The kit contains two carbapenems; Meropenem and Imipenem alone and in combination with Dipicolinic acid. This ensures that MBL activity in all species is identified due to otherwise varying resistance in some species to the two carbapenems. Imipenem + Dipicolinic acid detects MBL's in Pseudomonas spp. and Acinetobacter spp., whereas Meropenem + Dipicolinic acid detects MBL's in Enterobacteriaceae spp..
In addition Imipenem in combination with another MBL inhibitor EDTA is included to ensure that IMP-8 producing Enterobacteriaceae are detected. An organism with suspected MBL activity can easily be detected by the differences in the inhibition zones of Meropenem/Imipenem alone and in combination with the inhibitor.
Interpretation
Test only Ceftazidime resistant isolates.
Do not test Meropenem with non-fermenters.
Compare the zone of inhibition around Imipenem 10 μg to that of Imipenem + DPA and Imipenem +EDTA. If all zones are within 3 mm of each other, the organism is not expressing MBL activity. Look for synergism between DPA and Imipenem 10 μg and Meropenem 10 μg.
For Enterobacteriaceae: Compare the zones around Imipenem and Imipenem + DPA. If a difference in zone diameter of ≥ 5 mm (IM+DP-IMI10) is observed, report the isolate as expressing MBL activity. Synergism between DPA and Imipenem 10 μg or Meropenem 10 μg indicates that the isolate is MBL positive.
For Pseudomonas, aeruginosa/Acinetobacter spp.: Compare the zones around Imipenem and Imipenem + DPA. If a difference in zone diameter of ≥ 5 mm (IM+DP-IMI10) is observed, report the isolate as expressing MBL activity. MBL detection with the EDTA combination in Pseudomonas aeruginosa and Acinetobacter spp. If a difference in zone diameter is ≥ 8 mm, the isolate is MBL positive.
For Acinetobacter and oxacillinases: Oxacillinases are influenced by EDTA resulting in a weak synergism between Imipenem and EDTA, while DPA has no effect.