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Clinical and Epidemiologic Characteristics Cannot Distinguish Community-Associated Methicillin-Resistant Staphylococcus aureus Infection from Methicillin-Susceptible S. aureus Infection: A Prospective Investigation

  1. Loren G. Miller1,2,3,
  2. Franciose Perdreau Remington4,
  3. Arnold S. Bayer1,2,3,
  4. Binh Diep4,
  5. Nelly Tan3,5,
  6. Kiran Bharadwa6,
  7. Jennifer Tsui7,
  8. Joshua Perlroth1,2,3,
  9. Anthony Shay1,2,3,
  10. Grace Tagudar1,2,
  11. Uzoma Ibebuogu8, and
  12. Brad Spellberg1,2,3
  1. 1Division of Infectious Diseases, Torrance
  2. 2Los Angeles Biomedical Research Institute, Harbor—University of California—Los Angeles (UCLA) Medical Center, Torrance
  3. 3David Geffen School of Medicine, UCLA, Los Angeles
  4. 4Division of Infectious Diseases, University of California, San Francisco
  5. 5University of California, Riverside, California
  6. 6School of Medicine and Biomedical Sciences, State University of New York, Buffalo
  7. 7Mailman School of Public Health, Columbia University, New York, New York
  8. 8Medical College of Georgia, Augusta, Georgia
  1. Reprints or correspondence: Dr. Loren G. Miller, Div. of Infectious Diseases, Harbor-UCLA Medical Center, 1000 W. Carson St., Box 466, Torrance, CA 90509 (lgmiller{at}ucla.edu).

  1. Presented in part: 42nd Annual Meeting of the Infectious Diseases Society of America, Boston, Massachusetts, October 2004 (abstract LB-7).

 
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Abstract

Background. Community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA) infection has become common worldwide. Some researchers have argued that empirical therapy for MRSA should be given only to patients with suspected CA S. aureus infections who have risk factors for acquisition of MRSA. However, there are no prospective data examining this approach.

Methods. We prospectively enrolled consecutive patients who were hospitalized with S. aureus infection, administered a detailed questionnaire, and collected clinical and microbiological information.

Results. Of the 280 consenting patients, 180 were adults with CA S. aureus infection. Among these subjects, 108 (60%) had MRSA infection, and 78 (40%) had methicillin-susceptible S. aureus (MSSA) infection. MRSA infection was associated with younger age (P < .0001); skin/soft-tissue infection (P = .015); snorting/smoking illegal drugs (P = .01); recent incarceration (P = .03); lower comorbidity index (P = .01); more frequent visits to bars, raves, and/or clubs (P = .03); and higher frequency of laundering clothes in hot water (P = .05). However, the sensitivity, specificity, and predictive values for these factors for discriminating CA-MRSA infection from CA-MSSA infection were relatively poor. Post-hoc modeling revealed that, even in a 10% (i.e., low) MRSA prevalence population, patients lacking the 3 strongest MRSA risk factors would still have a 7% posttest probability of MRSA. Most MRSA strains belonged to the ST-8/USA300 genotype, contained SCCmec type IV, and shared virulence factors commonly found in the ST1:USA400 clone. MSSA strains were genotypically heterogeneous.

Conclusions. We found that clinical and epidemiological risk factors in persons hospitalized for CA S. aureus infection cannot reliably distinguish between MRSA and MSSA. Our findings have important implications for the choice of empirical antibiotic therapy for suspected S. aureus infections and for infection control.

Staphylococcus aureus is a common cause of community-associated (CA) and health care—associated (HCA) infections [1, 2]—especially infection of skin and soft tissues [2–4]. Until recently, CA strains of S. aureus were nearly uniformly susceptible to penicillinase-resistant β-lactams (e.g., methicillin, oxacillin, and cephalosporins). However, during this decade, outbreaks of CA methicillin-resistant S. aureus (MRSA) infection have been increasingly reported worldwide [5–13]. In many places, CA-MRSA infection has become endemic and more prevalent than CA methicillin-susceptible S. aureus (MSSA) infection [9, –17].

