Nitrite and nitrate in body fluids and tissues result from dietary source, endogenous nitric oxide (NO) production and from NO and its higher oxides (NO(x)) present as pollutants in the atmosphere. Nitrite and nitrate in human blood serum and plasma or urine are commonly used as biomarkers and measures of endogenous NO synthesis. In addition to dietary intake of nitrite and nitrate, our study indicates that NO(x) naturally present in the laboratory air may be an abundant source for nitrite and nitrate in human serum, plasma, and urine ex vivo. These artifacts can be effectively reduced by closing sample-containing vials during sample treatment.
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