title: Validity of the coding for herpes simplex encephalitis in the Danish National Patient Registry.
abstract: BACKGROUND
  Large health care databases are a valuable source of infectious disease epidemiology if diagnoses are valid. The aim of this study was to investigate the accuracy of the recorded diagnosis coding of herpes simplex encephalitis (HSE) in the Danish National Patient Registry (DNPR).
  
  
  METHODS
  The DNPR was used to identify all hospitalized patients, aged ≥15 years, with a first-time diagnosis of HSE according to the International Classification of Diseases, tenth revision (ICD-10), from 2004 to 2014. To validate the coding of HSE, we collected data from the Danish Microbiology Database, from departments of clinical microbiology, and from patient medical records. Cases were classified as confirmed, probable, or no evidence of HSE. We estimated the positive predictive value (PPV) of the HSE diagnosis coding stratified by diagnosis type, study period, and department type. Furthermore, we estimated the proportion of HSE cases coded with nonspecific ICD-10 codes of viral encephalitis and also the sensitivity of the HSE diagnosis coding.
  
  
  RESULTS
  We were able to validate 398 (94.3%) of the 422 HSE diagnoses identified via the DNPR. Hereof, 202 (50.8%) were classified as confirmed cases and 29 (7.3%) as probable cases providing an overall PPV of 58.0% (95% confidence interval [CI]: 53.0-62.9). For "Encephalitis due to herpes simplex virus" (ICD-10 code B00.4), the PPV was 56.6% (95% CI: 51.1-62.0). Similarly, the PPV for "Meningoencephalitis due to herpes simplex virus" (ICD-10 code B00.4A) was 56.8% (95% CI: 39.5-72.9). "Herpes viral encephalitis" (ICD-10 code G05.1E) had a PPV of 75.9% (95% CI: 56.5-89.7), thereby representing the highest PPV. The estimated sensitivity was 95.5%.
  
  
  CONCLUSIONS
  The PPVs of the ICD-10 diagnosis coding for adult HSE in the DNPR were relatively low. Hence, the DNPR should be used with caution when studying patients with encephalitis caused by herpes simplex virus.
authors.affiliation: Department of Infectious Diseases, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Laura Krogh
authors.initials: LK
authors.lastname: Jørgensen
authors.affiliation: Department of Infectious Diseases, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Lars Skov
authors.initials: LS
authors.lastname: Dalgaard
authors.affiliation: Department of Infectious Diseases, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Lars Jørgen
authors.initials: LJ
authors.lastname: Østergaard
authors.affiliation: Department of Clinical Microbiology, Odense University Hospital, Odense, Denmark.
authors.forename: Nanna Skaarup
authors.initials: NS
authors.lastname: Andersen
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Mette
authors.initials: M
authors.lastname: Nørgaard
authors.affiliation: Department of Infectious Diseases, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Trine Hyrup
authors.initials: TH
authors.lastname: Mogensen
keywords: DNPR; HSE; positive predictive value; sensitivity; validation studies; viral encephalitis