False-positive immunochromatographic CTX-M enzyme test from direct testing of a positive blood culture

Excerpt

The increasing prevalence of extended-spectrum β-lactamase (ESBL)-producing Enterobacterales in bloodstream infections is a major challenge given that these organisms are often resistant to cefuroxime, ceftriaxone, and amoxicillin-clavulanate which are commonly used as first line treatment in many settings [1, 2]. Because inappropriate antibiotic treatment in ESBL bloodstream infections are associated with increased treatment failure and poor outcomes, timely and reliable detection of ESBL-producing Enterobacterales are important to facilitate quick therapeutic management [1]. The NG-Test CTX-M MULTI assay is a lateral flow immunoassay that can detect CTX-M-type ESBLs from isolates in agar plates and from positive blood cultures in less than 15 minutes [1‐3]. The assay uses a cocktail of optimized mouse monoclonal antibodies for detection of 5 groups of CTX-M β-lactamases (group 1, 2, 8, 9, and 25) [1]. Several studies each evaluating ~100 to 300 isolates or blood culture samples have found the test to be highly sensitive and specific and no false-positive results were detected [1‐3]. Nonetheless, the manufacturer currently only recommends testing of isolates in agar plates [4]. Recently, OXY-1-type β-lactamase in Klebsiella oxytoca was found to cross-react with monoclonal antibodies contained in the NG-Test CTX-M MULTI assay, giving rise to false-positive results [5]. Here, we described a false-positive result in a blood culture positive for an Escherichia coli strain which is PCR negative for bla_CTXM gene. …