Nitric oxide as a challenge for the clinical chemistry laboratory

Nitric oxide (NO) is an important intra- and intercellular mediator. Although NO can be measured using many different chemical methods, the compound is challenging for a clinical chemistry laboratory, since its half-life in vivo in humans is only a few seconds. Most of the NO is oxidized to nitrite/nitrate, and the concentrations of these anions have been used as quantitative indices of NO production. The simplest and most widely used technique is spectrophotometric measurement of nitrite using the Griess reaction. Nitrate, the main metabolite of NO in blood and urine, must be reduced to nitrite before the colour reaction. Other methods used for measuring nitrite/nitrate in human blood or urine include high-performance liquid chromatography, gas chromatography-mass spectrometry, chemiluminescence, enzymatic assay with nitrate reductase and electron paramagnetic resonance. The reported mean concentrations of nitrite in the blood of healthy adults have varied from non-existent to 4.2 mumol l-1, and those of nitrate from 19.7 to 44 mumol l-1. The technical measurement of nitrite/nitrate is obviously reliable, but there are problematic preanalytical factors. Normal daily food contains more nitrate than that formed from NO, and thus diet-derived nitrate may contribute considerably to the concentration in blood. The problem may to some extent be solved with dietary restrictions, but it is questionable whether the confounding effect of diet-derived nitrate can be totally avoided. Therefore, better methods for measuring the production of NO in vivo would be very welcome.