Traditionally, risk factors for MRSA infection among patients with S. aureus infections have been exposures to health care settings, such as hospitals, nursing homes, and dialysis centers [11, 18]; among community dwellers, the risk factors have been injection drug use and diabetes [18–21], Recent outbreaks of CA-MRSA infection have occurred in populations with few or none of these risk factors, including athletes [6, 8, 22], prisoners [6, 23], and healthy children [9, 12, 24]. These outbreaks suggest that, among patients with CA S. aureus infection, exposure to specific non—health care environments may increase the likelihood that the infection is caused by MRSA.

The ability of traditional [11, 18–21] and more recently recognized [6, 8, 9, 12, 22–24] risk factors for CA-MRSA infection to distinguish between CA-MRSA infection and CA-MSSA infection is not well defined. Studies of the risk factors for acquisition of CA-MRSA infection have been gathered almost uniformly via retrospective case series, typically in the context of an outbreak among a well-defined population [7, 15, 17, 22, 25, 26]. Retrospective studies often lack comprehensive assessments of risk factors or may be subject to bias [27]. To our knowledge, only 1 investigation of pediatric patients has prospectively evaluated risk factors for CA-MRSA [9]. The lack of prospective data, especially among adults, hampers the ability of clinicians to properly determine which patients require empirical therapy for MRSA infection and which do not.

Los Angeles County, California, has been affected by numerous, well-described outbreaks of CA-MRSA infection among prisoners, athletes, and men who have sex with men [6, 25, 28]. Because populations at higher risk for acquiring CA-MRSA infection may exist in the county, it may be an ideal setting to investigate the clinical utility of CA-MRSA risk factors when evaluating patients with CA S. aureus infection. To this end, we performed a prospective investigation of patients with CA S. aureus infection at Harbor—University of California—Los Angeles (UCLA) Medical Center, a public tertiary care hospital in Los Angeles County, which has seen a rapid increase in the number of CA-MRSA infections.

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Methods

Surveillance Population

From 23 February through 15 October 2004, we screened the clinical microbiology laboratory at Harbor-UCLA Medical Center daily for new cultures of wound, blood, urine, or sputum specimens that were positive for S. aureus. Patients were considered eligible if the culture was positive for S. aureus, as determined using a rapid S. aureus–specific latex agglutination test (Staphaurex; Remel); the patient was still hospitalized; and the culture sample was obtained ≤72 h after hospital admission.

Patients were excluded from the study if they had previously been enrolled in the protocol or refused to participate. Study staff approached all eligible patients and attempted to complete the informed consent process. The study design was approved by the Institutional Review Board at Harbor-UCLA Medical Center.

Data Collection

A trained research assistant administered a standardized survey to all consenting patients. This detailed questionnaire explored a wide variety of exposures, demographic characteristics, and clinical characteristics that either have been classically associated with HCA-MRSA infection [18, 29] or are hypothesized to have an association with CA-MRSA infection on the basis of recent outbreaks (table 1) [5–7, 18–20, 25, 28, 30–32]. Measures incorporated survey items that were developed for investigation of previous outbreaks of CA-MRSA infection [25, 33], if available, and standard measures of severity of illness [34] and comorbidities [35]. We also obtained data on hygiene, bathing frequency, antibacterial soap use, and other habits or exposures (table 1), which were measured using Likert-type scales (e.g., bathing/showering more than once daily to bathing/showering less than once per week).

The in vitro susceptibility of S. aureus isolates was determined using the Vitek system (bioMérieux), according to Clinical and Laboratory Standards Institute (formerly the NCCLS) protocols. Enrollment was targeted to allow an 80% power to detect a ±;10% difference in prevalence for a given risk factor, assuming a risk factor prevalence of 20% in the MRSA group and 1 : 1 ratio of CA-MRSA infections to CA-MSSA infections. Interim analysis of enrollment was performed monthly, and recruitment ceased when the enrollment target was met.

