Objective: To develop a simple method for assessing endogenous nitric oxide (NO) production applicable to routine clinical practice in rheumatology.
Methods: NO production was assessed in patients with rheumatoid arthritis (RA) as serum nitrate levels and as the urinary nitrate:creatinine ratio in morning samples of urine following an overnight fast. The influence of dietary intake of nitrate on these measurements was investigated in healthy volunteers. The clinical value of the urinary nitrate:creatinine ratio was validated in patients with infectious gastroenteritis, in whom its production is known to be increased.
Results: Urinary nitrate:creatinine ratios were significantly elevated in patients with RA (average 3-fold elevation over controls; P < 0.005) or infectious gastroenteritis (average 10-fold elevation, P < 0.001). Serum nitrate was significantly elevated only in patients with infectious gastroenteritis (P < 0.001). Dietary intake of nitrate had no significant influence on the fasting morning urinary nitrate:creatinine ratio in the healthy volunteers, showing that this parameter is a useful indicator of endogenous NO production.