There are four main types of ELISA: direct ELISA, indirect ELISA, sandwich ELISA and competitive ELISA. Each has unique advantages, disadvantages and suitability.

In a direct ELISA, the antigen is immobilized to the surface of the multi-well plate and detected with an antibody specific for the antigen The antibody is directly conjugated to HRP or other detection molecules.

Advantages: ● Short protocol: saves time and reagents. ● No cross-reactivity from secondary antibody.

Disadvantages: ● Potential high background: all proteins in the sample bind to the surface. ● No signal amplification. ● Low flexibility: the primary antibody must be conjugated.

Indirect ELISA is a technique that uses a two-step process for detection, whereby a primary antibody specific for the antigen binds to the target, and a labeled secondary antibody against the host species of the primary antibody binds to the primary antibody for detection. As for direct ELISA assays, the antigen is immobilized to the surface of the multi-well plate. 

The method can also be used to detect specific antibodies in a serum sample by substituting the serum for the primary antibody.

Advantages: ● Signal amplification: several secondary antibodies will bind to the primary antibody. ● High flexibility: the same secondary antibody may be used for several primary antibodies.

Disadvantages: ● Long protocol if compared to direct ELISA. ● Potential cross-reactivity from secondary antibody.

Sandwich ELISA (or sandwich immunoassay) is the most commonly used format. This format requires two antibodies specific for different epitopes of the antigen. These two antibodies are normally referred to as matched antibody pairs. One of the antibodies is coated on the surface of the multi-well plate and used as a capture antibody to facilitate the immobilization of the antigen.  The other antibody is conjugated and facilitates the detection of the antigen.

Advantages: ● High specificity: involves two antibodies detecting different epitopes on the same antigen. ● Suitable for complex samples. ● High flexibility and sensitivity: both direct and indirect methods can be used.

Disadvantages: ● Demanding design: finding two antibodies against the same target that recognize different epitopes and work well together can be challenging at times.

Also known as inhibition ELISA or competitive immunoassay, competitive ELISA assays measure the concentration of an antigen by detection of signal interference. Each of the previous formats can be adapted to the competitive format. 

The sample antigen competes with a reference antigen for binding to a specific amount of labeled antibody. The reference antigen is pre-coated on a multi-well plate and sample is pre-incubated with labeled antibody and added to the wells. Depending on the amount of antigen in the sample, more or less free antibodies will be available to bind the reference antigen. This means the more antigen there is in the sample, the less reference antigen will be detected and the weaker the signal.

Some competitive ELISA kits use labeled antigen instead of a labeled antibody. The labeled antigen and the sample antigen (unlabeled) compete for binding to the primary antibody. The lower the amount of antigen in the sample, the stronger the signal due to more labeled antigen in the well.

Advantages: ● Depends on base ELISA selected. ● Suitable for small antigens.

Disadvantages: ● Depends on base ELISA selected