Case Definitions

Antimicrobial susceptibility was determined using the Vitek system (bioMérieux) in accordance with the protocols of the Clinical and Laboratory Standards Institute. Isolates that were resistant to oxacillin were considered to be MRSA; isolates that were susceptible to oxacillin were considered to be MSSA.

Subjects were considered to have CA infection if the culture specimen was not from a surgical site and if, in the past 12 months, the subject (1) had not resided in a long-term care facility, such as a nursing home or rehabilitation facility; (2) had no indwelling devices, such as intravenous catheters or venous access ports; (3) had not visited an infusion clinic; and (4) had not received peritoneal or hemodialysis. Any subjects who did not fulfill the aforementioned criteria were classified as having HCA infection. This definition of CA infection versus HCA infection is consistent with the Centers for Disease Control and Prevention's ABC criteria [33]. However, we considered patients who were hospitalized in the past 12 months to have CA infection, as pilot data suggested that prior hospitalization was typically unrelated to infection acquisition, and exclusion of this population would have limited the generalizability of our findings. Therefore, the criteria above allowed the categorization of all cases as CA-MRSA, CA-MSSA, HCA-MRSA, or HCA-MSSA (figure 1).

Figure 1
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Figure 1

Algorithm of patient recruitment and final subject categorization. In this investigation, 409 consecutive hospitalized patients from whom Staphylococcus aureus had been isolated from a wound, blood, urine, or sputum culture within 72 h after hospital admission were eligible. Of these, 326 were approached; the remaining 83 had been discharged, had died, or had transferred. Of the 326 patients approached, 280 consented, and 23 refused; in 23 cases, the informed consent process could not be completed, largely because of the patient's altered mental status. Once patients were enrolled and surveyed and clinical data were collected, the infections were categorized as either community associated or health care associated, and S. aureus isolates were determined to be methicillin resistant or methicillin susceptible (see Methods for details).

Strain Characterization

Genotyping. Molecular typing was performed on isolates from the first 123 patients enrolled at an independent site by investigators who were blinded to the clinical details (F.P.R. and B.D.); the number of patients was chosen on the basis of available resources. PFGE with SmaI [36] and multilocus restriction fragment typing genome [37–39] were used to characterize all S. aureus isolates. A PFGE group consisted of isolates with a 0–6-band difference from at least 1 other isolate in the group [40], and we used the genotype nomenclature of McDougal et al. [41]. Sequence types (STs) were assigned with reference to the MLST database (http://www.mlst.net). Strains were designated by ST and PFGE genotype to provide a simplified, uniform nomenclature for describing MRSA (e.g., ST8/USA300).

PCR assays for virulence genes and SCCmec allotypes. PCR-based assays for virulence factors (sea, seb, sec, sed, see, seg, seh, sei, sej, sen, seo, sem, tst, and lukM [42] and bsaA, lukE, sek, sel, sep, and seq [43]), adhesins (cna, fnbA, and icaA) [44], Panton-Valentine leukocidin (lukS-PV and lukF-PV) [45], and SCCmec types I-IV [46] were performed as described elsewhere. As a control for our PCR assay, we used a well-characterized strain collection from the National Institutes of Health Network for Antibiotic Resistance in Staphylococcus aureus (http://www.narsa.org) that contained all specified genes that were examined [42]. The number of strains analyzed for virulence factors was determined by initial findings and available resources.

Statistical Analysis

Because of the low rate of enrollment of children, the possibly distinct pathogenesis in this population, and the extreme rarity of children to have certain exposures (e.g., incarceration and injection drug use), all pediatric data were excluded from the primary analyses. Two secondary analyses were performed to evaluate the impact of (1) inclusion of pediatric data and (2) exclusion of adults who had been hospitalized within the past year from the population of CA-infected patients. For bivariate analyses, we examined the association of MRSA with 23 risk factors, which were chosen from a list of nearly 100 factors based on the literature at the time of analysis and on clinical judgment. Differences in the prevalence of risk factors between patients with CA-MRSA infection and those with CA-MSSA infection were compared using a χ2 test or Student's t test, as appropriate. Collinearity was assessed by Pearson correlation matrices and tolerance values for the independent variables. All risk factors with an association of P < .05 at the bivariate level were included in the multivariate logistic model predicting MRSA. Sensitivity, specificity, positive predictive value, negative predictive value, and positive and negative likelihood ratios of MRSA risk factors were calculated using established formulas [47, 48]. Data analysis was performed using SAS software, version 8.2 (SAS Institute).

