Adaptation of the nitrate reductase and Griess reaction methods for the measurement of serum nitrate plus nitrite levels.

Nitrite and nitrate determinations in biological fluids are increasingly being used as markers of nitric oxide production. We have modified a nitrate reductase and Griess reaction method for the measurement of serum nitrate and nitrite in ultrafiltrated samples using a microtitre plate. The recoveries of nitrate and nitrite were 95% (range = 86-113%) and 100% (range = 92-109%), respectively. The intra and inter assay coefficients of variation for nitrate plus nitrite in the concentration range 40-50 microM were 9.1% and 7.8%, and in the concentration range of 2.5-10 microM 23.4% and 25.5%, respectively. At its lower limit the assay is able to detect 125 pmoles of nitrate plus nitrite in 50 microL of sample (2.5 mumol/L). A mean serum nitrate plus nitrite level of 32.8 mumol/L (SD 12.3) was measured in 24 healthy adult volunteers (12 men and 12 women), no age or sex differences were noted.