Nitroprusside reaction

The nitroprusside reaction is a chemical test used for detecting the presence of free thiol groups of cysteine in proteins. Proteins with the free thiol group give a red colour when added to sodium nitroprusside with ammonium hydroxide. Some proteins test positive when denatured, indicating that thiol groups are liberated. The nitroprusside reaction is usually used for the identification of ketones in urine testing.