Spectrophotometric determination of plasma thiocyanate by formation of an ion-pair with methylene blue

We describe a sensitive method for measuring thiocyanate in 500 microliters plasma samples. This technique, although slower than the standard method, improves sensitivity. It requires the extraction in chloroform of an ion-pair formed between thiocyanate ions and methylene blue in acidic medium. Within-day precision had a coefficient of variation of 2.5% and between-day precision a CV of 4.75%. The results were well-correlated (r = 0.997). For 30 non-smokers, the mean thiocyanate level was < 55 mumol/l, and for 30 smokers 90 mumol/l (SD = 20). The method was successfully applied to seven fire smoke victims treated with hydroxocobalamin.