I am trying to measure nitrite content in serum samples by Griess assay. But I am facing problems like curdling of serum (possibly due to plasma contamination). Do I have to do some pre-treatment of the serum samples before actually doing the assay? I add serum sample and griess reagent in equal volumes and incubate them in dark for 10 mins at room temperature and take absorbance values at 548nm. please suggest any modifications in this procedure that are required.