ESBL/AmpC kits - a comparison
Laura Campbell - Royal Cornwall Hospital
The Royal Cornwall microbiology laboratory was interested in reviewing their method of detecting antibiotic resistance mechanisms.
The current method was not capable of detecting AmpC activity which is a growing concern in clinical microbiology.
As Mast's ESBL discs were in routine use the laboratory accepted the offer to try their combined ESBL/AmpC disc set. The discs were easy to use and a calculator programme made interpretation simple. However there was a significant number of organisms (20 out of 50) that the calculator classed as 'negative - further work required' (indeterminate) and the calculation software was found to be slow and prone to freezing.
These problems led us to trial the similar product recently introduced by Rosco. The Rosco product claims to be more stable with room temperature storage of in-use discs.
Altogether 50 organisms were put through the Mast ESBL/AmpC kit: 37 ESBL detected by conventional combination disc test, 10 cefpodoxime resistant organisms negative for ESBL by combination disc test and 3 controls TEM-3, TEM-10 and CTXM-15.
The organisms were stored then put through the Rosco ESBL/AmpC kit which also has a calculator programme.
Of 37 ESBL combination disc positive isolates, Mast ESBL/AmpC recognised 22 as ESBL; the remaining 15 were classified as indeterminate. Rosco detected 35 correctly. The reamining 2 failed to grow.
Of 10 ESBL combination disc positive isolates tested Mast identified 6 as AmpC (confirmedby the reference laboratory) and 4 as indeterminate. Rosco correctly identified 5 as AmpC (confirmed by the reference laboratory), 2 as 'negative', 3 isolates failed to grow on subculture.
Of the 3 control organisms Rosco correctly identified all 3 whilst Mast classified the Tem-3 and Tem-10 as indeterminate.
*some isolates failed to grow on subculture
10 isolates were sent to the HPA ARMRL: 5 were confirmed as AmpC and 5 were confirmed as ESBL. Of the 5 AmpC Rosco has correctly identified 5 whilst Mast had identified 1 with 4 being reported as indeterminate.
We found the Rosco ESBL/AmpC kit easy to use. It correctly identified all the organisms tested and allows us to report ESBL & AmpC with confidence. The Rosco kit was more cost effective than the Mast kit because it reduces the number of isolates that would either require more work or need referring to the HPA ARMRL Colindale.