Inoculum Size and False-Positive Detection of NDM- and OXA-48-Type Carbapenemases Using Two Multiplex Lateral Flow...

Area of Science:

• Clinical microbiology
• Molecular diagnostics
• Antimicrobial resistance

Background:

• Carbapenemase-producing Enterobacterales (CPE) are a major public health threat.
• Lateral flow assays (LFAs) offer rapid detection of carbapenemases.
• Accurate performance evaluation of diagnostic tests is crucial.

Purpose of the Study:

• To evaluate the impact of inoculum size on the performance of two carbapenemase-detecting LFAs.
• To assess the reliability of NG-Test CARBA 5 and Carbapenem-resistant K.N.I.V.O. Detection K-Set.
• To identify potential sources of false-positive results in LFAs.

Main Methods:

• Testing 27 Enterobacterales isolates with NG-Test CARBA 5 and Carbapenem-resistant K.N.I.V.O. Detection K-Set LFAs.
• Varying bacterial inoculum sizes during assay performance.
• Whole-genome sequencing (WGS) as the reference standard for carbapenemase gene detection.

Main Results:

• False-positive NDM results observed with NG-Test CARBA 5 at high inoculum in *Serratia* spp. and *M. morganii*.
• False-positive NDM and/or OXA-48-like results with K.N.I.V.O. K-Set at large inoculums in *M. morganii*, *Serratia* spp., and *K. pneumoniae*.
• WGS confirmed absence of carbapenemase genes in isolates with false-positive LFA results.

Conclusions:

• Inoculum size is a critical factor affecting LFA performance for carbapenemase detection.
• High bacterial loads can lead to inaccurate results, particularly false positives.
• Diagnostic evaluation of LFAs must rigorously assess the influence of bacterial concentration.