Kinetic centrifugal analyzer and manual determination of serum urea nitrogen, with use of o-phthaldialdehyde reagent.

o-Phthaldialdehdye reagent was used in assaying urea with a miniature centrifugal analyzer. Seventeen samples can be assayed kinetically in 2 min at a current cost of less than a penny per test. The reaction is initiated in the instrument upon simultaneous mixing of the o-phthaldialdehyde reagent containing 10 microliter of sample with dilute sulfuric acid/N-(1-naphthyl)ethylenediamine dihydrochloride reagent. The reaction is monitored kinetically at 546 nm during the initial 2 min, when the rate is nearly constant. The reaction of o-phthaldialdehyde with urea reaches equilibrium by about 7 or 8 min, but the method cannot be used for end-point analysis because of interference by the blank reaction. A 1.2 g/liter solution of o-phthaldialdehyde used in conjunction with 2 mol/liter sulfuric acid solution was found to be most suitable for centrifugal analyzers. I also describe a manual version of the procedure that gives a linear standard curve up to 1.0 g of urea nitrogen per liter.