Let's perform a thought experiment. At St. Eligius Hospital there are two ICUs. These two ICUs have the same number of beds, the same number of patient days (12,000/year), and the same case mix index. In fact, they're essentially identical, except that ICU A has an annual CLABSI rate of 2.7/1,000 central line days and ICU B has a CLABSI rate of 5.0/1,000 central line days. Which ICU is better performing with regards to CLABSI? Well, without any other data to consider, we'd be greatly tempted to conclude that ICU A is the better performer since it's CLABSI rate is nearly one-half that of ICU B. Now, let's add another piece of information: ICU B focused on reducing central line placement as a safety intervention--so at year's end, ICU A had 7,500 central line days and ICU B had 3,000 central line days. This means that ICU A finished the year with 20 CLABSIs, and ICU B had 15. Now it's clear that ICU B is the better performer despite having the higher rate.
This is not just a theoretical problem. During my first rotation on the Infectious Diseases Consultation Service at the University of Iowa last year, I was struck by the low prevalence of central lines in the medical ICU. Turns out my perception was spot on--when I looked at our NHSN data, I saw that 3 of our 5 adult ICUs have central line utilization ratios less than the 15th percentile nationally. This is not an accidental occurrence; clinicians in those ICUs have worked hard to avoid placement of devices that are associated with infection. The problem is that the central lines that do get placed in these units are concentrated in a group of patients that are sicker and more likely to develop CLABSI, since the less sick patients will be managed without a central line. Moreover, the denominator is reduced. And the result is higher CLABSI rates. Here, no good deed goes unpunished.
But there's an easy fix. Instead of using device days as the denominator, use patient days. In our thought experiment, we would see that ICU A would have a CLABSI rate of 1.7/1,000 patient days and ICU B would have a rate of 1.2/1,000 patient days. The better performer (ICU B) will now have the lower rate, as expected. Makes sense, no? CDC should move to address this given the financial penalties hospitals now face based on CLABSI rates. Changing the denominator would provide an incentive for hospitals to aggressively reduce device insertion. And since NHSN has collected patient days for decades, there would be no loss of long-term trending. Lastly, use of patient-days as a denominator produces a patient-centered metric. Think about it: do we really care at what rate catheters become infected? No! Our focus should be on what rate of and how many patients become infected, which is also more intuitive for providers at the sharp edge of patient care.