As a post-hoc analysis, we sought to define the utility of using the 3 strongest risk factors (as measured by positive likelihood ratios) in calculating the posttest probability of an adult being infected with MRSA (using a pretest probability of 60%, which is the probability of MRSA infection in the cohort). In addition, we used our data to develop 4 hypothetical cohorts with various proportions of MRSA infection—specifically, 10%, 20%, 30%, and 40%. Data from each subject from the original cohort were duplicated at varying ratios to achieve the desired percentage of MRSA infection within each new hypothetical cohort. For example, to create the hypothetical cohort in which 30% of subjects are infected with MRSA (and 70% with MSSA), we used 2 copies of data from MRSA-infected subjects (60% of the total enrollment) and 7 copies of data from each MSSA-infected subject (40% of the total enrollment) to achieve a cohort with exactly 30% MRSA cases and 70% MSSA cases (i.e., [60% MRSA] × [2] = 1.2; [40% MSSA] × [7] = 2.8; 1.2/[1.2 + 2.8] = 30% MRSA cases; 2.8/[1.2 + 2.8] = 70% MSSA cases). The posttest probability of MRSA infection for subjects with 0, 1, 2, and 3 risk factors in each hypothetical cohort was then calculated.

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Results

A total of 409 patients had cultures positive for S. aureus within 72 h after hospital admission; 326 (80%) of these patients were queried for study enrollment (figure 1). The remaining 83 patients (20%) were unavailable to study personnel because they had been discharged from the hospital (n = 74), had died (n = 6), or had transferred hospitals (n = 3). The informed consent process was not completed by 23 of the subjects queried for study enrollment, largely as a result of altered mental status. Of the 303 patients who completed the informed consent process, 280 subjects (92%) consented and 23 (8%) refused participation. Seventy-eight subjects (28%) were determined to have HCA S. aureus infection, and 202 were determined to have CA S. aureus infection, including 180 adults, 108 (60%) of whom had CA-MRSA infection and 72 (40%) of whom had CA-MSSA infection.

The median age of the adult population was 45 years, and most patients (78%) were male (table 1). Fifty-one percent of the patients were Hispanic, 26% were white, 18% were African American, and 6% were of other/mixed race. A minority of patients (24%) had been hospitalized within the prior 12 months, and most patients (89%) presented with skin or soft-tissue infection. The median duration of symptoms prior to presentation was 5 days. Additional clinical details are outlined in table 1. Antimicrobial susceptibility tests demonstrated a high degree of resistance among CA-MRSA isolates to fluoroquinolones and macrolides, a high degree of susceptibility to clindamycin and tetracycline, and a very high degree of susceptibility to trimethoprim-sulfamethoxazole, rifampin, and vancomycin (table 2).

Figure 2
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Figure 2

PFGE patterns for selected methicillin-resistant Staphylococcus aureus strains. Digestion of chromosomal DNA was by SmaI for selected strains from study subjects: 1, a USA200 strain; 2, a USA400 strain; 3, a USA100 strain; 4, 5, and 7, methicillin-resistant USA300 strains; and 6, a methicillin-susceptible USA300 strain. The band corresponding to the loss of 24kb of DNA containing the type IV SCCmec is indicated by an asterisk.

Table 1
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Table 1

Characteristics of adult patients with community-associated (CA) Staphylococcus aureus infection and clinical and epidemiologic risk factors for methicillin-resistant S. aureus (MRSA).

Table 2
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Table 2

Antimicrobial susceptibilities of community-associated methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) isolates.

