Staphylococcus aureus

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Staphylococcus aureus

ARTICLESthe previous epoch. We assumed anunstructured working correlationstructure and used Huber-White sandwichestimates of variance. 23,24 Afterdemonstrating that an ego-alter paircomposed of siblings of whom 1 wasMRSA-positive had significantly higherodds of the uncolonized sibling becomingcolonized compared with nonsiblingego-alter pairs, we removed siblingego-alter pairs from subsequentmodels. The individual patients remainedin the model as egos and altersfor other patients. Therefore, although34% of births were products of multiplegestation, only 1% of data (in whichan ego-alter pair was composed of siblings)was lost. We attempted to analyzesibling ego-alter pairs separatelybut, because of small numbers, wewere unable to draw statisticallymeaningful conclusions from the data.Univariate descriptors of NICU-widecharacteristics were compared by usingStudent’s t tests. Variables found tohave P values of 0.2 were consideredfor inclusion in subsequent multivariatemodels. Logistic regression wasused to construct multivariate modelspredicting an epoch having an incidentcase of MRSA colonization at the end ofthe epoch.Analyses were performed using Stata10.1 (Stata Corp, College Station, TX)and SAS 9.1 (SAS Institute, Cary, NC). Pvalues of .05 were considered statisticallysignificant. Networks were visualizedin Pajek, 25 a freely available softwarepackage for drawing socialnetworks by using the algorithm describedby Kamada and Kawai. 26FIGURE 1Schematic diagrams of some of the patient ties (connections) analyzed. Room numbers correspond tophysical locations of rooms relative to one another (that is, room 1 is immediately next to room 2,whereas there are 3 rooms between rooms 1 and 5). Details of tie definitions are presented in the text.Pt indicates patient; RN, nurse. A, Black lines represent ties between patient 1 and 4 other patients whowere in the NICU with patient 1. From the perspective of patient 1, patient 4 is a roommate, patient 2is 1 room away, patient 5 is 2 rooms away, and patient 3 is 2 rooms away. B, Patients 2 and 3 areconnected through a direct nursing connection (red circle). As a result, indirect nursing connections(dotted lines) exist between patient 3 and patients 1, 4, and 5, because they are all within 1 room ofpatient 2. C, The normalized group degree centrality in this diagram is 0.33, because only 1 of 3possible direct nursing connections exists between MRSA-negative infants (yellow text) and 1MRSA-positive infant (red text).RESULTSWe identified 3488 patients who weretreated in the NICU during 1 nursingshift during the study period. Of those,2620 infants (75%) were screened forMRSA at least once on 272 distinctscreening dates, for a total of 9678screening results. One hundred thirtyfivepatients (5.2%) were found to becolonized during their hospitalization.Of these, 30 patients tested positive oninitial screening and thus were censoredas egos because it was not possibleto determine their networkconnections at the time of their colonization;they remained in the model asalters. Patient characteristics are presentedin Table 1. The median weeklyPEDIATRICS Volume 128, Number 5, November 2011Downloaded from pediatrics.aappublications.org at Univ of California San Diego on May 12, 2012e1185


