title: Acromegaly according to the Danish National Registry of Patients: how valid are ICD diagnoses and how do patterns of registration affect the accuracy of registry data?
abstract: BACKGROUND
  The incidence of acromegaly is uncertain, since population-based studies are few. In the absence of a specific acromegaly registry, the Danish National Registry of Patients (DNRP) becomes a potential source of data for studying the epidemiology of acromegaly, by linking all hospital discharge diagnoses to the personal identification numbers of individual Danish inhabitants. The validity of the DNRP with respect to acromegaly, however, remains to be tested. The aim of this study was to validate the International Classification of Diseases (ICD) codes for acromegaly (ICD-8: 25300, 25301. ICD-10: E22.0) as used in the DNRP, and to assess the influence of various registration patterns on the accuracy of registry data.
  
  
  METHODS
  We identified patients registered with ICD codes for the diagnosis of acromegaly or other pituitary disorders during the period 1991-2009. Data on the institutional origin of each registration and the number of relevant DNRP registrations were recorded, and systematic patient chart reviews were performed to confirm the diagnosis.
  
  
  RESULTS
  In total, 110 cases of acromegaly were confirmed, compared with 275 registered cases, yielding a positive predictive value (PPV) of 40%. When restricting the search to the regional highly specialized department of endocrinology, the PPV increased to 53% with no loss of cases with confirmed acromegaly. With a requirement of at least one, two, or three DNRP registrations, the PPV increased, but with a concurrent loss of confirmed cases.
  
  
  CONCLUSIONS
  The DNRP seems to be a useful source for identifying new cases of acromegaly, especially when restricting the search to a relevant regional highly specialized department. The PPV of DNRP data used for this purpose can be increased by including only cases with several registrations. A similar approach may be successfully applied to other rare diseases in which continuity of care is provided by highly specialized departments.
authors.affiliation: Department of Endocrinology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Jakob
authors.initials: J
authors.lastname: Dal
authors.affiliation: Department of Endocrinology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Nikolaj
authors.initials: N
authors.lastname: Skou
authors.affiliation: Department of Endocrinology, Aalborg University Hospital, Aalborg, Denmark.
authors.forename: Eigil Husted
authors.initials: EH
authors.lastname: Nielsen
authors.affiliation: Department of Endocrinology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Jens Otto Lunde
authors.initials: JO
authors.lastname: Jørgensen
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Lars
authors.initials: L
authors.lastname: Pedersen
keywords: Danish National Registry of Patients; acromegaly; epidemiology; incidence; validation