Risk factors for MRSA infection in adults. Adults infected with CA-MRSA were significantly younger than adults infected with CA-MSSA (OR for CA-MRSA, 0.952; 95% CI, 0.928–0.977 for each decade of age; P < .001). (table 1) Subjects with CA-MRSA had a lower Charlson comorbidity index (OR, 0.76; 95% CI, 0.61–0.94; P = .01) and were less likely to present with non—skin/soft-tissue infection (6% vs. 17%; OR, 0.28; 95% CI, 0.10–0.82; P = .015). Patients infected with CA-MRSA were more likely to smoke or snort illicit drugs (28% vs. 12%; OR, 2.9; 95% CI, 1.2–6.8; P = .01) and to have been incarcerated in the previous 12 months (20% vs. 8%; OR, 2.8; 95% CI, 1.1–7.3; P = .03). Of note, all but 1 subject who had been incarcerated had been in the Los Angeles County jail, the site of an ongoing outbreak of CA-MRSA infection [6]. Patients infected with CA-MRSA more frequently laundered clothes in hot water (P = .05) and more frequently visited bars, raves, or clubs (P = .03).

Sensitivity, specificity, predictive values, and likelihood ratios for these risk factors were very limited in their ability to distinguish patients with CA-MRSA infection from those with CA-MSSA infection. For example, the sensitivities of most risk factors for MRSA were <30%, and specificities ranged from 65% to 90% (table 3). All positive likelihood ratios were <3 (with ≥5 generally considered to be clinically useful) (table 3) [49].

Table 3
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Table 3

Test characteristics of risk factors for methicillin-resistant Staphylococcus aureus (MRSA) infection among 180 adults with community-associated S. aureus infection.

Secondary analyses of MRSA risk factors. In a secondary analysis of adults (82 of whom were infected with MRSA and 55 of whom were infected with MSSA) with “definite” CA infection (i.e., excluding patients who had undergone surgery within the past 12 months), there were minimal differences from our primary analysis. MRSA infection was associated with snorting/smoking illicit drugs (OR, 5.6; 95% CI, 2.0–15.6; P = .004), close contact with an injection drug user (OR, 3.2; 95% CI, 1.2–8.6; P = .02), frequently hospitalized household contacts (P = .04), more frequent laundering of clothes in hot water (P = .002), and more frequent use of antibacterial soap (P = .03).

The results of the secondary model, which included pediatric subjects, were likewise similar to the primary model. Although recent contact with someone with a known skin infection trended toward being more common among adult patients with CA-MRSA infection (OR, 2.5; 95% CI, 0.9–7.2; P = .08), the trend became significant when children were included (OR, 3.5; 95% CI, 1.3–9.7; P = .01). We found no other significant differences between this model and our primary model.

Multivariate analyses. In a multivariate model of adults with CA S. aureus infection (table 4), only younger age was associated with MRSA infection (OR per decade of age, 0.96; 95% CI, 0.93–0.99; P = .02). The results of a model that included children also found only younger age to be an independent predictor of MRSA (P = .02). A multivariate model of definite CA S. aureus infection found that younger age (P = .01) and snorting or smoking illicit drugs (OR, 5.0; 95% CI, 1.4–18.0; P = .01) were independently associated with MRSA.

Table 4
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Table 4

Multivariate analysis predicting methicillin-resistant Staphylococcus aureus (MRSA) among persons with community-acquired S. aureus infection.

Modeling for populations with a lower prevalence of MRSA infection. We modeled the 3 strongest risk factors for MRSA in adults (specifically, close contact with someone with a skin infection, snorting or smoking illicit drugs, and incarceration). Overall, patients who had 0, 1, 2, or 3 of these risk factors had 52% (60 of 116), 71% (32 of 45), 81% (13 of 16), or 100% (3 of 3) probabilities of being infected with MRSA, respectively (table 5). To determine the clinical utility of our findings in populations with a lower prevalence of MRSA infection, we modeled these risk factors in hypothetical cohorts with 10%, 20%, 30%, and 40% pretest probabilities of MRSA infection. In these cohorts, the posttest probability of MRSA infection was only marginally changed (e.g., from 10% to 7%) if no MRSA risk factors were present, regardless of the pretest probability.

