active , population - based surveillance for invasive mrsa infections is ongoing in 9 us states ( california , colorado , connecticut , georgia , maryland , minnesota , new york , oregon , and tennessee ) through the active bacterial core surveillance system in the emerging infections program at the centers for disease control and prevention ( cdc ) .
personnel in each state actively collect laboratory reports of positive mrsa cultures from normally sterile sites ( e.g. , blood ; cerebrospinal , joint , or pleural fluid ) of residents in their catchment areas to identify cases . in 2005 ,
the estimated combined population under surveillance was 16.3 million , according to data from the us bureau of the census . to report a case
, personnel must link a laboratory report to the patient 's medical record . during record reviews , personnel abstract information about the following hrfs :
culture obtained > 48 hours after admission ; presence of an invasive device ( e.g. , vascular catheter , g - tube ) ; and history of mrsa infection or colonization , surgery , hospitalization , dialysis , or residence in a long - term care facility in the 12 months preceding the culture .
for this analysis , we used information from the record review to classify cases into 3 mutually exclusive groups : 1 ) case - patients with classic healthcare - associated infections ( ha ) whose culture was obtained > 48 hours after admission with or without other hrfs ; 2 ) case - patients with hrfs but with community onset ( i.e. , whose cultures were obtained < 48 hours after admission ) ( haco ) ; and 3 ) case - patients with community - associated ( ca ) infections without hrfs , according to medical record review .
a subset of isolates from case - patients was collected from laboratories that voluntarily submitted them for microbiologic characterization . of the isolates received at cdc by october 2005
first , isolates were stratified by emerging infections program site ; none were available from maryland .
second , all isolates from tissues other than blood were selected from each emerging infections program site . to ensure 1213
isolates per site , we selected blood isolates from case - patients classified as ca and obtained the remainder from samples from ha and haco case - patients .
isolates were tested by pfge ; patterns were analyzed by using bionumerics ( applied maths , austin , tx , usa ) .
isolates were grouped into pfge types using dice coefficients and 80% relatedness ( 10 ) .
we considered isolates with pfge types usa300 , 400 , or 1000 to be of community origin and those with types usa100 , 200 , and 500 to be of healthcare origin as previously described ( 10 ) . statistical analysis consisted of comparisons of proportions between ca and ha and between ca and haco cases using  pairwise comparisons .
of 9,147 cases of invasive mrsa infection investigated from january 2004 through february 2006 , 2,535 ( 28% ) were ha , 5,353 ( 59% ) were haco , and 1,259 ( 14% ) were ca .
the median age of case - patients with ha and haco was significantly higher than that of case - patients with ca ( table 1 ) .
ca case - patients were 1 ) more likely to have pneumonia than haco but not ha case - patients ; 2 ) more likely to have endocarditis than either ha or haco case - patients ; and 3 ) less likely to die during this hospital stay than were ha or haco case - patients . * mrsa , methicillin - resistant staphylococcus aureus ; hrfs , healthcare - related risk factors . 
patients with community - associated infections were those who did not have hrfs ; these patients were used as reference . 
p<0.05 for  test for categorical variables ; wilcoxon rank sum for age . of the 100 isolates selected for initial testing ,
29 were from ha case - patients , 44 were from haco case - patients ( including 1 isolate of a unique pfge type ) , and 27 were from ca case - patients ( including 1 isolate that could not be typed ) ( table 2 ) .
of the haco isolates , 6 ( 14% ) were usa300 , 1 ( 2% ) was usa400 , and 1 ( 2% ) was usa1000 .
thus , 18%28% of isolates in patients with hrfs ( ha and haco ) had pfge patterns typical of community strains . of the 27 isolates from ca case - patients , 5 ( 19% )
were usa100 and 2 ( 7% ) were usa500 ; thus , 7 ( 26% ) of isolates among ca case - patients were strains typically considered to be of healthcare origin .
* mrsa , methicillin - resistant staphylococcus aureus ; pfge , pulsed - field gel electrophoresis . 
mrsa strains such as usa300 , which were initially a cause of mrsa infections in the community , have migrated into healthcare settings .
the results from this multisite project are consistent with observations from individual facilities , where usa300 isolates caused illness in patients whose infection was healthcare associated ( 11,12 ) .
although age and frequency of endocarditis still differed between case - patients with hrfs ( ha and haco ) and those without hrfs ( ca ) , pfge testing indicated that 18%28% of patients with hrfs were infected with a " community - associated " strain of mrsa , primarily usa300 . furthermore
, 26% of patients without hrfs had a " healthcare - associated " strain , typically usa100 .
 to determine frequency of methicillin - resistant staphylococcus aureus infections caused by strains typically associated with community - acquired infections ( usa300 ) among persons with healthcare - related risk factors ( hrfs ) , we evaluated surveillance data . of patients with hrfs , 
 18%28% had a " community - associated " strain , primarily usa300 ; of patients without hrfs , 26% had a " healthcare - associated " strain , typically usa100 . 
The Study
Conclusions