Denominators matter

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.


  1. The thought experiment works with the assumption these two ICUs are indistinguishable except for the frequency of CVC use. Historically, I think the justification for comparative rates using CVC denominators was a no brainer. These devices were critical to saving lives, and the variations in device utilization probably reflected differences in patient populations, even within similar types of locations. accounting for the overwhelming primary risk (the CVC) made sense, since these devices were critical to care. The problem your outlining now is very real -- as the clinical environment has proven that a lot of the variations in CVC use may in fact be personal preference. Just like the argument with CAUTIs (where foley use is deemed less critical to care) to use a PD denominator are strong, we may be at a time where the CLABSI argument is as strong. Improving the classification of ICU types, by more objective criteria than currently used in NHSN (i.e. the 80% rule), would really advance the comparative metric substantially, and likely provide more valid risk adjustment with PD denominators than we currently have with these archaic classification schemes (e.g., "med-surg icu"). advancing the use of composite administrative data to classify patient locations to a more objective, reliable, and granular level, based on fractions of patient-days that have key underlying diagnosis, procedures, etc. is greatly needed.

  2. In order for patient days to be an appropriate denominator, you could only include the patient days of those patients who actually had a central line as only these patients were at risk for a CLABSI. If you include all patients, you falsely represent the risk of CLABSI by diluting the patients who were at risk with those who had no risk. In the example above, although ICU B had numerically fewer CLABSIs, the number of patients at risk was significantly lower than those in ICU A because they inserted fewer lines. Based on the fact that ICU B had half as many central line days, but almost the same number of CLABSIs, I would argue that ICU A is doing a better job. Bottomline, denominators do matter, and we must be very careful to ensure that the measures used are appropriate to the assessment being made.

  3. But doesn't this get us right back to CVC days as a denominator? I view every patient in an ICU as being "at risk" for a CLABSI, because during the course of their care they are at risk for CVC placement. A unit that successfully lowers or eliminates that risk by caring for the patient without the use of a CVC should be rewarded for that, no? Just as we have decided that for CAUTI, it is unfair to punish hospitals who successfully reduce catheter days (removing lower-risk cath days from the denominator and inflating the rate per cath days while reducing it per patient days), shouldn't we do the same now for CLABSI--if we think there is value in reducing CVC utilization?

  4. The Keystone Initiative proved that zero is possible. The argument presented here sounds a bit like the "our patients are sicker" argument we often get from surgery colleagues as a justification for SSI. It's an assumption that an ICU with fewer central line days or lower device utilization is really making that determination only because of patient risk or severity of illness. As it has been said previously, a lot of the individual determination is style of practice. Perhaps a denominator of (central line days * patient days) or some variation thereof would better account for the problem of eliminating the lower risk patients from the denominator and reward units with lower device utilization, while still acknowledging that every individual CLABSI is preventable.


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