Thiocyanate in smokers interferes with the Nova magnesium ion-selective electrode.

Thiocyanate found in serum ordinarily is the metabolite of cyanide that is inhaled with tobacco smoke and ingested with cyanogenic foods. We investigated the effect of the thiocyanate ion (SCN-) on the ionized magnesium (iMg) and ionized calcium (iCa) results determined with the AVL and Nova magnesium and calcium ion-selective electrodes (ISEs). We analyzed saline and pooled serum with added SCN-, and serum from apparently healthy nonsmokers (n = 20) and smokers (n = 20). The mean (and range) of the measured serum SCN- concentration was 0.019 (0.008-0.046) mmol/L for nonsmokers and 0.077 (0.020-0.138) mmol/L for smokers. Only the Nova iMg results decreased with increasing SCN- concentration, and the change was dependent on the baseline iMg concentration. In the absence of Mg, SCN- decreased the voltage response of the Nova Mg ISE to calcium ions. At apparently normal serum iMg and iCa concentrations, the interference by SCN- appeared to be equimolar (iMg = -1.04 x SCN- + 0.52). Thus, the serum SCN- commonly found in smokers causes a significant (P < 0.0001) decrease in the Nova iMg results.