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Clindamycin-susceptibility Rates of Methicillin-resistant Staphylococcus aureus Varies by Infection Type in Pediatric Patients

Li, Amanda PharmD; Selvarangan, Rangaraj BVSc, PhD, D(ABMM); Ogden, Richard PharmD; French, Brandon PharmD; Yu, Diana PharmD

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The Pediatric Infectious Disease Journal: August 2016 - Volume 35 - Issue 8 - p 927-928
doi: 10.1097/INF.0000000000001200
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Methicillin-resistant Staphylococcus aureus (MRSA)-associated infections in the community have increased over time.1 Community-associated MRSA (CA-MRSA) has been known to cause infections in children, especially skin and soft tissue infections (SSTIs). Differences in susceptibility have been recognized based on epidemiologic classification: CA-MRSA isolates are typically susceptible to clindamycin and sulfamethoxazole-trimethoprim, whereas healthcare-associated MRSA (HA-MRSA) isolates are more likely to be resistant to those agents.2 Additionally, some studies have found that the bacteria display different susceptibility patterns depending on the clinical site from which they were isolated.3,4 Antibiograms are utilized to guide selection of empiric antibiotics for treatment of MRSA and should be institution specific. Traditionally, antibiograms include aggregate information from the first isolate per patient; however, specific antibiograms based on epidemiologic classification, infection type and underlying medical conditions may not be readily available. The primary objective was to determine whether there are differences in MRSA susceptibility patterns based on infection type and epidemiology of pediatric patients.

This was a retrospective, descriptive study at a single, free-standing pediatric academic hospital. MRSA isolates were identified through clinical microbiology laboratory culture results from January 1 to December 31, 2011. The first culture obtained from each site for each patient was included. Cultures from patients with cystic fibrosis and nasal surveillance cultures were excluded. Specific antibiograms were created for infection type, epidemiologic classification, unit, immunocompromised patients and patients transferred from an outside facility. The infection type was determined through review of the patient’s profiles and cross-referenced with International Classification of Diseases, 9th revision codes. The following data were collected from the patient’s profile: patient history, hospital location, specimen site susceptibilities, history of immunosuppression, prior hospitalizations and prior MRSA infections. The institutional review board of Children’s Mercy approved this study.

MRSA isolates were classified into 3 epidemiologic classifications. CA-MRSA was defined as cases that had the following criteria5: (1) diagnosis in an outpatient setting or by a culture positive for MRSA within 48 hours after admission to the hospital; (2) no medical history of MRSA infection or colonization, no medical history in the past year of hospitalization, admission to a nursing home, skilled nursing facility or hospice, dialysis or surgery; (3) no permanent indwelling catheter or medical devices that pass through the skin into the body. Healthcare-associated hospital onset (HA-MRSA-HO) was defined as cases with a positive culture result from a normally sterile site obtained >48 hours after hospital admission. Healthcare-associated community onset (HA-MRSA-CO) was defined as cases diagnosed in an outpatient facility or within 48 hours after admission to the hospital and have at least 1 of the following criteria: (1) presence of an invasive device at time of admission; (2) history of MRSA infection or colonization; (3) history of surgery, hospitalization, dialysis or residence in a long-term care facility in the 12 months preceding culture date. Immunocompromised patients were defined as medical history including an oncologic process, use of prolonged steroids or use of immunomodulators.

This study identified 1415 MRSA isolates, and 865 isolates were included in the analysis. The antibiotics reported for MRSA susceptibility included clindamycin, gentamicin, rifampin, tetracycline, linezolid, sulfamethoxazole-trimethoprim and vancomycin. The antimicrobial susceptibilities were similar throughout all the different subcategories, ranging from 97% to 100% susceptible, except for clindamycin. The overall clindamycin susceptibility of MRSA and categories is listed in Table 1. Categories that were identified to have a higher rate of clindamycin resistance (ie, >10% resistance) included non-SSTI type infection, non-CA-MRSA, MRSA isolated outside of the emergency department and MRSA isolated from immunocompromised patients.

Clindamycin Susceptibility by Infection Type and Origin

The institution-wide antibiogram did not appear to be entirely reflective of clindamycin susceptibilities based on the selected categories. This suggests that specific antibiograms may be able to shed light on factors associated with higher clindamycin resistance. Current guidelines from the Infectious Diseases Society of America recommend an alternative antibiotic for SSTIs if local clindamycin resistance rate is greater than 10%.6 Using the overall susceptibility, the rate of resistance was 13%; however, by breaking down the susceptibility into infection type, clindamycin resistance was 10% in SSTI. Thus, clindamycin would remain an adequate empiric therapy. Higher clindamycin resistance was noted for non-SSTIs, and alternative therapies can be considered. These differences in susceptibility are better highlighted through a more specific antibiogram, which may be helpful in selecting appropriate initial therapy for suspected MRSA infection. Early appropriate empiric therapy is critical because initiation of inadequate therapy has been associated with poor clinical outcomes.7

For MRSA isolates that are classified as CA-MRSA, clindamycin had a high susceptibility rate compared with HA-MRSA-CO and HA-MRSA-HO isolates (91% vs. 78% and 44%, respectively; P < 0.001 for both), which is consistent with previous reports.2 Isolates that were identified from the pediatric intensive care unit had a lower susceptibility rate compared with the isolates from other locations, which is consistent with prior literature: unit-specific antibiograms demonstrated that intensive care units have increased resistance in comparison with the general population.8,9

The study was limited by the small number of isolates in some categories, so the susceptibility rates may be imprecise. This was a retrospective study, so it was dependent on accurate patient profiles and patient recall of past medical history. There was a risk of misclassifying the isolates into the CA or HA classification and clinical manifestations. The study also does not take into account prior antibiotic exposure which can have an effect on resistance rates. There are no consensus guidelines that provide recommendations on how to create an antibiogram based on clinical manifestation or epidemiologic definitions.

This study demonstrates that when MRSA isolates are classified according to clinical manifestation, they display different resistance profiles to antibiotics, especially clindamycin. This can be utilized along with an institution-wide antibiogram and clinical judgment to help health care providers select empiric antibiotic therapy.


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methicillin-resistant Staphylococcus aureus; antibiogram; clindamycin

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