What endogenous substance may cause a positive interference in the urease/glutamate dehydrogenase assay?

What endogenous substance may cause a positive interference in the urease/glutamate dehydrogenase assay?

• A. Ammonia
• B. Creatinine
• C. Glucose
• D. Cholesterol

Answer: (A)

In this assay, urea is first split by urease into ammonia, which then participates in the glutamate dehydrogenase reaction to form glutamate, with NADH being oxidized and measured. The amount of NADH consumed reflects the urea present. If there is already ammonia in the sample, it will drive the GDH reaction on its own, causing NADH consumption and a signal that mimics higher urea. This is positive interference because the measured signal falsely suggests more urea than is actually present. The other substances listed do not feed into this enzymatic sequence, so they don’t cause the same interference.

In this assay, urea is first split by urease into ammonia, which then participates in the glutamate dehydrogenase reaction to form glutamate, with NADH being oxidized and measured. The amount of NADH consumed reflects the urea present. If there is already ammonia in the sample, it will drive the GDH reaction on its own, causing NADH consumption and a signal that mimics higher urea. This is positive interference because the measured signal falsely suggests more urea than is actually present. The other substances listed do not feed into this enzymatic sequence, so they don’t cause the same interference.