Which reagent is not utilized in a coupled enzymatic method to quantify serum creatinine?

• A. Picric acid
• B. Chromogenic dye
• C. Creatinine amidohydrolase
• D. Sarcosine oxidase

Answer: (A)

In a coupled enzymatic method, the signal comes from a cascade of enzymes that converts creatinine through intermediate compounds to a detectable reporter, typically hydrogen peroxide, which is then captured by a chromogenic system. Creatinine amidohydrolase first converts creatinine to creatine. The pathway continues with enzymes that transform creatine to sarcosine, and sarcosine oxidase then oxidizes sarcosine to produce hydrogen peroxide. This hydrogen peroxide reacts with a chromogenic dye in the presence of peroxidase, yielding a color proportional to the creatinine level. Picric acid is used in the Jaffe reaction, a non-enzymatic method, not in this enzymatic cascade. Therefore picric acid is not utilized in a coupled enzymatic creatinine assay, while chromogenic dye, creatinine amidohydrolase, and sarcosine oxidase are.