An immunoassay is a diagnostic method that uses the reaction of an antibody to an antigen to diagnose a disease. Here is how diagnostic immunoassays work;
When a patient is infected with a pathogen, the pathogen can be detected by measuring the presence of antigens. They could also work by measuring the presence of antibodies to detect the patient’s response to a pathogen.
A brief recap of biology class would remind us that a body’s natural response to pathogens is producing antibodies. Immunoassays are based on the tendency of these antibodies, produced as a natural immune response, to bind with high specificity to a corresponding antigen. The antigen in question usually is a component of the pathogen. In immune based diagnostics, antibodies or antigens can be used, depending on the disease in question.
Using Antibodies
Antibody detection only indicates that a patient was at some point infected with an infection and does not provide any useful treatment guidance. Tests that use antibodies can only measure the one-time existence of an infection and are hereby unqualified in determining whether an infection is still active or if it has cleared.
This makes the tests unsuitable for some tests like the covid 19 severity test. You could have had an infection years ago and still have the antibodies for the disease now whether or not the pathogens are still around.
An alternative to it would be using diagnostic methods that directly detect the presence of pathogens in lieu of measuring a host’s immune response.
Using Antigens
Antigen capture tests detect the presence of antigens in a host. These tests are not as reliable either and they can sometimes return false positives in the case of carriers (patient) with the pathogens but the pathogens don’t affect them in any way. They can also be misleading by giving false negatives in the case of low levels of infection. When the presence of pathogen-related antigens in the blood is low and difficult to detect, we get false negatives.
How Medical Practitioners Interpret Results
After diagnostic immunoassay, the test has already established the presence of antibodies/antigens or lack thereof for that matter.
There now must be some way to give a readout that your health worker or lab technician can interpret the results. Immune based diagnostics generally include some sort of reagent-activated dye or any other label that outputs an amplified signal that can now be visualized ocularly or using a microscope, plate reader or any other detection method.
Advantages and Limitations of Immune Based Diagnostics
There are tons of diagnostic immunoassays out there each for a specific disease or infection. With that in mind, they all have different advantages and disadvantages depending on the test subject.
The common baseline is they cannot detect the level of infection in patients, making them unsuitable for tests like a covid 19 severity test.
Rapid Antibody Test (RAT)
Advantage
It’s low cost, fast and only requires a small amount of blood to test.
Limitation
The sensitivity and specificity of the test is highly dependent on the antigen used.
Montenegro Skin Test
Advantage
It is low cost and enables detection of T cell immunity.
Limitation
It may not detect cases of leishmaniasis in some stages of the malady in the sense that it can neither tell the difference between infection and disease nor active and progressive disease.
Flow Cytometry
Advantage
It has better sensitivity and specificity vis a vis all other methods. On top of that, a small amount of blood can differentiate between infection, disease, as well as cured patients.
Limitation
The cost associated with the reagents and the equipment needed. There are also few studies as of yet.
