The present invention describes methods and kits for determining the level of S-nitrosothiols in biological fluid samples. The method includes separating tissue or cellular matter from the biological fluid sample and then contacting the biological fluid sample with paraformaldehyde in an amount sufficient to fix free thiols thereby essentially eliminating diaphorase activity of the free thiols. The biological fluid sample is then contacted with NADPH and nitro blue tetrazolium (NBT) wherein S-nitrosothiols in the biological fluid sample facilitate the reduction of NBT to NBT formazan or diformazan in the presence of paraformaldehyde. The amount of reduced NBT is measured and the determined levels correlate to the amount of S-nitrosothiols in the biological fluid sample.