We studied the recently developed Abbott Laboratories fluorescent polarization immunoassay (FPIA) for salicylates in serum. The cross-reactivity test of the assay was performed with 20 substances that showed a similar chemical structure as salicylic acid. A chemical substitution on the 5- position of the salicylate formula enhanced the cross-reactivity. 5-Methylsalicylic acid, diflunisal, salazosulfapyridin, and 5-aminosalicylic acid (5-ASA) showed a significant cross-reactivity with 1200%, 222%, 153% and 122%, respectively. Diflunisal and salazosulfapyridin at a similar therapeutic serum level as salicylic acid can produce false positive salicylate results, in case of unknown medication of these compounds.
