included. The NHDS is a nationally representative annual sample of discharge records from ≈475 nonfederal short- stay hospitals (8). The survey is based on a stratified, mul- tistage probability design; the sampled hospital discharge records are weighted to produce national estimates. The database includes ≤7 principal discharge diagnoses. We identified S. aureus–related discharge diagnoses by using the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes specif- ic for S. aureus infection: 038.11 (S. aureus septicemia), 482.41 (S. aureus pneumonia), and 041.11 (S. aureus infection in conditions classified elsewhere or of unspeci- fied site). A discharge record listing multiple S. aureus- related diagnoses was counted only once. Septicemia was preferentially included, followed by S. aureus–related pneumonia. Next, the percentage of isolates resistant to oxacillin was determined. To simplify terminology, resistance to methicillin and oxacillin hereafter are used interchange- ably. Oxacillin is used as a proxy for testing of susceptibil- ity to all β-lactam antimicrobials, including methicillin. The Surveillance Network (TSN) Database-USA (Focus Technologies, Herndon, VA, USA) was the source of antimicrobial susceptibility testing results. TSN is a repos- itory of quantitative and qualitative susceptibility results collected from >200 microbiology laboratories in the United States. These laboratories make up a nationally rep- resentative sample based on associated hospital bed size, patient population, and geographic region as determined by the US Bureau of the Census (9,10). Susceptibility testing of patient isolates is conducted on site by each participating laboratory as part of routine diagnostic testing; only isolates judged as clinically significant are included. Data are gen- erated by using Food and Drug Administration–approved testing methods. S. aureus antimicrobial susceptibility to oxacillin was classified as susceptible, intermediate, or resistant according to NCCLS breakpoint criteria; we clas- sified intermediate isolates as methicillin-susceptible S. aureus for purposes of this analysis. Data were stratified by site of infection, i.e., bloodstream, lung, and other sites. Duplicate isolates were removed if the initial and subse- quent isolates were cultured within 30 days of each other. The number of hospital discharges listing S. aureus–specific diagnoses was multiplied by the propor- tion of methicillin resistance at each corresponding infec- tion site to determine the total number of MRSA infections. Infections also were stratified by geographic region and age. The frequency of primary diagnosis and the 10 most frequent secondary (all-listed) diagnoses were abstracted from hospitalizations that included S. aureus–specific diagnoses. Results for the years 1999–2000 were determined by calculating data specific to each year and then averaging. MRSA Hospitalizations, United States Data on resistance rates were stratified first by region and then by age; for each stratification, a chi-square test was used to determine whether differences were significant. The Cochran-Armitage test, a nonparametric method, was used to determine the trend in MRSA hospitalization rate by age category. The effects of region and age on the incidence rate of MRSA were assessed by calculating relative rates and their associated 95% confidence intervals, with the lowest rates designated as comparison groups. Since the rate of S. aureus and the MRSA proportion were estimated separate- ly and then multiplied to obtain the MRSA hospitalization rate, the variance of the MRSA rate was calculated by using the delta method (11). The variance of the methi- cillin resistance proportions was determined under the assumption that the antimicrobial susceptibility data reflected those that would have been derived from a ran- dom sample of all S. aureus isolates in the United States in that time period. Variance estimates were calculated using SUDAAN software (Research Triangle Institute, Research Triangle Park, NC, USA). For both S. aureus rates and methicillin resistance proportions, variances were estimat- ed separately for 1999 and 2000, and the larger of the vari- ance estimates was used in subsequent calculation of 95% confidence intervals for relative rates. Results We estimate that 291,542 hospital discharges with S. aureus infection-related diagnoses occurred annually from 1999 to 2000 (Table 1). A diagnosis of S. aureus infection occurred in 9.13 of every 1,000 hospital discharges. The overall rate of methicillin resistance for all S. aureus infec- tions was reported to be 43.2%. MRSA rates for sep- ticemia, pneumonia, and other infections increased with patient age. An estimated 125,969 hospitalizations with 1 or more discharge diagnoses associated with MRSA infec- tion occurred annually, accounting for 3.95 of every 1,000 hospital discharges. For all sites, most diagnoses occurred in persons ≥65 years of age. In hospitalizations in which S. aureus septicemia and pneumonia were listed as discharge diagnoses, these con- ditions were primary diagnoses in 34.3% and 49.3% of dis- charges, respectively. For S. aureus infection in conditions classified elsewhere and in an unspecified site, a diagnosis intended only for secondary listing, the most frequent pri- mary diagnoses were postoperative (e.g., wound) infection (10.1%), cellulitis or abscess (9.9%), infection from an implanted device or graft (7.3%), and urinary tract infec- tion (3.6%). The largest proportion of S. aureus–related discharge diagnoses occurred in patients from the South, followed by the Midwest, Northeast, and West (Table 2). For both, the rate of S. aureus discharge diagnoses and methicillin Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 6, June 2005 869