Bacteraemia is associated with high morbidity and mortality rates, so it is essential to have an accurate diagnosis and an early introduction of effective antibiotic therapy1.
As the production of carbapenemases is one of the main mechanisms of resistance in Gram-negative bacilli, methods are needed for rapid carbapenemase detection directly from clinical samples.
The BD MAX™ Check-Points CPO assay is an automated qualitative real-time PCR designed to detect the carbapenemase genes blaKPC, blaNDM, blaVIM/blaIMP and blaOXA-48 from swabs from patients with suspected rectal colonisation.
The objective of this study was to assess the performance of the BD MAX™ Check-Points CPO assay directly on blood culture bottles positive for Gram-negative bacilli. We studied patients admitted with suspected bacteraemia and with risk factors for infection by carbapenemase-producing Gram-negative bacilli. The risk factors considered to be associated were: history of admission in the previous 30 days; patients referred from other healthcare institutions; history of colonisation or previous infection with carbapenem-resistant Gram-negative bacilli.
For positive culture bottles where the Gram stain showed the presence of Gram-negative bacilli, we performed the conventional culture and the BD MAX™ Check-Points CPO assay simultaneously. The conventional culture plates were incubated for 18 h at 37 °C. The identification and antibiotic sensitivity of the isolates were studied using the Phoenix™ automated system (Becton Dickinson). The tests and interpretation of the results were carried out according to the recommendations of the Clinical and Laboratory Standard Institute (CLSI)2. Phenotypic screening for carbapenemases was carried out using synergy with boronic acid and EDTA discs (Britania), and the CARB BLUE Kit® (ROSCO) method3.
To perform the BD MAX™ Check-Points CPO assay, 50 μl was taken from the bottle, a 1/50 dilution was made, 10 μl of which was then entered into the kit’s sample buffer. The test was then performed following the manufacturer’s instructions.
A total of 59 flasks positive for gram-negative bacilli were studied, from which 61 isolates were recovered by conventional culture.
A total of 33 isolates were non-susceptible to carbapenems by culture. In 21, the presence of carbapenemases was detected by inhibition with boronic acid (19), EDTA (2) and/or the CARB BLUE Kit® method.
The following results were obtained with the BD MAX™ Check-Points CPO: 32 negatives and 29 positives (17 blaKPC, 8 blaOXA-48, 2 blaNDM and 2 were positive for both blaKPC and blaOXA-48) (Table 1). Only one strain was positive for OXA-48, with no evidence of resistance to carbapenems.
Results obtained with the direct BD MAX™ Check-Points CPO assay of positive blood cultures.
| Germ | No. | Culture | BD MAX™ | ||||
|---|---|---|---|---|---|---|---|
| No sensitivity to carbapenems | Sensitivity to carbapenems | blaKPC | blaOXA-48 | blaNDM | blaKPC blaOXA-48 | ||
| Klebsiella pneumoniae | 31 | 25 | 6 | 17 | 4 | 2 | 2 |
| Escherichia coli | 9 | – | 9 | – | – | – | – |
| Acinetobacter baumannii | 4 | 4 | – | – | – | – | – |
| Enterobacter cloacae | 3 | 2 | 1 | – | 2 | – | – |
| Proteus mirabilis | 3 | – | 3 | – | – | – | – |
| Pseudomonas aeruginosa | 3 | – | 3 | – | – | – | – |
| Klebsiella oxytoca | 2 | – | 2 | – | – | – | – |
| Providencia stuartii | 2 | – | 2 | – | 1 | – | – |
| Serratia marcescens | 2 | 1 | 1 | – | 1 | – | – |
| Aeromonas veronii | 1 | 1 | – | – | – | – | – |
| Citrobacter koseri | 1 | – | 1 | – | – | – | – |
| Total | 61 | 33 | 28 | 17 | 8 | 2 | 2 |
The results showed that BD MAX™ Check-Points CPO detected 100% of the carbapenemase-producing strains, blaKPC, blaNDM and blaOXA-48. The only case where the results of the culture and BD MAX™ Check-Points CPO differed was a strain of Providencia stuartii, in which blaOXA-48 was detected with no evidence of resistance to carbapenems, although this could be due to the lack of expression of the enzyme in said isolate.
There are immunochromatography methods that enable the detection of carbapenemases from isolated colonies where the sensitivity and specificity are 100%4. There are no reports on the use of such methods on direct samples in our setting, and the costs of these determinations are similar to those of molecular methods.
There is evidence of the use of BD MAX™ on direct blood culture samples for the detection of methicillin-resistant Staphylococcus aureus5–7. In these studies, sensitivity and specificity were in the range of 98%–100%.
To our knowledge, there are no reports in the literature evaluating the BD MAX™ Check-Points CPO assay using direct samples. Our study has shown that the assay detected 100% of the blaKPC, blaNDM and blaOXA-48 carbapenemase-producing Gram-negative bacilli compared to traditional culture methods. One limitation of our study is the fact that because of the low incidence in our environment no blaVIM/blaIMP was detected in the period analysed.
In this study we analysed the efficiency of the BD MAX™ Check-Points CPO method in the detection of blaKPC, blaNDM and blaOXA-48 in direct samples from blood culture bottles in an estimated time of 3 h.
Becton Dickinson Argentina provided the assays to carry out the study.
Please cite this article as: Wisner B, Herrero M, Serruto G, Zarate MS. Detección directa de genes blaKPC, blaNDM y blaOXA-48 a partir de hemocultivos positivos. Enferm Infecc Microbiol Clin. 2022;40:455–456.
