NITRITE AND NITRATE DETERMINATIONS IN PLASMA - A CRITICAL-EVALUATION

Plasma nitrite and nitrate determinations are increasingly being used in clinical chemistry as markers for the activity of nitric oxide synt hase and the production of nitric oxide radicals. However, a systemati c evaluation of the determination of nitrite and nitrate in plasma has not been performed. In this study the recovery and stability of nitri te and nitrate in whole blood and in plasma, the relation between nitr ite and nitrate concentrations in plasma, and possible sources of arti facts were investigated. The main conclusions are: (a) Recovery of nit rite and nitrate from plasma is near-quantitative (87%) and reproducib le; (b) nitrite and nitrate are stable in (frozen) plasma for at least 1 year; (c) nitrite in whole blood is very rapidly (>95% in 1 h) oxid ized to nitrate, and therefore plasma nitrite determination alone is m eaningless; (d) the ranges of nitrite and nitrate concentrations in pl asma samples of 26 healthy persons are 1.3-13 mu mol/L (mean 4.2 mu mo l/L) and 4.0-45.3 mu mol/L (mean 19.7 mu mol/L), respectively; (e) pla sma nitrite and nitrate concentrations were not correlated (nitrite as % of total nitrite + nitrate varied from 3.9% to 88% in plasma sample s); and (f) plasma samples should be deproteinized, and background con trols for each sample should be included in the assay, to avoid measur ing artifactually high nitrite and nitrate concentrations in plasma.