Most colorimetric protein assay methods can be divided into two groups: those involving protein-copper chelation with secondary detection of the reduced copper and those based on protein-dye binding with direct detection of the color change associated with the bound dye.

The Thermo Scientific Pierce BCA and Modified Lowry Protein Assays are based on copper chelation. The Thermo Scientific Pierce 660 nm and Coomassie (Bradford) Protein Assays are based on dye binding. All are well-characterized, robust assays that offer consistent, reliable results. Collectively, they represent a state-of-the-art set for colorimetric detection and quantitation of total protein.

Colorimetric protein assay selection guide

  660nm Protein Assay BCA Assays Bradford Assays Modified Lowry
Reaction scheme
  • Shift in absorption max to 660 nm when dye-metal complex binds protein
  • Dye changes from reddish-brown to green
  • Based on the reduction of copper followed by BCA chelation and complex formation
  • Dye changes from green to purple (standard BCA assay) or from green to yellow (Rapid Gold BCA Protein Assay)
  • Coomassie dye binds directly to protein
  • Dye changes from brown to blue
  • Based on the reduction of copper followed by biuret reagent chelation
  • Dye changes from clear to blue (Folin-Ciocalteu reagent enhances blue color)
  • Ideal for measuring total protein concentration in samples containing both reducing agents and detergents
  • Used for quick yet accurate estimation of protein
  • Measures peptides as small as 2,500 Da
  • Less protein-to-protein variation, compatible with most detergents
  • Various accessory reagents and alternative versions of these assays accommodate many other particular sample needs
  • Measures peptides as small as 2,000 Da
  • Used for quick estimation of protein concentration
  • Used to measure protein in the presence of reducing agents
  • Measures protein of > 3,000 Da
  • Substitute for “homemade” preparations of the Lowry reagents
  • For the determination of a broad range of proteins after TCA precipitation
  • Compatible with any published method for sample treatment modification of the Lowry Assay
Pierce 660 nm Protein Assay

The Thermo Scientific Pierce 660nm Protein Assay is a ready-to-use, detergent- and reducing agent-compatible assay reagent to quickly measure (A660nm) total protein concentration compared to a protein standard. Although the Pierce 660 nm Protein Assay produces a higher level of protein-to-protein variation (37%) than other assays, such as the BCA Protein Assay, it is more linear than Coomassie-based Bradford assays and compatible with higher concentrations of most detergents, reducing agents and other commonly used reagents. The accessory Ionic Detergent Compatibility Reagent (IDCR) provides for even broader detergent compatibility, making this one of the only protein assays that is suitable for samples containing Laemmli SDS sample buffer with bromophenol blue.


We offer easy-to-use colorimetric or fluorescent peptide assays that are designed specifically to improve the sensitivity and reproducibility of quantitation of peptide mixtures. The kits include simple protocols with a representative standard enclosed. The kits have been fully validated with representative samples that have been processed and analyzed using Thermo Scientific Mass Spectrometers