Abstract: Test means, method for making same and method for assaying the urea content of fluids such as blood serum, plasma, urine and spinal fluid are disclosed. The preferred method for making the test means involves a three stage application of reagents separated from each other on an acid modified carrier matrix. One reagent is o-phthalaldehyde and the other reagent is 3-hydroxy-1,2,3,4-tetrahydrobenzo[h] quinoline, 3-acetoxy-1,2,3,4-tetrahydrobenzo[h]quinoline or their dihydrochloride salts. Stability of the reagents is achieved and precipitation of serum proteins is avoided. Moreover, the interfering color due to the presence of bilirubin is minimized when the urea content of serum is determined.