The Folin phenol reagent can be used to measure proteins in samples only when they are in a pure form and not in complex mixtures.

The Folin phenol reagent is actually a general protein‑detector that reacts with aromatic and reducing groups, so it can be used in complex mixtures, not just pure proteins. However, substances such as phenols, sugars, and detergents can also reduce the reagent, giving false high readings. Therefore, while it can measure proteins in mixtures, careful controls or cleanup steps are needed to avoid interference. For example, a crude cell lysate will give a higher absorbance than a purified protein, but that extra signal comes from other reducing compounds in the lysate.