Arcus Biologicals offers 3 different ELISA formats:

The procedure of the DIRECT and the INDIRECT ELISA are very similar. Technically the difference relies on the detection step.

Both ELISA have a precoated capture antibody in the wells. The samples, the standard and the controls are added to the wells and incubated.

Subsequently, the plate is washed and the detection antibody is added, which differs depending on the assay format used. Whereas in the direct ELISA, as the name says, only one detection step, involving a conjugated secondary antibody, is required, a two step detection is used in the indirect ELISA.

The antibody used in the direct ELISA is usually only the F(ab')2 fragment of the detection antibody. This confers a higher specificity as the Fc part is missing and is the main source for unspecific binding. In contrast the indirect ELISA uses a double detection step, using a biotinylated secondary antibody followed by a further washing step and an incubation with an HRP conjugated Biotin molecule.

This method enhances slightly the signal and therefore also lowers the detection limit.
After the incubation with the respective detection system the plates are washed again and the substrate is added to the plate resulting in a clorimetric reaction for a subsequent quantification in an ELISA reader.
Arcus Biologicals designed the direct ELISA to measure serum and plasma samples especially for diagnostic use.

The ready ELISA is a further development of the indirect ELISA.
The assay is designed for routine use in research and diagnostics. The main benefit using ready ELISA is the reduced hands-on time down to 15 min since all components are pre-lyophylized in the plate including the standard that comes prediluted in 4 separate strips. First the pre-lyophylized components are rehydrated and subsequently the samples are added an incubated. After incubation a washing step is required followed by the detection with the colorimetric reaction.