Incidence of skin and soft tissue infections in ambulatory and inpatient settings, 2005–2010

We performed an examination of both ambulatory- and inpatient-treated SSTIs as part of a single investigation in a large cohort of patients. We analyzed data from an administrative database and identified over 2.3 million SSTIs between 2005 and 2010. We found that the incidence of SSTIs is substantial and approximately twice of that of UTIs and tenfold of that of pneumonia. In our study period, on average, over 4 SSTIs occurred per 100 persons aged 0–64 years old annually. We also found that SSTIs were about twenty times more likely to be ambulatory-treated than inpatient-treated. Finally, our findings suggest that the rapid increase in SSTI incidence observed earlier during this Century [1, 2, 5] has leveled off, and SSTI incidence has been relatively stable across our study period.

We used ICD-9 codes to identify SSTI occurrences, which is a research identification method similar (or identical) to previous SSTI investigations [1, 2, 5]. While diagnosing SSTIs using ICD-9 coding has limitations (i.e., potential misclassification, coding errors), we observed that our SSTI incidence was comparable to other studies that used different administrative claims databases. Hersch et al. estimated outpatient infections in the U.S. and found that the visit rate among patients aged 18–44 years increased from 13.1 visits per 1,000 population in 1997 to 27.1 in 2005 [1]. Their 2005 estimates compare to our findings of 42.9 ambulatory-treated infections per 1,000 population per year between 2005 and 2010 in this age group. However, they used data from the National Ambulatory Medical Care Survey, which did not sample non-physician clinician visits in office-based practices, which may in part explain their lower incidence [16]. Edelsberg et al. found that an estimated 289,715 admissions occurred for SSTIs among patients aged 45–64 years in 2004 [5]. Our estimate of SSTIs with inpatient-treated of 3.9 per 1,000 PY in the 45–64 age category represents about 277,000 episodes during 2004, based on U.S. census population estimates for this year [17]. These results suggest that our findings of SSTI incidence are valid in comparison to other investigations employing claim-based methodologies. Finally, our SSTI incidence over the study period of 47.9 SSTIs/1000 person years is also similar to that the 49.6 SSTIs/1000 person year rate found in a population of Kaiser Permanente Northern California patients from 2009 to 2011 when analyzing electronic medical records and administrative databases [18].

Our study is novel as it compares SSTI incidence to other common infectious diseases. The reasons for these comparisons were twofold. First, if a trend was noted SSTI incidence, we wanted to ensure that these represented true incidence changes rather than secular changes in the HIRD population or other methodological factors. However, SSTI incidence changes observed were relatively modest and likely represented year to year fluctuations rather than true trends observed between the late 1990s and 2005 in other populations [1, 2, 5]. The second reason for comparing SSTI trends to other common infectious diseases is descriptive. We chose pneumonia and UTIs given they are relatively common infections seen in both inpatient and ambulatory settings and affect both children and adults. We found an SSTI incidence approximately twice of that of UTIs and 10 fold of that of pneumonia. The UTI incidence of 17.3–20.0 UTIs per 1,000 PY is comparable to findings from outpatients <65 years in Veteran Administration (VA) clinics in 2001. Among females, our UTI incidence was 31.43 UTIs/1,000 PY (data not shown), which is comparable to the estimated 55.1–60.6 UTIs per 1,000 PY in VA clinics [11]. Among males, UTI incidence was 5.23 UTIs/1,000 PY (data not shown), which is comparable to the estimated 16.7–22.7 UTIs per 1,000 PY men [12]. Our lower rate may be related to the fact that the VA does not care for children, in which the incidence of UTI is very low [11, 12], and the VA population may not be representative of insured adults. When these facts are considered, the rates may be more comparable. Our pneumonia incidence we found was comparable to ICD-9 code data from the very large MarketScan Commercial database, in which the pneumonia incidence of 4.89 episodes per 1,000 PY among those aged 18–64 [13] is comparable to our incidence of 4.10–5.03 per 1,000 PY.

Our study has several strengths. First, we studied trends in a demographically diverse population that included a very large group of persons with and without significant co-morbidities. Second, the time period of our investigation was novel in that it examined SSTI incidence in the period after a rapid rise in SSTIs that occurred in the first 5 years of the 21^st Century [1, 2]. Third, we used databases that included both inpatient and outpatient treated SSTIs. Previous large studies in the U.S. captured skin infections only in ambulatory settings, and used sample sizes that were a thousand-fold smaller than the present research to create estimates of SSTI [1, 19]. SSTI investigations in hospitalized patients that used HCUP data utilized a stratified random sample of only inpatient stays for SSTI [5, 6]. Unlike these previous efforts, our investigation directly measured SSTI rates rather than analyzing a subset of cases or a convenience sample.

There are limitations to our investigation. First, due to the nature of the commercially insured data used, we did not include persons over 65, the majority of them who utilize a Medicare or fee-for-service reimbursement model instead of commercial insurance plans. If we were able to quantify SSTI incidence in older Americans, it is likely the rates of infection would be substantially higher in this age group compared to younger persons. Older Americans typically have a higher prevalence of co-morbidities associated with increased risk for SSTI [20] and they have a higher risk of many other infections probably due co-morbidities and declines in host defenses [21, 22]. Second, we did not have microbiologic correlations for each SSTI. Thus we could not assess if the relationship between CA-MRSA trends and SSTI incidence in our cohort. Nevertheless, other investigations have shown that CA-MRSA appears to be the primary cause of increasing community-associated SSTI incidence in the beginning around 2000 [23, 24]. Third, although we were able to categorize diagnoses of skin infection by clinical type, the accuracy of these designations is unclear as diagnoses were derived from billing codes that may or may not have reflected accurately the pathology of the actual infection seen by clinician. Fourth, our designation of uncomplicated and complicated may not necessarily be consistent with definitions used by others [14, 15]. Regardless, there is no standardized definition distinguishing complicated from uncomplicated infection and, when developed, such distinctions may only be useful for clinical trials in which clinicians are prospectively determining definitions based on clinical presentation [25]. Finally, our study population is selected from all the U.S. regions and some regions are more represented than others. However, overall the HIRD population is geographically diverse and covers most regions of the U.S.