Bill McConnell:

Urea
Definitive Method: Isotope Dilution Mass Spectrometry.

Reference Method: Coupled Enzymatic Urease/GLDH with Zinc Sulphate and Barium Hydroxide.

Common Method: Coupled Enzymatic Urease/GLDH
Example Shown Component of Roche 917 Autoanalyser.

Urea + H₂O =urease==> 2NH₄⁺ + CO₂

a-Ketoglutarate + NH₄⁺ + NADH =GLDH==> L-Glutamate + NAD⁺ + H₂O

Decrease in absorbance due to consumption of NADH is measured Kinetically at 340 nm.
1-2% CV.

Other Methods:

Coupled Enzymatic Urease/GLDH	Quantitative Endpoint Kinetic	Rapid and Specific Suits autoanalyser.
Conductimetric	Quantitative Kinetic assay	Specific, Rapid Highly Accurate.
Indicator Dye/Dry Slide	Urease/Ammonia Indicator Dye	Very Specific. Automated.
Ion selective electrode	New POC Units (ISTAT) Urease/Ammonia ISE potentiometry	Point of Care

Conductimetric:

Excellent method. Quite commonly used in Beckman analsyers.

Sample containing urea is added to the reaction mix. A conductivity electrode monitors the mixture. As urease in the mixture catalyses the conversion of urea to ammonium carbonate the change in conductivity in the solution may be measured. The rate of change of conductivity is proportional to the amount of urea in the sample.