Direct renin assay and plasma renin activity assay compared.

substances produced an interference 5% compared with the serum diluted with drug cocktail diluent. The list of drugs tested appears in Supplement 1, which accompanies the online version of this Technical Brief at http:// www.clinchem.org/content/vol50/issue11/. Ion suppression was assessed as recommended by Annesley (12 ). Briefly, the response of calibrators injected into the mobile phase was compared with the same amount of cotinine added to unextracted and extracted samples. To assure that slight differences in matrix were not causing suppression, we used 20 different cotininefree serum samples. We observed a small decrease in signal, but because the internal standard and cotinine signals coincided, the ion suppression present was corrected when concentrations were calculated. Additionally, cotinine (10 g/L) was infused at a constant rate into the mass spectrometer, and extracts from 20 different cotinine-free serum samples were injected. Signal decreased by 10% when serum samples were injected. The total time to complete this assay is substantially shorter than for other methods that require extensive extractions before assay (11, 13). The simple extraction into methylene chloride and a 3-min assay time combine to allow 100 samples to be analyzed in 8 h. Additionally, the 50L sample size makes this an ideal assay for use in large research studies or epidemiologic surveys where multiple analytes and limited sample volumes are common. The described assay is simple, sensitive, precise, and requires only a small serum sample volume. Without the need for a HPLC column, analysis time is rapid (3 min), and is sensitivity suitable for the qualitative detection of recent passive nicotine exposure. We conclude that this method is ideally suited for analysis of cotinine in passive and active smokers for both clinical and research purposes.