ARTICLESTABLE 3 Univariate Comparison of NICU-wide Statistics in Epochs With 1 Incident Case of MRSAColonization and Those Without Any and Multivariate Model Predicting That Epoch WouldHave 1 Incident Case of MRSA Colonizationepoch. In addition, the patient/nurseratio showed a trend toward statisticalsignificance (OR: 3.4 [95% CI: 0.89–13.1]; P .07). The average daily censuswas not included in the final modelbecause of colinearity with the patient/nurseratio.IncidentMRSANo IncidentMRSAOR (95% CI)Univariate —Daily census, mean SD, No. of patients 33.8 0.56 33.0 0.46 — .17Patient/nurse ratio, mean SD 2.9 0.03 2.8 0.02 — .004Network diameter, mean SD, No. of ties 4.5 0.11 4.4 0.08 — .45Network density, mean SD, % 0.21 0.004 0.21 0.003 — .96Normalized GDC, mean SD 0.39 0.03 0.28 0.02 — .0001MultivariatePatient/nurse ratio — — 3.4 (0.89–13.1) .07Normalized GDC — — 18.1 (3.6–90.0) .0004GDC indicates group degree centrality.DISCUSSIONThe results of this study indicate thatspecific connections between colonizedand uncolonized patients have a significantimpact on the likelihood that MRSAnegativepatients will become colonized.To our knowledge, this study representsthe first application of social networktheory to infection-control data in aninpatient setting. Despite currentinfection-control strategies, includingcontact precautions and physical separationof MRSA-positive infants, we foundthat persistent connections to these infantsthrough sibship and nursing caresignificantly increased uncolonized patients’odds of becoming colonized. Ofnote, these risk factors are both potentiallymodifiable through changes instaffing patterns, cohorting practices,improvements in hand hygiene, andinfection-control practices focused onmultiple gestations for parents and staffmembers.Network analysis conceptualizes a systemas being composed of objects andthe connections that exist betweenthem. 27,28 Networks can be examinedby using graphical visualization or assessmentof quantitative metrics. 19 Althougha wide variety of analytic measuresand methods are available toexamine network characteristics atsingle points in time, methods to analyzequantitatively longitudinal networkchanges are only beginning to bewidely used. 21,29It is not surprising that we found that aMRSA-positive infant would be significantlymore likely to put his or her siblingat risk of colonization, comparedwith a nonsibling infant. Several mechanismsmay underlie this finding.Khoury et al 6 found that multiple gestationwas a significant independent riskfactor for MRSA colonization, even withcontrolling for lower birth weight andgestational age. Those authors proposedthat 2 factors may contribute tothe risk, that is, inherent vulnerabilityof products of multiple gestation andsiblings sometimes sharing the samebed, thus facilitating transmissionamong them. Of note, in our NICU, siblingswere not moved to separaterooms when 1 was found to be MRSApositive,unlike nonsibling roommates.Although cobedding was not practicedwhen 1 sibling was known to be colonizedand the other was not, a siblingpair might have cobedded before theircolonization status was known (giventhat screening was performed on aweekly basis). Parents also may have aparticular role in spreading coloniza-Ption among siblings, and health careworkers may practice infectioncontrolprocedures differently whenmoving among siblings.We found that a MRSA-negative patienthad significantly increased odds of colonizationwhen that patient was connectedto a MRSA-positive infantthrough a nurse, compared with a situationin which the 2 patients had nonursing care in common. Moreover,when the common nurse was caringfor the MRSA-negative infant directly,compared with caring only for that infant’sroommates or neighbors, andthe exposure was presumably more intense,the relative odds of MRSA colonizationwere higher. The increasedrisk of shared nursing assignmentsfor the development of new MRSA colonizationremained even for connectionsto infants who were already designatedfor contact precautions on thebasis of a previous week’s MRSAscreening culture results (although itwas not possible to determine compliancewith this infection-control measurefor any particular patient in thisretrospective study). The increasedrisk observed with a shared nursingconnection is not simply a reflection ofcohorting practices, because our outcomeof interest, incident MRSA colonization,affects only infants not previouslyknown to be MRSA-positive.From a NICU-wide perspective, wefound that a larger proportion of connectionsbetween colonized and uncolonizedinfants through sharednursing connections was an independentrisk factor (indeed, the only riskfactor that remained statistically significantin the multivariate model) foran incident case of MRSA in that epoch.Therefore, cohorting of nurses andother health care workers such thatthey care only for colonized or uncolonizedinfants may be beneficial, particularlyduring MRSA outbreaks. Furthermore,when MRSA colonizationPEDIATRICS Volume 128, Number 5, November 2011Downloaded from pediatrics.aappublications.org at Univ of California San Diego on May 12, 2012e1187


