approximately 30 microliters of a sample of body fluid are added to a reaction tube and to it simultaneously are added approximately 1 8 ml of each of the above described reagents and the resultant mixture is thoroughly mixed to insure homogeneity the reaction mixture is then incubated at 37 u00b0 c to further hasten the color development for a period of three minutes the reaction mixture is then transferred to a spectrophotometer and the absorbance of the resulting color is measured at a wavelength of 520 nm the urea content of the patient 39 s sample is then automatically calculated from the absorbance reading of a standard solution of urea which has been treated in an identical manner this particular reagent system and application has been shown to give linear results with serum samples containing as much as 150mg dl of urea nitrogen and to be virtually unaffected by sulfa drugs commonly used in the treatment of kidney dysfunctions the o phthalaldehyde reagent is prepared by dissolving approximately 2 grams of o phthalaldehyde in one liter of 3 5 n sulfuric acid which contains 4 ml l of triton x 100 and 1 ml l of polyoxyethylene lauryl ether brij 35 both of which are surfactants the receptor reagent is prepared by dissolving 1 8 gm of 1 3 dihydroxynaphthalene in one liter of 5n sulfuric acid containing 15 ml l of triton x 100 in this example 20 microliters 0 02 ml of a body fluid containing an unknown amount of urea is added to.