directly, by measuring the presence of antigens. Alternatively, the host immune response to a pathogen can be detected by measuring the presence of antibodies.

Overview

Immunoassays are based on the tendency of antibodies, produced as part of the natural host immune response, to bind with high specificity to a corresponding antigen, generally a component of an infectious organism. The choice of whether to use an antibody or an antigen to diagnose a disease depends on the specific disease.

Tests that detect the presence of antibodies in a patient can measure only the one-time existence of an infection, and are not useful for determining whether an infection is active or has cleared. For instance, patients who are infected with the parasite that causes Chagas disease have antibodies to the infectious organism for many years after infection whether or not the parasites are still there. Therefore, antibody detection in the blood indicates only that the patient was at one time infected with the disease and does not provide any useful treatment guidance. As such, immunoassays in development for Chagas are based on antigen-capture technology and other diagnostic methods that directly detect the presence of parasites rather than measure a host immune response.

Antigen-capture tests detect the presence of antigen in a patient. These, too, can be misleading— antigen detection tests can sometimes return false positives in the case of a disease carrier or a latent infection. For example, detection of pneumococcal bacterial antigen in urinary samples often leads to false positives in the cases of children who are nasopharyngeal carriers but do not have an active pneumonia. Antigen-based immunoassay can also result in false negatives when there is only a low level of infection. Again in the case of chronic Chagas disease, parasites are present in low levels and sequestered in the internal organs. Therefore, the presence of parasite-associated antigens in the blood is extremely low and difficult to detect.

After the test establishes the presence or lack of an antibody/antigen, there must be some means of producing a signal or readout that a health care worker or laboratory technician can interpret. Immunoassays generally include some sort of reagent-activated dye or other label that produces an amplified signal that can then be visualized by eye or quantified using a plate reader, microscope, or other detection method.

Common Immunoassays

Assay Strengths Weaknesses
Enzyme-linked immunosorbent assay (ELISA) Widely-used, high-throughput, inexpensive Requires temperature control, more qualitative than quantitative, time-and-labor intensive
Western blot Highly sensitive and specific Complicated laboratory diagnostic
Immunofluorescent antibody assay (IFA) Relatively simple to perform Less sensitive than ELISA, requires microscopy to evaluate
Electrochemoluminescence Sensitivity More applicable to commercial laboratory setting
Magnetic immunoassays Quantitative Requires a magnetic reader that may limit its field application

Point-of-Care Immunoassays

One of the primary advantages of immunoassay-based diagnostics is that this method lends itself to point-of-care tests. For diseases where the antibody or antigen level in a patient is high enough (i.e., where the antibody/antigen reaction in the diagnostic can produce a color change that is visible by eye) it is possible to conduct the diagnostic assay at the point-of-care. Point-of-care immunoassays for use in non-clinic settings are able to overcome some of the challenges of traditional immunoassays, such as personnel training and equipment requirements, by including all of the reagents as part of the test and producing a robust visual readout that is easy to interpret. More information on point-of-care immunoassays, also called rapid diagnostic tests, is available in the Rapid Diagnostic Test technology profile.

Existing Products

Immunoassays as Non-Neglected Tropical Disease Diagnostics

Immunoassays are considered the gold standard laboratory assays for a variety of diseases that involve natural host immune responses.1,2 These include, but are not limited to, detection of infectious diseases, autoimmune diseases, and certain forms of cardiovascular disease. In general these diagnostic assays are performed in centralized laboratories by highly skilled laboratory personnel.

Immunoassays as Neglected Tropical Disease Diagnostics

The immunoassays, commonly ELISA assays, are a staple for laboratory diagnosis of neglected tropical diseases, and Western blot is the gold standard for HIV diagnosis. However, laboratory assays are not always practical for use in low-resource settings that lack the equipment and trained manpower to perform them. Immunoassays using dipsticks, card agglutination, and lateral flow strips have proven extremely useful at diagnosing malaria, leishmaniasis, onchocerciasis, schistosomiasis and lymphatic filariasis, among others at the point-of-care.3-6 These rapid diagnostic tests are covered in more detail in the Rapid Diagnostic Test technology profile.

References

  1. Andreotti et al. (2003). “Immunoassay of infectious agents"Biotechniques 35.
  2. P. van Lode (2005). “Point-of-care immunotesting: Approaching the analytical performance of central laboratory methods”Clinical Biochemistry 38.
  3. Bosompen KM et al. (1997). “A monoclonal antibody-based dipstick assay for diagnosis of urinary schistosomiasis”Transactions of the Royal Society of Tropical Medicine and Hygiene 91: 554-556.
  4. WHO (2010). Progress report 2000-2009 and strategic plan 2010-2020 of the global programme to eliminate lymphatic filariasis: halfway towards eliminating lymphatic filariasis.
  5. WHO. Malaria Rapid Diagnostic Tests.
  6. Medscape. Onchosariasis

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Pipline

Analysis

Immunoassays are in development for numerous neglected diseases. However, immunoassays that are specifically in development for rapid diagnosis at the point-of-care are profiled separately in the Rapid Diagnostic Test technology profile.

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To learn how you can get involved in neglected disease drug, vaccine or diagnostic research and development, or to provide updates, changes, or corrections to the Global Health Primer website, please view our FAQs.

There are numerous publications and manuals with detailed protocols and explanations of the principles of immunoassays, especially from vendors who sell protein purification equipment and accessories.

  • "The Immunoassay Handbook", 3rd Edition, David Wild, Ed., Elsevier, 2008
  • Learning Guides from Abbott Laboratories on Immunoassay (English): Part 1 Part 2 Part 3 Part 4 Appendix

More information on the availability of tools for immunoassay for diagnosis of specific neglected diseases is available in each of the specific disease profiles.

Get Involved

To learn how you can get involved in neglected disease drug, vaccine or diagnostic research and development, or to provide updates, changes, or corrections to the Global Health Primer website, please view our FAQs.