REFERENCES1. Carey AJ, Duchon J, Della-Latta P, Saiman L.The epidemiology of methicillin-susceptibleand methicillin-resistant Staphylococcusaureus in a neonatal intensive care unit,2000–2007. J Perinatol. 2010;30(2):135–1392. GerberSI,JonesRC,ScottMV,etal.Managementof outbreaks of methicillin-resistant Staphylococcusaureus infection in the neonatal intensivecare unit: a consensus statement. InfectControl Hosp Epidemiol. 2006;27(2):139–1453. Gregory ML, Eichenwald EC, Puopolo KM.Seven-year experience with a surveillanceprogram to reduce methicillin-resistantStaphylococcus aureus colonization in aseems to be increasing in a NICU, cohortingnursing assignments to separateareas of the unit may be beneficial.This notion is supported by themathematical modeling of the spreadof Mycoplasma pneumoniae in an inpatientsetting described by Ancel Meyerset al, 30 which suggested that limitingthe diversity of interactionsbetween different caregivers and patientsmay be the most effectiveinfection-control intervention.This investigation carries the inherentlimitations of a retrospective approach.Although our models controlledfor lower gestational age, wedid not have more precise measures ofillness severity. 31,32 Other unmeasuredconfounders, such as adherence tohand hygiene guidelines, also may exist.To bias our results, however, suchconfounders would need to be associatednot only with becoming colonizedwith MRSA but also with being linked toa MRSA-positive infant through thespecific tie of interest. Severing such atie (eg, a nursing connection) between2 infants still represents a potentialpoint of intervention for curbing thespread of MRSA colonization. As suggestedby our results, even a 10% reductionin the proportion of potentialconnections between the colonizedand uncolonized groups of infantswould be predicted to be associatedwith a 25% (95% CI: 12%–36%) reductionin the odds of developing newcases of MRSA colonization. These improvementscould be expected withoutthe need to decrease average dailycensus counts or to increase staffinglevels through increased attention tothe design of nursing care assignmentson the basis of patients’ MRSAstatus. Our model’s central estimate ofthe effect of the patient/nurse ratio onMRSA spread would suggest a need toincrease nurse staffing by 9% (eg, anadditional 1 or 2 nurses per shift) toachieve a similar result. This study wasconducted in a single NICU, and the resultsmay not be generalizable to othersettings. Finally, this study examinedonly 1 type of connection among patientsthrough health care workers,namely, nurses. We chose to examineconnections through nursing care becausenursing documentation translatesdirectly into contact with patientsat particular times and data on nursingassignments were readily availablein our EHR. Future studies mayuse expanded data sources, such asnotes from bedside clinician groupsand computerized provider order entry,to identify other health care workersinvolved in patient care.CONCLUSIONSIn agreement with other studies, 3 ouranalyses revealed the generally low incidenceof MRSA colonization and progressionto clinical infection that canbe associated with routine surveillanceprograms and isolation of colonizedinfants as part of an overallinfection-control strategy. We havealso demonstrated the importance ofconsidering an infant’s ties to othercolonized patients, in addition to individualpatient characteristics, whenexamining the spread of MRSA colonization.Despite current infectioncontrolpolicies, MRSA-negative infantsare at significant risk of colonizationthrough connections to colonized siblingsand to any other colonized patientwith whom they share a nursingconnection. Use of neglected metadatacontained in the EHR and network analyticmethods allowed us to identifymodifiable risk factors for MRSA colonization.These methods, which are nottraditionally used in health care research,are likely to prove useful in futurequality improvement and epidemiologicalendeavors.ACKNOWLEDGMENTSThis work was supported in part by agrant from the National Library ofMedicine (grant 1 G08 LM 010703-01)and by the Center for Integration ofMedicine and Innovative Technology(US Army Medical Research AcquisitionActivity Cooperative AgreementsDAMD17-02-2-0006, W81XWH-07-2-0011,and W81XWH-09-2-0001).We thank Elise Robinson, PhD, for invaluablestatistical advice.neonatal intensive care unit. Pediatrics.2009;123(5). Available at: www.pediatrics.org/cgi/content/full/123/5/e7904. Back NA, Linnemann CC Jr, Staneck JL, KotagalUR. Control of methicillin-resistantStaphylococcus aureus in a neonatalintensive-care unit: use of intensive microbiologicsurveillance and mupirocin. InfectControl Hosp Epidemiol. 1996;17(4):227–2315. Jernigan JA, Titus MG, Gröschel DH,Getchell-White S, Farr BM. Effectiveness ofcontact isolation during a hospital outbreakof methicillin-resistant Staphylococcusaureus. Am J Epidemiol. 1996;143(5):496–5046. Khoury J, Jones M, Grim A, Dunne WM Jr,Fraser V. Eradication of methicillinresistantStaphylococcus aureus from aneonatal intensive care unit by active surveillanceand aggressive infection controlmeasures. Infect Control Hosp Epidemiol.2005;26(7):616–6217. Saiman L, Cronquist A, Wu F, et al. An outbreakof methicillin-resistant Staphylococcusaureus in a neonatal intensive careunit. Infect Control Hosp Epidemiol. 2003;24(5):317–321e1188GEVA et alDownloaded from pediatrics.aappublications.org at Univ of California San Diego on May 12, 2012

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