Stability and Reproducibility of the Measurement of Plasma Nitrate in Large Epidemiologic Studies.

Inorganic nitrate has emerged as a therapeutic agent for cardiovascular disease; however, nitrate can also metabolize to carcinogenic nitrosamines under pathologic conditions. Few large epidemiologic studies have examined circulating levels of nitrate in relation to cardiovascular disease and cancer. Data on the validity of nitrate measurement in blood samples collected in typical epidemiologic settings are needed before nitrate can be evaluated as an exposure in large epidemiologic studies. We measured plasma levels of nitrate in three pilot studies to evaluate its laboratory variability, stability with delayed processing, and reproducibility over time among women from the Nurses' Health Study and healthy female volunteers. Laboratory variability of nitrate levels was fairly low, with a coefficient variation (CV) of 7%. Plasma nitrate levels in samples stored as whole blood on ice for up to 48 hrs before processing were very stable; the overall intra-class correlation (ICC) from 0 to 48 hours was 0.89 (95%CI, 0.70-0.97). The within-person reproducibility over a one-year period was modest, with an ICC of 0.49 (95% CI, 0.33- 0.94). Our results indicate that measurement of nitrate in plasma is reliable and stable in blood samples with delayed processing up to 48 hours. Within-person reproducibility was modest but data from this study can be used for measurement error correction in subsequent analyses. The measurement of nitrate cannot be widely used in epidemiologic research without the documentation of its stability and reproducibility.