Since collagen has an unique amino acid composition which is rich in Glycine and Proline, general protein assays do not show correct collagen levels when BSA or IgG is used as a standard.
For example, a bicinchoninic acid (BCA) protein assay and a Coomassie Brilliant Blue protein assay (a Bradford protein assay) for collagen samples tend to show lower protein levels with collagen than other proteins. With the Bradford protein assay, adding sodium dodecyl sulphate at 0.0035% in the Coomassie Brilliant Blue increases assay sensitivity 4-folds (1).
Instead of the protein assays, we have to use hydroxyproline assay for determining collagen levels because collagen specifically contains approximately 13.5% of hydroxyproline in their amino acid composition. Just use of collagen weight works however we have to consider that collagen powder contains 20% of moisture (water) in their weight.
On the other hand, coomassie blue is a popular staining dye to visualize SDS-gel. However, it stains collagen lower than other proteins. Therefore, when we compare collagen and contaminants, collagen levels tend to be underestimated (higher contaminants) by a densitometry.