Unusual analytical interference caused by benzathine penicillin G

: Benzathine penicillin G is one of the antibiotics most often used in ENT practice. In spite of potential allergic or hypersensitivity complications, the restrictions for its administration are scant and its antibacterial spectrum often coincides with the pathogenic flora of the upper airways. A curious analytical interference secondary to its use was detected in two patients seen in our emergency unit. This phenomenon, not observed with other beta-lactamics or even other penicillins, consisted of a continuous false positive result in the urinary detection of amphetamine and its metabolites that lasted up to 50 days after the antibiotic was administered. This finding not only seems to be specific to the benzathine salt, but also to the enzymoimmunoanalysis used to detect drug abuse.