Based on HCUP data, which is comprised of 20% of all US hospitalizations between 2001 and 2009, we found an increase in S. aureus associated hospitalizations consistent with prior reports [8, 9, 18]. Importantly, we found that among S. aureus infection the proportion of hospitalizations with a diagnosis code for SSTI also increased, indicating that the rise in S. aureus infections was at least in part driven by SSTIs. By 2009, half of all S. aureus -hospitalizations were associated with a SSTI code.
We also found that between 2001 and 2009 costs associated with S. aureus-SSTI hospitalizations increased 26%, which would imply an increase of national cost from $3.36 to $4.22 billion in constant dollars. Trends in S. aureus-SSTI hospitalization cost increases may be explained, at least in part, by changes in the age group demographics of hospitalization, hospital length of stay, and/or average hospitalization cost. For example, by 2009, patients aged 0–44 years accounted for a larger proportion of hospitalizations than in 2001 (36% vs. 26%, respectively). Furthermore, the average length of stay and cost of hospitalization dropped within these age groups more than in the older age strata (Table 2). Therefore, the fact that the national annual cost of S. aureus-SSTI hospitalizations did not increase even higher was because the higher S. aureus-SSTI hospitalization rates were attenuated by increasingly lower age of those hospitalized (who have less expensive hospitalizations) as well as shorter length of stay in all age groups over time. We found a mean cost of S. aureus-SSTI of $11,622 in 2009, which falls into the cost/charge range found in the literature from $6,800 to $40,046 [12].
The increase in SA-SSTIs seen disproportionately in the younger age groups has occurred at a time of an increased incidence of infections due to CA-MRSA caused by the USA 300 strain, which has also been noted in the younger age groups [18, 19].
Our findings on the increased incidence of S. aureus-SSTI hospitalizations are generally in agreement with prior studies. Klein, et al., showed a 62% increase in the incidence of S. aureus hospitalization between 1999 and 2005 [8], which is similar to the 55% increase between 2001 and 2005 we have found. Interestingly, there are recent data demonstrating decreases in invasive MRSA infections between 2005 and 2011 [20]. However, we did not see decreases, most likely because the decreases noted by Dantes, et al., appear to be driven largely by decreases in healthcare-associated MRSA infections, likely due to hospital-based infection prevention efforts [20], whereas our data focused on SSTIs, which are typically community associated and thus less likely to be affected by hospital-based prevention efforts.
Our study has several limitations. First, we calculated the estimated costs of S. aureus-SSTIs using associated cost estimates, rather than excess cost estimates. Thus, we have likely overestimated the cost of S. aureus associated SSTIs because our analysis includes the costs of concomitant treatment of comorbidities. On the other hand, we may have underestimated the S. aureus-SSTI associated hospitalization cost because HCUP provides information on direct hospital costs only, and not costs associated with physicians and other medical professionals that provided care but which are billed separately. Second, diagnoses of S. aureus-SSTI were based solely on ICD-9 codes and were not validated with chart reviews. Overall, our estimates of S. aureus-SSTI hospitalization incidence was (~30%) higher than those of Klein, et al. [8], during the overlapping periods of the two studies. This difference may be due to the databases used (we used HCUP while Klein, et al., used the National Hospital Discharge Survey). In addition, we adopted a previous ICD-9 coding method [8] that included some diagnoses which are not universally classified as SSTIs, such as device infections and wound infections.
Another limitation is that the incidence of S. aureus-SSTI hospitalizations is only a fraction of all the burden of S. aureus-SSTIs in the population. For example, SSTIs are a major reason for emergency department (ED) visits [4, 21]. Even though ED visits are associated with a lower cost than hospitalizations, they are high in volume, and may still account for a large national cost associated with S. aureus-SSTIs. Therefore, as suggested above, our methods could have led to an underestimate of total costs attributable to S. aureus-SSTIs. Finally, it should be noted that contemporary data from NIS are typically 3 years old making descriptions of current or past year trends impossible.