What is the primary reagent used in the Jaffe method for measuring creatinine?

The Jaffe method for measuring creatinine primarily utilizes saturated picric acid in an alkaline solution (with sodium hydroxide). This method is based on the reaction between creatinine and picric acid, which forms a colored complex. The intensity of this color is directly proportional to the concentration of creatinine in the sample, allowing for quantitative measurement through spectrophotometry.

The use of picric acid is pivotal because it is specifically reactive with creatinine, thereby ensuring accurate results. The alkaline environment is necessary for facilitating the reaction and enhancing the color development. This method is one of the oldest and most widely used techniques for creatinine measurement in clinical laboratories, leveraging the distinct chemical properties of creatinine when it interacts with picric acid.

Understanding the pivotal role of picric acid and sodium hydroxide helps clarify why this combination is essential for the Jaffe method, making it the correct choice in the context of the question.