<< Emergency Department Infections In The Era Of Community-Acquired MRSA

Diagnostic Studies

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Diagnostic Studies

Diagnostic Studies

The current IDSA guidelines for SSTI treatment4 recommend that patients with signs and symptoms of systemic toxicity (abnormal vital signs such as fever or hypothermia, tachycardia, or hypotension) have a laboratory evaluation that includes:

  • Complete blood cell count and differential
  • Blood cultures
  • Creatinine level
  • Bicarbonate level
  • Creatine phosphokinase level
  • C-reactive protein level

The society also recommends that patients with abnormalities in these variables be admitted for aggressive evaluation and treatment, including surgical consultation, to exclude a deep invasive infection. (See Necrotizing Infections section, and Table 6.)

A risk stratification tool for predicting a necrotizing infection based on several laboratory parameters— the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC)—has been developed. Points are assigned to different laboratory values and a total score is obtained. The maximum score is 13. A score greater than or equal to 6 has a positive predictive value of 92% for necrotizing fasciitis (95% CI, 84.3%-96.0%) and a negative predictive value of 96% (95% CI, 92.6%-97.9%). The Laboratory Risk Indicator has not been prospectively validated, and its ability to identify necrotizing fasciitis early in its course is unproven.63,64


Ultrasonography can be a useful adjunct to help identify deep abscesses, differentiate cellulitis from an abscess, and guide drainage.65,66 (See Figure 1.)


Should Abscesses Be Cultured?

No studies have answered this question, and expert opinion alone currently guides practice. Obtaining a culture during incision and drainage was considered unnecessary before the emergence of CA-MRSA.6Although many experts now advocate obtaining cultures routinely during incision and drainage,2,4,5,48 obtaining cultures for every purulent infection is controversial. Abrahamian et al have recommended not sending cultures when the results will not change management (eg, when antibiotics for CA-MRSA will be given regardless), pointing out that patients should not be billed for studies done solely for epidemiologic surveillance.7,67 Wallin et al have recommended sending cultures from the ED for a complicated abscess (defined by fever, lymphangitis, or significant surrounding cellulitis), for patients requiring admission, and for immunocompromised hosts.9

Is It Possible To Culture Cellulitis?

As mentioned previously, CA-MRSA’s role in nonpurulent SSTIs (ie, cellulitis without purulence) is unknown. Importantly, in the EMERGEncy Net ID study demonstrating a high prevalence of CA-MRSA, all of the patients had purulent infections for which cultures were easily obtained.2 Cellulitis generally cannot be cultured, and when a culture is drawn, it has low yield.8Yields for needle aspirations and more invasive punch biopsies in this disorder are also low and are generally not recommended.4

When Should Blood Cultures Be Sent?

With cellulitis, blood cultures are positive in less than 5% of cases.68 Cultures are not routinely recommended for simple SSTIs but are recommended when there are more serious signs and symptoms or concerns about an invasive infection.4,8

Special Considerations

Severe Community-Acquired Pneumonia

Although HA-MRSA has been identified as a pathogen in nosocomial and ventilator-associated pneumonias for many years, it is not a community-acquired pneumonia (CAP) pathogen. CA-MRSA, on the other hand, can cause severe CAP. In 2002, Gillet et al published a report of 8 cases of CAP due to PVL-positive S aureus in France in which there were 6 fatalities.28 In recent years, reports of CA-MRSA causing serious CAP have increased.69-73 Many of these cases have involved previously healthy children and young adults.

Characteristic findings in CA-MRSA pneumonia include occurrence during an influenza outbreak or a preceding flulike illness and rapid progression to severe illness. The nationwide Emerging Infections Network identified 51 cases of CA-MRSA during the 2006-2007 influenza season.74 Of these, 47% of patients had a documented viral syndrome preceding identification, and 33% had confirmed influenza. The overall mortality rate was 51%.

Radiographic characteristics include patchy or homogeneous bronchopneumonia, cavitary infiltrates, pneumatoceles, pleural effusion, and empyema. Studies have identified the isolates as SCCmec type IV, USA300, and PVL positive for the exotoxin gene. Patients in these studies had high rates of respiratory failure and circulatory collapse as well as high mortality rates (25% to 60%).69-73 The possibility of CA-MRSA must be considered in any patient presenting to the ED with severe CAP. (See Table 7.)

Publication Information

Denise Nassisi

Publication Date

October 1, 2010

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