雙縮脲檢驗蛋白質無水硫酸銅有極強的吸水性，把它投入95%乙醇或含水有機物，即吸收水分而恢復為藍色結晶體。硫酸銅中的銅離子能破壞蛋白質的立體結構，使之變性。測定蛋白質濃度時常在蛋白質中加入堿，再加入硫酸銅溶液，此時溶液會變為紫色，這個反應被稱為雙縮脲反應。

Biuret test showed that protein anhydrous copper sulfate had strong water absorption. It was injected into 95% ethanol or water-containing organic matter, that is, absorbed water and restored to blue crystals. Copper ions in copper sulfate can destroy the stereostructure of proteins and denaturate them. When the concentration of protein is determined, alkali is often added to the protein and copper sulfate solution is added. At this time, the solution becomes purple. This reaction is called biuret reaction.