Table 5
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Table 5

Model of strongest risk factors for methicillin-resistant Staphylococcus aureus (MRSA) infection among persons with community-associated S. aureus infection.

Molecular characteristics of strains. Of the 123 S. aureus isolates examined, 62 were CA-MRSA, 40 were CA-MSSA, 13 were HCA-MRSA, and 8 were HCA-MSSA. Among the CA-MRSA strains, 57 (92%) of 62 were associated with ST8/USA300 clonal complex (figure 2). Of note, the USA300 strain also accounted for 6 (46%) of 13 HCA-MRSA isolates. The remaining 7 HCA-MRSA isolates belonged to 5 distinct strain types (ST5, ST36, ST72, ST1, and ST8C [USA500]), 3 of which also carried the SCCmec type IV element. Of the 48 MSSA isolates, 13 (27%) demonstrated closely related PFGE profiles (designated USA500), which also belong to the ST8 clonal complex. No other MSSA strain type accounted for >2 isolates.

The PFGE profiles of 2 MSSA isolates showed striking similarity to the profiles for the prototypical USA300 MRSA strain (figure 2). The spontaneous excision of the methicillin resistance—encoding determinant—the type IV SCCmec element (∼25 kb in size)—from the chromosome of the methicillin-susceptible USA300 strain can be observed as a shift to a lower molecular weight SmaI band (figure 2).

The predominance of ST8/USA300 and ST8/USA500 strains among patients with CA S. aureus infection suggested that the ST8 clonal complex may carry genetic features that facilitate the persistence of these strains in the community. To test this hypothesis, we further analyzed the distribution of 26 virulence genes in a representative set of 32 MRSA and MSSA isolates. All 17 USA300 and USA500 isolates tested positive for the bsa gene encoding a bacteriocin (table 6), compared with only 3 of 15 isolates from other PFGE strain types. USA300 differed from USA500 and other PFGE strain types with regard to its carriage of the genes encoding Panton-Valentine leukocidin and 2 recently described enterotoxins, Q and K—2 pyrogenic toxin superantigens that lack emetic activity [50, 51].

Table 6
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Table 6

Distribution of virulence factors and methicillin resistance determinants in 32 selected isolates of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA), grouped by strain type.

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Discussion

Given the increasing number of CA-MRSA outbreaks and infections [5–12, 14–16], clinicians increasingly face a challenging clinical dilemma: should empirical therapy for CA S. aureus infection include β-lactam regimens, which are traditionally effective against only MSSA, or should therapy against MRSA be included? One approach is to utilize the presence or absence of risk factors for MRSA to determine the empirical treatment regimen. However, we found that, in our population of patients hospitalized with CA S. aureus infection, clinical and epidemiologic factors lacked the predictive capacity to distinguish patients with CA-MRSA infection from those with CA-MSSA infection.

Our results have important clinical implications. First, our findings suggest that, even if patients with CA S. aureus infection lack exposures associated with MRSA in contemporary outbreaks of infection, the probability of MRSA infection is still sufficiently high to warrant administering empirical therapy against MRSA. For example, in our population, lacking the top 3 MRSA risk factors would change the pretest probability of MRSA from 60% to a posttest probability of 52%. Modeling our data to a population in which MRSA comprise 10% of CA S. aureus isolates, lacking the top 3 risk factors only decreases the posttest MRSA probability to 7%. Thus, risk factors and exposures should be neither used to justify inclusion or omission of empirical antibiotics targeting MRSA nor used to exclude infected patients from receiving infection-control measures for controlling MRSA spread.

