title: Prevalence dependent calibration of a predictive model for nasal carriage of methicillin-resistant Staphylococcus aureus.
abstract: BACKGROUND
  Published models predicting nasal colonization with Methicillin-resistant Staphylococcus aureus among hospital admissions predominantly focus on separation of carriers from non-carriers and are frequently evaluated using measures of discrimination. In contrast, accurate estimation of carriage probability, which may inform decisions regarding treatment and infection control, is rarely assessed. Furthermore, no published models adjust for MRSA prevalence.
  
  
  METHODS
  Using logistic regression, a scoring system (values from 0 to 200) predicting nasal carriage of MRSA was created using a derivation cohort of 3091 individuals admitted to a European tertiary referral center between July 2007 and March 2008. The expected positive predictive value of a rapid diagnostic test (GeneOhm, Becton & Dickinson Co.) was modeled using non-linear regression according to score. Models were validated on a second cohort from the same hospital consisting of 2043 patients admitted between August 2008 and January 2012. Our suggested correction score for prevalence was proportional to the log-transformed odds ratio between cohorts. Calibration before and after correction, i.e. accurate classification into arbitrary strata, was assessed with the Hosmer-Lemeshow-Test.
  
  
  RESULTS
  Treating culture as reference, the rapid diagnostic test had positive predictive values of 64.8% and 54.0% in derivation and internal validation corhorts with prevalences of 2.3% and 1.7%, respectively. In addition to low prevalence, low positive predictive values were due to high proportion (> 66%) of mecA-negative Staphylococcus aureus among false positive results. Age, nursing home residence, admission through the medical emergency department, and ICD-10-GM admission diagnoses starting with "A" or "J" were associated with MRSA carriage and were thus included in the scoring system, which showed good calibration in predicting probability of carriage and the rapid diagnostic test's expected positive predictive value. Calibration for both probability of carriage and expected positive predictive value in the internal validation cohort was improved by applying the correction score.
  
  
  CONCLUSIONS
  Given a set of patient parameters, the presented models accurately predict a) probability of nasal carriage of MRSA and b) a rapid diagnostic test's expected positive predictive value. While the former can inform decisions regarding empiric antibiotic treatment and infection control, the latter can influence choice of screening method.
authors: Institute for Hygiene and Microbiology, University of Würzburg, Josef Schneider-Strasse 2, Würzburg, 97080, Germany. jelias@hygiene.uni-wuerzburg.de
authors: Johannes
authors: J
authors: Elias
authors: Peter U
authors: PU
authors: Heuschmann
authors: Corinna
authors: C
authors: Schmitt
authors: Frithjof
authors: F
authors: Eckhardt
authors: Hartmut
authors: H
authors: Boehm
authors: Sebastian
authors: S
authors: Maier
authors: Annette
authors: A
authors: Kolb-Mäurer
authors: Hubertus
authors: H
authors: Riedmiller
authors: Wolfgang
authors: W
authors: Müllges
authors: Christoph
authors: C
authors: Weisser
authors: Christian
authors: C
authors: Wunder
authors: Matthias
authors: M
authors: Frosch
authors: Ulrich
authors: U
authors: Vogel
mesh_terms: D000293:Adolescent; D000328:Adult; D000369:Aged, 80 and over; D002138:Calibration; D002353:Carrier State / Q000175:diagnosis / Q000453:epidemiology / Q000382:microbiology*; D015331:Cohort Studies; D003661:Decision Support Techniques; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000302:isolation & purification*; D008875:Middle Aged; D008954:Models, Biological; D009296:Nasal Cavity / Q000382:microbiology*; D017711:Nonlinear Dynamics; D011237:Predictive Value of Tests; D015995:Prevalence; D015203:Reproducibility of Results; D012307:Risk Factors; D013203:Staphylococcal Infections / Q000175:diagnosis / Q000382:microbiology* / Q000517:prevention & control