Enzyme-linked immunosorbent assay (ELISA) simplified

Why was the ELISA test sad after a negative result? Because it is very sensitive! HAHAHA! Sensitive, get it? HAHAHA!
Diagnostics can be a tricky little place to be in. Consider this, Your patient has been coughing his lungs out and you can't quite put a finger on what's causing it. A la house M.D., you have 20 different differentials scribbled on a white board/notepad. But the question still remains: What is the mysterious little creature coursing the airway of your patient? A zillion coffee mugs later you are not very much closer to IDing the rogue.

Phew! There there. Enter Mr.(Ms./Mrs./ Dr.? ) Microbiologist to the rescue. A few diagnostic procedures and voila, he presents to you the little tyrant. You thank your stars for LabDs and move on to the next patient.

Now that we've established the importance of lab diagnostics, you must also know that there are many procedures involved. However few of these tests are as sensitive, economic  and as easy to perform as the Enzyme immunoassay (EIA). ELISA ( Enzyme linked immunosorbent assay), an heterogeneous EIA, is quite a popular diagnostic method. The principle behind it is fairly simple and easy to grasp.

Whenever you have an infection, the B-lymphocytes in your body produce antibodies which then bind to the antigen (infection causing agent) and a series of events ensue which eventually help in getting rid of the infection. These antibodies are specific to the antigen that has opened attack, which means, it will bind to that infectious agent and none other (most of the time). Thus, If an antigen is exposed to its specific antibody, they are bound to bind!

This insane specificity is what drove scientists to develop the ELISA. A fairly easy concept which gave rise to a fairly easy lab procedure. Coat a plate with the antigen, put a patient's serum in it. Add a substrate and some enzyme tagged anti-antibody. If this magic diagnostic mix develops/changes color, antibodies are swarming the serum in significant quantities. 

As with other procedures there are errors involved. However, most of these are overcome by setting up positive and negative controls which help you to know if the color has actually changed and to what extent. To quantitatively analyse, color intensity meters are available. Infections are sometimes missed if the concentration of antibodies is not significant enough or if antibodies have not yet been formed. This is overcome by repeating the test at regular intervals in suspected patients.
However, modifications in the ELISA (ALL HAIL SUPERELISA) leave little scope for false negatives. ELISAs and modified ELISAs are very rampantly used now to detect anything from adulterants to pregnancies. This only proves that even a very basic concept if understood can go on to create a revolution, also millions in patent rights.

So hit the books hard, who knows you might just create the next ELISA.

Some terminologies (roughly put);
Sensitivity:  The ability of the test to detect even the most minute quantities. A very sensitive test will not give you any false negatives.
Specificity: This basically means that the test will give a positive for only that antigen and none other. This means no cross reactions and thus no false positives.

This post was written by a friend of mine. Since it was her first blog post, she was very nervous. Please say something nice and encourage her to write more! =)


  1. wrote well.... keep it coming.... liked the way u presented the matter.... tell your friend to start blogging regularly now :)

  2. very well put, easy to understand.