What is the ELISA test?
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Enzyme-linked immunosorbent assays (ELISAs) are a biochemical lab technique where you first capture an antigen or antibody from the sample, and then quantify the bound amount via an enzyme-driven colour-changing reaction. Usually that’s the enzyme HRP with its substrate TMB, but I’m sure there are other enzyme-substrate pairs that can be made to work.
The preceding reactions make sure that an amount of HRP proportional to the analyte you’re looking for gets bound. Thus, when you add a fixed amount of TMB and allow the reaction to proceed for a fixed time before stopping it with sulfuric acid, the amount of TMB that has been processed by HRP is again proportional to your original analyte concentration. And because processing by HRP turns TMB from colourless to bright blue, you can measure the exact “blueness” with a photospectrometer. You take that optical density value, do a bit of math, and out comes your original analyte concentration.
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