Select phenotypic tests for carbapenemase detection in clinical isolatesa

Test parameterModified Hodge test (10,1824)Carba NP test and variants (10, 2429, 32)mCIM (7, 10, 32, 43)Lateral flow immunoassays (1116)Targeted carbapenemase assays (4753)MALDI-TOF MS hydrolysis approach (28, 5461)
Carbapenem-resistant organismsEnterobacteriaceaeEnterobacteriaceae, Pseudomonas aeruginosaEnterobacteriaceae, P. aeruginosaEnterobacteriaceaeGenerally EnterobacteriaceaeEnterobacteriaceae, P. aeruginosa, A. baumannii
AccuracyFor Enterobacteriaceae, sensitivity, ∼95% and specificity, ∼91%; false-positive results with ESBL- or AmpC-positive isolates with porin defects; false-negative results with MBLsFor Enterobacteriaceae, sensitivity, ∼84% and specificity, ∼100%; for P. aeruginosa, sensitivity, 98% and specificity, 98%; with change in lysis buffer and starting pH, sensitivity increases to close to 99%; false-negative results with OXA-48-like producers or mucoid isolatesFor Enterobacteriaceae, sensitivity, 97% and specificity, 99%; for P. aeruginosa, sensitivity, 98% and specificity, 95%; mCIM improves detection of OXA-48-like producers over CIM; false-positive results may occur with AmpC-hyperproducing E. cloacae; addition of eCIM identifies MBLs in EnterobacteriaceaeFor Enterobacteriaceae, sensitivity, 100% and specificity, ≥95%; false-positive results with some noncarbapenemase OXA-enzymesAccuracy varies by test, e.g., OXA-48 disk test sensitivity, 96% and specificity, 98%; KPC gradient strips, sensitivity, 92% and specificity, 100%; MBL gradient strips, sensitivity, 94% and specificity, 95%; BD Phoenix CPO Detect, sensitivity, 97% and specificity, 69%For Enterobacteriaceae, sensitivities, 77%100% and specificities, 94%100%
Ease of useEasy to perform, no special reagents or media necessaryFor manual versions, fresh reagents need to be prepared frequently due to limited shelf-life of imipenem solution; pH meter needed; commercially available tests remove the requirements for reagent preparation and need for a pH meterEasy to perform, no special reagents or media necessaryEasy to perform, no special reagents or media necessary besides the test kitAll supplies generally readily available except gradient strips or diskComplexity varies based on approach
Interpretation of resultsIndentation of zone diam of E. coli toward carbapenem disk along streaked isolate; reading of zone indentation can be subjectiveColor change from red to yellow or blue to yellow depending on the pH indicator; color change can be subjective for intermediate resultsPositive, zone diam of 6–15 mm; indeterminate, 16–18 mm; negative, ≥19 mm; requires initial setup and then plating of disk onto lawn of E. coli following 4 h of incubationPositive results based on the presence of visible lines specific for carbapenemase-typeVaries by test; for MBL Etest, only used for MBL detection; ratio of the IP/IPI MIC of ≥8 or presence of phantom zone or deformation of ellipseComplex to interpret; requires MALDI-TOF instrument settings different from those traditionally used for FDA-approved microbial identification
Total time to perform test15 min for setup and 2 min to read the following day40 to 170 min (varies based on test)5 min for initial setup, 10 min to inoculate plate, and 2 min to read the following day5 min for initial setup and 2 min to readVaries by test; for MBL Etest, 10-min setup and <5 min to read the following day34 h
Turnaround time1824 hSame-day result; 30 min to 2 h18–24 h; shorter TAT has been described with shorter incubation of the disk with the indicator strainSame-day result; 15 min1824 hSame-day result; 4 h
Estimated cost<$1.00For manual versions, ≥$2.00; cost increases if imipenem-based reagents are not used within 3 days; commercial versions are more expensive, at ∼$2.50–$15.00 (varies based on test)<$1.00Prices have not yet been establishedVaries by test; ranges from <$1.00$6.00; MBL gradient strips, ∼$6.00<$1.00 if MALDI-TOF MS instrument already available
Regulatory statusLDT; no longer CLSI endorsedLDT; except the Rapidec Carba NP is FDA cleared for use with Enterobacteriaceae and P. aeruginosa; manual Carba NP is CLSI endorsedLDT; CLSI endorsedRUORUORUO
  • a IP, imipenem; IPI, imipenem inhibitor (imipenem plus EDTA); RUO, research use only; LDT, laboratory-developed test. Table modified from reference 10.