Why do colorimetric protein assays require a standard curve?

A standard curve relates color response to concentration under the current reaction conditions. Standards and samples should use consistent dispensing, reagents, incubation time and temperature, with matrices matched where practical.

Assess blanks, replicates, deviations at individual levels and whether samples lie within the working range, not only the fit statistic. Dilute and reassay out-of-range samples according to the method instead of extrapolating.

A smooth curve does not exclude sample interference. Differences in protein response or components such as reducing agents and detergents can cause systematic bias; check the specific assay compatibility guidance.