Assessment of incidence of severe sepsis in Sweden using different ways of abstracting International Classification of Diseases codes: Difficulties with methods and interpretation of results

OBJECTIVE: To compare three International Classification of Diseases code abstraction strategies that have previously been reported to mirror severe sepsis by examining retrospective Swedish national data from 1987 to 2005 inclusive. DESIGN: Retrospective cohort study. SETTING: Swedish hospital discharge database. PATIENTS: All hospital admissions during the period 1987 to 2005 were extracted and these patients were screened for severe sepsis using the three International Classification of Diseases code abstraction strategies, which were adapted for the Swedish version of the International Classification of Diseases. Two code abstraction strategies included both International Classification of Diseases, Ninth Revision and International Classification of Diseases, Tenth Revision codes, whereas one included International Classification of Diseases, Tenth Revision codes alone. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: The three International Classification of Diseases code abstraction strategies identified 37,990, 27,655, and 12,512 patients, respectively, with severe sepsis. The incidence increased over the years, reaching 0.35 per 1000, 0.43 per 1000, and 0.13 per 1000 inhabitants, respectively. During the International Classification of Diseases, Ninth Revision period, we found 17,096 unique patients and of these, only 2789 patients (16%) met two of the code abstraction strategy lists and 14,307 (84%) met one list. The International Classification of Diseases, Tenth Revision period included 46,979 unique patients, of whom 8% met the criteria of all three International Classification of Diseases code abstraction strategies, 7% met two, and 84% met one only. CONCLUSIONS: The three different International Classification of Diseases code abstraction strategies generated three almost separate cohorts of patients with severe sepsis. Thus, the International Classification of Diseases code abstraction strategies for recording severe sepsis in use today provides an unsatisfactory way of estimating the true incidence of severe sepsis. Further studies relating International Classification of Diseases code abstraction strategies to the American College of Chest Physicians/Society of Critical Care Medicine scores are needed.