Overview of Urea and Creatinine

Jose H. Salazar, MS, MLS(ASCP)CM

Disclosures

Lab Med. 2014;45(1):e19-e20. 

In This Article

Testing Methods

Creatinine may be measured either chemically or enzymatically. The chemical method known as the Jaffe reaction involves creatinine reacting with picric acid in an alkaline environment to produce an orange-red colored compound. The disadvantage to this method is that the reaction is non-specific for creatinine. Other sources in the patient sample such as ascorbic acid, acetone, or cephalosporins may erroneously produce an orange-red color as they react with the picric acid. The chemical method is time consuming and not widely used in automated instruments.[5]

Creatinine may be also measured enzymatically. Multiple enzymatic methods utilizing creatiniase have been used to measure creatinine. These enzymatic methods employ the use of a spectrophotometer to measure NADH to NAD+ or H2O2 to H2O.

BUN is most frequently measured using enzymatic methods. The first step involves the enzyme urease to hydrolyze urea, thereby producing ammonium. The second step involves the quantitative measurement of ammonium using a variety of methods to determine the amount of urea in the sample. The various methods include the glutamate dehydrogenase (GLDH) method which measures NADH to NAD+, measurement of ammonium ion conductivity, or an indicator dye that reacts with ammonium ions.[6]

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