Which protein measurement method is most suitable for detecting low concentrations of proteins in a sample?

The bicinchoninic acid (BCA) assay is the best choice for detecting very low protein levels because it uses a color change that is highly sensitive to copper ions reduced by proteins, allowing detection down to about 0. 5 µg/mL. In the assay, proteins reduce Cu²⁺ to Cu⁺, which then reacts with BCA to form a purple complex that can be measured at 562 nm; this reaction is more tolerant of detergents and reducing agents than other methods. Because the color change is strong even for tiny amounts of protein, the BCA assay gives reliable readings when the sample contains only a few micrograms per milliliter. For example, a 2 µg/mL protein solution will produce a measurable purple color that can be quantified against a standard curve, making it ideal for low‑concentration samples.