I am talking here about Multi-Drug Resistant Organisms in the bacterial sense, ie MRSA, ESBLs, VRE etc.
The first thing to say is that in New Zealand we don’t see that many of them, certainly compared with other countries in the world. Click here for more about this.
But what I want to focus on here is the measurement of MDRO rates. There are two main ways to measure these rates.
- 1) Crude Rates: The total number of MDRO isolates per population. e.g number of MRSA isolates per 100,000 population. This is the type of measurement preferred by reference laboratories as they often only receive the resistant isolates, not the susceptible ones…. However this type of rate varies widely depending on how much screening is done at any one institution. The more MDROs are looked for, the more that will be found. The amount of MDRO screening performed in New Zealand varies dramatically from one hospital to another dependent on contractual and other factors.
- 2) As a percentage of total isolates. The MDRO rate can also be measured as a percentage of the total number of bacterial isolates, both susceptible and resistant. e.g The MRSA percentage rate is calculated by calculating the number of MRSA isolates as a percentage of the total Staphylococcus aureus isolates (both methicillin susceptible and resistant). A further example is the percentage of ESBL producing E.coli as a percentage of total E.coli.
I prefer the latter method. I think it is more comparable across areas and it also allows better comparison of rates over time. The Crude MDRO rate measurement is too biased by the amount of testing done and is also susceptible to population fluctuation.
I am not saying that the percentage method is perfect. Like all these things it is susceptible to bias also…..
Currently in our laboratories (NZ): (2012)
- 10% of our total Staph aureus isolates are methicillin resistant (MRSA)
- 1% of our E.coli isolates are ESBL producing.
- 11% of our K. pneumoniae isolates are ESBL producing.
When I return to work in Europe it will interesting to compare the percentage rates with what I am currently used to and speculate as to the differences.