A chemical test to distinguish smokers and nonsmokers is important in many epidemiologic studies. We have developed an automated method for determining serum thiocyanate by use of the reaction between thiocyanate and ferric ions to form a colored complex. Our data show that this method distinguishes cigarette smokers and nonsmokers: of 197 healthy individuals studied, 1.8% false-positive and 6.7% false-negative results were obtained when 85 µmol of SCN- per liter was the critical value used to distinguish the two populations.

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