MRSA Hospitalizations, United States Control and Prevention, unpub. data). Our estimates for 1999 to 2000 are similar for S. aureus infections but are higher for MRSA. Although ICD-9-CM coding accuracy for S. aureus infections has not been specifically examined, the accura- cy of coding for sepsis from all causes has been reviewed, and has demonstrated a sensitivity >75% for any sep- ticemia or bacteremia code and positive and negative pre- dictive values >80% for the code specific for Staphylococcus spp. septicemia (ICD-9-CM 038) (16,19). However, the relationship between true S. aureus infec- tions and ICD-9 discharge coding should be further assessed to validate this method as a tool for monitoring national trends. We found associations between MRSA rate and both region and age. This finding is consistent with previously published data showing an association between age and both the incidence of invasive S. aureus infection and the rate of methicillin resistance (17,20). We also demonstrat- ed a significant difference in MRSA discharge rates between the South and West. Although past microbiologic surveys also have reported higher rates of methicillin resistance in the South compared with other regions, the reasons for this variation are unclear (21,22). These differ- ences may need to be assessed as community-associated MRSA infection becomes more common. Our estimate is subject to a number of limitations that most likely underestimated hospitalizations associated with MRSA infection. First, S. aureus infections may not have been accurately represented by the ICD-9-CM dis- charge code; colonization may have been inadvertently included; and more likely, true infections may not have been identified, since these diagnoses require laboratory culture confirmation. Since only 7 principal diagnoses are included in NHDS, infections listed less prominently may have been excluded. Duplicate isolates were excluded when identified within 30 days of each other; thus, unusu- al scenarios, such as multiple infections during a hospital- ization or infections present for >30 days, were not included. We were not able to distinguish between commu- nity- and healthcare-acquired infection. However, this analysis was designed to measure the overall incidence of disease associated with acute care hospitalization, regard- less of acquisition site, and did not include disease man- aged in the outpatient setting. Although previously pub- lished region and age stratification groups were used, which reduces risk of bias, unmeasured confounders may have affected calculated trends. Finally, although both NHDS and TSN data aim to represent nationally represen- tative samples based on similar factors, methods may have differed, which could have skewed our results. For all data used, institutional settings, such as long-term care or cor- rectional facilities, were not included. In summary, our estimates indicate that the national burden of serious MRSA disease is quantifiable and sub- stantial. Measurement of trends in S. aureus disease, such as the increasing incidence of antimicrobial resistance associated with certain age groups and geographic regions, will have implications in the development of prevention programs, both in the healthcare and community settings. Our method provides a simple way to estimate trends of magnitude of hospitalization associated with S. aureus infection in the United States and could complement meth- ods currently in place for national surveillance. Acknowledgments We appreciate invaluable technical assistance provided by Maria Owings, Jean Kozak, Ron Master, and Daniel Sahm. Dr. Kuehnert is a medical epidemiologist at the National Center for Infectious Diseases, Centers for Disease Control and Prevention. His research interests have included antimicrobial resistance surveillance and now focus on improvement of blood, organ, and other tissue safety. References 1. National Nosocomial Infections Surveillance (NNIS) system report, data summary from January 1992–June 2001, issued June 2001. Am J Infect Control. 2001;29;404–21. 2. Lowy FD. Staphylococcus aureus infections. N Engl J Med. 1998;339:520–32. 3. Finland M. Emergence of antibiotic resistance in hospitals. Rev Infect Dis. 1979;1:4–21. 4. Jernigan JA, Clemence MA, Stott GA, Titus MG, Alexander CH, Palumbo CM, et al. Control of methicillin-resistant Staphylococcus aureus at a university hospital: one decade later. Infect Control Hosp Epidemiol. 1995;16:686–96. 5. Chang S, Sievert DM, Hageman JC, Boulton ML, Tenover FC, Downes FP, et al. Infection with vancomycin-resistant Staphylococcus aureus containing the vanA resistance gene. N Engl J Med. 2003;348:1342–7. Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 6, June 2005 871