Data quality of the monoclonal gammopathy of undetermined significance diagnosis in a hospital registry

OBJECTIVE: To estimate the positive predictive value (PPV) and completeness of the monoclonal gammopathy of undetermined significance (MGUS) diagnosis coding in a hospital registry within a population-based health-care setting. PATIENTS AND METHODS: Through the Danish National Patient Registry (DNPR), we identified 627 patients registered with MGUS in two Danish regions during the period January 2001–February 2011. We reviewed the medical records of all patients registered with MGUS at the Department of Hematology, Aalborg University Hospital, and a sample of patients registered at the other three hematological departments in the two regions. We estimated the PPV of the MGUS diagnosis based on this sample of 327 medical records. We also estimated the completeness of the DNPR by linking data from the DNPR and data from a previously validated MGUS cohort of 791 patients identified through the laboratory system covering North Jutland Region. RESULTS: The diagnosis of MGUS was confirmed in 231 patients and assessed as probable in an additional 38 patients, corresponding to a PPV of 82.3% (95% confidence interval [CI] 78.1%–86.4%). By contrast, 58 (17.7%) of the patients did not definitively meet the diagnostic criteria for MGUS. When we excluded patients registered with malignant monoclonal gammopathy recorded prior to or within the first year after registration of MGUS in the DNPR, the PPV increased to 88.3% (95% CI 84.5%–92.1%). The DNPR only registered a diagnosis of MGUS in 133 of the 791 MGUS patients identified through the laboratory system, corresponding to a completeness of 16.8% (95% CI 14.1%–19.6%). CONCLUSION: The PPV of the diagnosis coding for MGUS in the DNPR is high and can be further improved by simple data restriction. However, the low completeness raises concern that MGUS patients registered in the hospital system may be highly selected.