This study demonstrates the application of two commercially available molecular-method-based syndromic panels for off-label detection of bacterial targets in LRT samples. Though the number of negative samples is small, we did not detect any false positivity of the assays (100 % specificity). The analytical sensitivity however, differed for the three bacterial targets tested and seemed to depend mainly on the bacterial load in the samples (based on LDT CT values). This finding is in line with previous studies that evaluated the detection of viral pathogens in clinical LRT samples using multiplex assays [5, 8, 10, 11].
As L. pneumophila is an important pathogen for community acquired respiratory infections, with a specific treatment regimen, accuracy and speed of diagnosis is crucial. The ePlex® RPP assay did not detect four of the L. pneumophila positive LRT samples while the QIAstat-Dx® RP assay detected all of them. The L. pneumophila samples that were missed by the ePlex® RPP assay were all sputa. With a reported limit of detection of 30 CFU/ml (package insert), the viscosity of the material or the extraction method of the assay might have affected detection of the pathogen. Previous studies have shown that caution should be exercised when interpreting test results from the ePlex® RPP assay that are derived from sputum samples [5, 8]. Basically, a positive result is positive, but interpretation of a negative result may not rule out a Legionella infection.
Both assays demonstrate moderate/impaired sensitivity for the detection of M. pneumoniae in the evaluated clinical samples with lower bacterial loads (CT > 30). Previous studies have shown high positive percentage agreement for detecting M. pneumoniae with the ePlex® RPP assay compared to the BioFire® FilmArray® (93.3 %) (6) and with the BioFire® FilmArray® compared to the SOC FilmArray RP (95.8 %) (2). This discrepancy might be caused by: (1) the use of different materials in this study (as the other studies only used nasopharyngeal swabs), (2) by lower bacterial loads in our samples, (3) by a difference in the extraction methods, or by (4) differences in the analytical sensitivities of the LDT method used in these comparisons. However, since clinical diagnosis of M. pneumoniae is often difficult and DNA positivity in infected patients has been reported to be very short , coincidental detection of this pathogen using syndromic panels might still be useful [13, 14].
The difference in the detection of B. pertussis between the QIAstat-Dx® RP assay and the ePlex® RPP assay is remarkable (100 vs. 66.7 % respectively). The target with the greatest analytical sensitivity to detect B. pertussis is the insertion sequence IS481 . The utilization of a single-copy pertussis toxin promotor target (ptxP) in several multiplex panels has been shown to be less sensitive for the detection of B. pertussis compared to assays based on the multicopy IS481 insertion sequence . The ePlex RPP panel uses a specific single gene (potentially ptxP) as target, while the QIAstat-Dx® RP assay uses the IS481 multicopy sequence, which is less specific as it can be present in other Bordetella species as well. In addition, the input volume of the QIAstat-DX® RP assay is 300 µl versus 200 µl for ePlex®. One other study evaluating the Filmarray for respiratory pathogens in children found complete correlation (100 %) with an LDT for B. pertussis for nine clinical samples, but these were all nasopharyngeal swabs with a high bacterial load .