This is an excellent review, recently published in JAMA.
It needs to be read in the original but here are a few key points:
When is MRSA coverage needed?
From the article:
There has been increasing concern about antibiotic-resistant bacteria, such as community-acquired methicillin-resistant S aureus (MRSA), which is reflected in the increased use of anti-MRSA antibiotics (eg, vancomycin, trimethoprim-sulfamethoxazole, doxycycline, clindamycin) and broad-spectrum gram-negative antibiotics (eg, β-lactam/β-lactamase inhibitors, levofloxacin, ceftriaxone) during the past decade.40 However, most cases of cellulitis do not involve gram-negative organisms, and in cases of nonpurulent and uncomplicated cellulitis, the addition of antibiotics against community-acquired MRSA did not improve outcomes.41 As such, narrow-spectrum antibiotics against Streptococcus and methicillin-sensitive S aureus remain appropriate. In purulent cellulitis (presence of a pustule, abscess, or purulent drainage), S aureus infection is more likely, as demonstrated by a study of 422 patients who presented with “purulent skin and soft tissue infections” to 11 emergency departments throughout the United States, in which skin surface swab cultures revealed MRSA in 59% of patients, methicillin-sensitive S aureus in 17%, and β-hemolytic streptococci in 2.6%.42 Because methicillin-sensitive S aureus and MRSA can be difficult to differentiate according to clinical features alone,43 MRSA should be considered for purulent infections in known high-risk populations, such as athletes, children, men who have sex with men, prisoners, military recruits, residents of long-term care facilities, individuals with previous MRSA exposure, and intravenous drug users.44
The authors also recommend MRSA coverage for non purulent cellulitis in certain situations: severe systemic manifestations, rapid spread and immune compromise.
When should the spectrum be extended beyond staph and strep (eg anaerobes, gram negatives)?
Severe systemic manifestations, immune compromise, rapid spread.
What if purulent cellulitis is mild and can be treated with oral antibiotics?
Consider confining the coverage to strep and MSSA (but not MRSA). But if MRSA is deemed important to cover TMP/SMX, doxy or clinda may be acceptable (as well as, of course, linezolid).
What are some unusual organisms to consider in special situations?
Traditional (pharmacologic) immunosupression, HIV: strep pneumo, Mtb, gram negatives, crypto species.
Chronic liver disease, CKD: vibrio species ( including vulnificus), gram negatives (including pseudomonas).
When should nec fash (and other complications) be considered?
From the article:
In cases of suspected necrotizing fasciitis, early surgical assessment is recommended; however, laboratory testing may help differentiate cellulitis from early evolving necrotizing fasciitis. Wall et al75 found in a modeling study that a white blood cell count greater than 15 400 cells/mm3 or serum sodium level less than 135 mEq/L could suggest a diagnosis of necrotizing fasciitis with a sensitivity of 90%, specificity of 76%, positive likelihood ratio of 3.75, and negative likelihood ratio of 0.13. Similarly, Wong et al76 developed the Laboratory Risk Indicator for Necrotizing Fasciitis score according to white blood cell count and levels of C-reactive protein, hemoglobin, serum sodium, creatinine, and serum glucose, which had a sensitivity of 90%, specificity of 95%, positive likelihood ratio of 19.95, and negative likelihood ratio of 0.10. Finally, Murphy et al77 identified that for necrotizing fasciitis among cases in their series, a serum lactate level of 2.0 mmol/L had a sensitivity of 100%, specificity of 76%, positive likelihood ratio of 4.17, and negative likelihood ratio of 0. All of these tests are offered as adjunctive tools, along with history, physical examination, and surgical exploration, to guide diagnosis of necrotizing fasciitis.
Imaging studies are not diagnostic of cellulitis but can help distinguish it from more severe forms of infection and can identify drainable fluid collections, such as abscesses. Osteomyelitis can sometimes complicate cellulitis and when suspected can be best ruled out with magnetic resonance imaging or radiography, if chronic. Furthermore, magnetic resonance imaging or computed tomography can help differentiate cellulitis from necrotizing fasciitis or pyomyositis.78 The appearance of gas on computed tomography scan in the absence of soft tissue trauma or a rim-enhancing fluid collection, as would be found with an abscess, is considered pathognomonic of, but not requisite for, a diagnosis of necrotizing fasciitis.79- 81 A recent study evaluating the utility of modern-day computed tomography scanners demonstrated a positive predictive value of 76% and a negative predictive value of 100% and found that only 36% of cases of necrotizing fasciitis included gas.82
In cases of non response to treatment consider cellulitis mimics and possible derm consultation
Mimics include stasis dermatitis, calciphylaxis, erythema migrans and other conditions.