A second important implication of our investigation relates to findings from our molecular studies. We identified a combination of 4 genes uniquely associated with USA300 that included the genes encoding bacteriocin, Panton-Valentine leukocidin, and enterotoxins Q and K. This 4-gene combination was found only in 1 non-USA300 isolate in our strain population, the USA400/MW2 strain (figure 2) [52], consistent with findings of Orwin et al. [53]. The presence of seq and sek in the 2 predominant CA-MRSA strains found in this country, USA300 and USA400/MW2, suggests that production of enterotoxins may be sufficient to promote S. aureus colonization and transmission [54]. Production of bacteriocin may further facilitate the spread of USA300 and USA400/MW2 in community settings because of its antimicrobial activity against normal bacteria flora [55]. Finally, the spontaneous excision of the type IV SCCmec element from the chromosome of the methicillin-susceptible USA300 strain (figure 2) suggests that some USA300 isolates may have lost their type IV SCCmec methicillin-resistance determinant and, thus, become MSSA. Alternatively these strains may represent the methicillin-susceptible progenitors of the USA300 strain. This phenomenon has also been seen with the USA400/MW2 strains [56].

There are limitations to our study. First, our investigation was performed at a single health care center. Therefore, the findings may not be generalizable to other areas where circulating strain types and patient exposures differ. Nevertheless, Los Angeles County has been affected by outbreaks of CA-MRSA infection in well-defined populations (e.g., athletes, prisoners, and men who have sex with men) [6, 28, 57, 58]. In areas that lack outbreaks that involve these populations, the link between these risk factors and CA-MRSA infection may be even weaker. Second, our study examined patients hospitalized for CA S. aureus infection. However, because of the limitations of our County health care system, patients treated as outpatients in other settings are frequently hospitalized. Therefore, our findings may be generalizable to some outpatient populations. Finally, our definition of CA infection included patients hospitalized in the past year and differs from the ABC criteria [33]. Nonetheless, in our post-hoc analyses, which used a stricter definition of CA infection, excluding patients who had been hospitalized within the past 12 months, our findings did not significantly differ from our primary analysis.

There are strengths to our investigation. First, our study was prospective, and epidemiologic data were obtained quickly (typically 1 day after hospitalization). At the time of interview, the susceptibilities of S. aureus isolates were generally not known. In most other studies, MRSA risk factor surveys were performed as part of an outbreak investigation and conducted long after diagnosis and treatment [7, 15, 22, 25, 26], potentially resulting in recall bias [27]. Second, we obtained data on a wide range of potential risk factors that are generally not recorded in medical records (e.g., hygiene and recent incarceration). Third, the participation rate among eligible subjects (92%) was very high.

In conclusion, in this prospective investigation, we found no reliable epidemiological or clinical risk factors that could distinguish patients infected with CA-MRSA from those infected with CA-MSSA. From an infection-control perspective, our data suggest that, in areas where a significant proportion of patients hospitalized for CA S. aureus infection carry MRSA, contact isolation should be given to all patients with suspected CA S. aureus infection and should be continued until pathogens are identified. From a clinical standpoint, given the morbidity and mortality associated with delayed treatment of MRSA infection [59, 60], it is prudent to include MRSA coverage in empirical antibiotic regimens in settings where a significant proportion of patients hospitalized for CA S. aureus infection have MRSA.

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Acknowledgments

We would like to thank Claudia Morales for her critical contributions with subject recruitment and Kelli Matayoshi and James Lee for their assistance with data management. We wish to thank Dr. Robert Yoshimori and the Clinical Microbiology Laboratory at Harbor-UCLA for their tremendous assistance and patience. We also thank Elizabeth Bancroft and Nolan Lee for their valuable contributions to the survey. We are indebted to the patients who participated in the study and took the time to speak with us about their illnesses.

Financial support. Centers for Disease Control and Prevention (R01/ CCR923419 to L.G.M.); National Institute of Health (K23AI0183 to L.G.M.); and Short Term Training Program at the David Geffen School of Medicine at the University of California, Los Angeles to N.T.

Potential conflicts of interest. All authors: no conflicts.

  • Received August 4, 2006.
  • Accepted November 6, 2006.
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