Reminder/ICYMI: the SIR is the Standardized Infection Ratio. It compares the observed number of infections to a risk-adjusted number of predicted infections.
SIR = sum(observed infections) / sum(predicted infections).
This allows us to compare surgeries with different risk levels to each other on an equitable basis. Your goal is to have an SIR under 1.0, and ideally as low as possible.
Introducing the CAD
Another metric we can use is the Cumulative Attributable Difference, or CAD. Whereas the SIR is the ratio between observed and expected infections, the CAD is the difference. In mathematical terms,
CAD = MAX(0, sum(observed infections) – sum(predicted infections)).
The CAD is largely an operational tool, so to make it even more obvious, we exclude any values less than zero. We use the CAD to help allocate our limited infection prevention resources to the places that have the most need of improvement. Here you can see a dashboard showing CAD broken out along various categories:
Issues with benchmarking against the SIR
You may have noticed the “SIR Goal” parameter in the graph above. While the SIR is a great metric, there is a serious time lag. Right now (as of December 2018), the most up-to-date SIR data provides a benchmark based on 2015 data. The problem with this is that everyone has improved since then. Even if our SIR is consistently under 1.0, that doesn’t mean we’re doing better than the national average, even with risk adjustment. With this in mind, the CDC recommends using a “SIR Goal,” i.e. a number less than 1.0 that is your institution’s SIR benchmark.
Here’s a comparison of the various metrics we’ve discussed using an SIR Goal of 1.0:
And here’s what it looks like if we use an SIR Goal of 0.8:
While performance certainly looks better when using an SIR Goal of 1.0, this is unrealistically optimistic. In my experience, it’s generally better to look worse but hold yourself to a higher standard. The idea is to have a constantly evolving SIR Goal. When you start consistently meeting your goal, it’s time to re-assess and lower your goal again!
To briefly recap, the CAD is a nice complementary metric to the SIR, as each shows a different side of the same question. The SIR is perhaps a fairer comparison, but the CAD is better at showing the pure number of excess infections, which is quite handy for operational improvement.
That’s it for now, thanks for reading. If you’d like to see an interactive version of these dashboards, click here. More to come soon!