The performance of each facility in preventing Surgical Site Infections has been adjusted to reflect the higher risk of infections associated with certain procedures. Adjusting for these risk levels allows for fairer comparisons among the facilities. If a facility has a high infection rate after the adjustment, viewers of the data can have more confidence that the higher rate is the result of a problem with surgical site infections and not merely the result of treating a large number of high-risk patients.
Here is a detailed look at the basis for the adjustments and how they are calculated:
Risk factor scores for a procedure can vary from 0 to 3, with 3 representing the highest probability of an infection. Risk scores are the sum of scores for three component factors, each of which contributes a 0 or 1 to the total score. The component risk factors that are used in adjusting a facility’s performance are:
- the degree of contamination of the wound at the time of the operation
- the duration of the procedure
- the American Society of Anesthesiologists (ASA) score. This score is an evaluation of the patient’s physical condition.
Occasionally overall risk factor scores are merged, as in “2, 3”. For these surgical procedures, the Centers for Disease Control found that SSI rates were similar whether the overall risk score was a 2 or a 3.
One component, the duration of the procedure, has changed over time. Prior to 2006, the risk score was assigned based on the number of hours needed to complete the procedure. For example, a hip prosthesis procedure that lasted more than 2 hours was assigned a 1, which was then combined with the other two factors to come up with that patient’s total risk score.
Beginning with 2006 and 2007 data, the standard was changed to the 75th percentile of the distribution of durations to determine the point at which a duration factor would contribute a 1 to the overall risk factor score. Under the new standard, a hip prosthesis procedure that lasted more than 123 minutes resulted in a ‘1’ being contributed to the overall risk score. (This data was published in November 2008, Edwards JR, et al, National Health Care Safety Network report, data summary for 2006 through 2007. Am J Infect Control 2008; 36:609-26.)
Missouri does not collect procedure duration data in minutes from the facilities and therefore continues to use the pre-2006 standard based on hours. Data are collected on three procedures for hospitals – hip prosthesis, abdominal hysterectomy and coronary artery bypass with chest and donor incision.
For coronary bypass surgery, the 75th percentile standard published in 2008 is the same as the previous standard – 300 minutes or 5 hours. For hip prosthesis surgery, the national standard is 3 minutes longer than in Missouri -- 123 minutes vs. 2 hours. For abdominal hysterectomy, the standard was 18 minutes longer than in Missouri – 138 minutes vs. 2 hours.
Since Missouri uses the prior standard, this means that Missouri facilities would have slightly higher risk scores for abdominal hysterectomy and hip prosthesis procedures than they would if the 75th percentile standard had been used. For example, a hip prosthesis procedure lasting 122 minutes would result in a ‘1’ being added to the risk score for Missouri facilities because the procedure lasted more than two hours. Facilities using the 123-minute standard would have a ‘0’ added, and would therefore have a lower overall risk score for the same procedure. At some point in the future, we plan to follow NHSN’s lead by collecting procedure duration data in minutes.
To put Missouri’s data into perspective, users often want to make comparisons to U.S. data. For that reason, a column is included in our HAI tables labeled “Hospital Performance Compared with Facilities in U.S.” This column is based on the annual National Health Care Safety Network publications, known as NHSN.
However, hospitals that report to NHSN are not a random sample of hospitals in the U.S., and NHSN’s annual publications do not cover the same time periods as Missouri data, which are updated quarterly. And, as described above, Missouri and NHSN no longer define duration risk the same way. These differences make NHSN data a less-than-optimal basis for comparison. Nevertheless, the department’s staff believes that including a comparison with their data in our tables still helps users put Missouri data into a reasonable context and perspective.
