Validity of the recorded codes of gonadotropin-releasing hormone agonist treatment and orchiectomies in the Danish National Patient Registry

PURPOSE Large-scale observational studies based on existing medical databases may have an important role in studies of long-term effects of different treatments in prostate cancer patients if the coding of the treatment is valid. We therefore estimated the positive predictive value (PPV) and negative predictive value (NPV) of hospital codes for gonadotropin-releasing hormone (GnRH) agonist treatment and orchiectomies in the Danish National Patient Registry (DNPR). PATIENTS AND METHODS From Danish prostate cancer patients we selected 100 patients who were registered as users of GnRH agonists, 100 patients who were registered as nonusers of GnRH agonists, 50 patients who were registered as bilateral orchidectomized, and 50 patients who were not registered as orchidectomized in the DNPR between January 1, 2002 and December 31, 2008. From the patients' medical files we recorded codes for GnRH agonist treatment and orchiectomies, including dates of treatment from date of first prostate cancer diagnosis and onward. RESULTS The PPV of GnRH agonist treatment coding in the DNPR was 93% (95% confidence interval [CI]: 86.1-97.1), and the NPV was 94% (95% CI: 87.4-97.8). Both the PPV and NPV of orchiectomy coding in the DNPR were 100% (97.5% CI: 92.9-100). CONCLUSION We measured the validity of codes for GnRH agonist treatment and orchiectomies in the DNPR among prostate cancer patients and found high PPV and NPV. Thus, the DNPR remains a valuable tool for clinical epidemiological studies of GnRH agonist treatment and orchiectomies in the treatment of prostate cancer.