Determination of inorganic nitrate in serum and urine by a kinetic cadmium-reduction method

Abstract

Nitrate in serum and urine was assayed by a modification of the cadmium-reduction method; the nitrite produced was determined by diazotization of sulfanilamide and coupling to naphthylethylene diamine. After samples were deproteinized with Somogyi reagent, the nitrate was reduced by Cu-coated Cd in glycine buffer at pH 9.7 (2.5 to 3 g of Cd granules for a 4-mL reaction mixture). The reduction followed pseudo-first-order reaction kinetics, a convenient time interval for assay being 75 to 90 min. Maximum reduction (85%) occurred at about 2 h. Detection limits in urine or serum were 2 to 250 μmol/L. This method does not require the reaction to go to completion, does not require expensive reagents or equipment, and can assay several samples simultaneously. Repeated assays of two serum pools gave CVs of 9.0% and 4.7% for nitrate concentrations of 31.4 and 80.2 μmol/L, respectively (n = 20 each). The mean concentration of nitrate was 1704.0 ± 1294 (SD) μmol/L (n = 21) in untimed normal urine, 81.8 ± 50.1 μmol/L in serum of 38 renal dialysis patients, and 51.2 ± 26.4 μmol/L in serum of 38 controls.