Use of explicit ICD9-CM codes to identify adult severe sepsis: impacts on epidemiological estimates

Abstract

Background: Severe sepsis is a challenge for healthcare systems, and epidemiological studies are essential to assess its burden and trends. However, there is no consensus on which coding strategy should be used to reliably identify severe sepsis. This study assesses the use of explicit codes to define severe sepsis and the impacts of this on the incidence and in-hospital mortality rates.

Methods: We examined episodes of severe sepsis in adults aged ≥18 years registered in the 2006-2011 national hospital discharge database, identified in an exclusive manner by two ICD-9-CM coding strategies: (1) those assigned explicit ICD-9-CM codes (995.92, 785.52); and (2) those assigned combined ICD-9-CM infection and organ dysfunction codes according to modified Martin criteria. The coding strategies were compared in terms of the populations they defined and their relative implementation. Trends were assessed using Joinpoint regression models and expressed as annual percentage change (APC).

Results: Of 222 846 episodes of severe sepsis identified, 138 517 (62.2 %) were assigned explicit codes and 84 329 (37.8 %) combination codes; incidence rates were 60.6 and 36.9 cases per 100 000 inhabitants, respectively. Despite similar demographic characteristics, cases identified by explicit codes involved fewer comorbidities, fewer registered pathogens, greater extent of organ dysfunction (two or more organs affected in 60 % versus 26 % of cases) and higher in-hospital mortality (54.5 % versus 29 %; risk ratio 1.86, 95 % CI 1.83, 1.88). Between 2006 and 2011, explicit codes were increasingly implemented. Standardised incidence rates in this cohort increased over time with an APC of 12.3 % (95 % CI 4.4, 20.8); in the combination code cohort, rates increased by 3.8 % (95 % CI 1.3, 6.3). A decreasing trend in mortality was observed in both cohorts though the APC was -8.1 % (95 % CI -10.4, -5.7) in the combination code cohort and -3.5 % (95 % CI -3.9, -3.2) in the explicit code cohort.

Conclusions: Our findings suggest greater and increasing use of explicit codes for adult severe sepsis in Spain. This trend will have substantial impacts on epidemiological estimates, because these codes capture cases featuring greater organ dysfunction and in-hospital mortality.

Keywords: Epidemiology; Health services research; Incidence; Outcome; Severe sepsis; Trends.

Fig. 1

Cases of severe sepsis identified using explicit and combination codes across the study interval. From 2006 to 2011, the number of cases rose from 12 652 to 16 748 in the combination code cohort and from 13 156 to 30 026 in the explicit code cohort. Within this cohort, cases coded 995.92 (severe sepsis) rose from 2697 to 11 233 and those coded 785.52 (septic shock) rose from 10 459 to 18 793

Fig. 2

Trends in age-adjusted incidence of severe sepsis according to assigned ICD-9-CM codes. Values are adjusted annual rates. In the explicit code cohort, adjusted incidence rates increased from 36 cases per 100 000 inhabitants in 2006 to 73.6 per 100 000 in 2011, and from 34.6 to 40.9 cases per 100,000 inhabitants in the combination code cohort. Within the cohort of explicit codes, the adjusted incidence rate of cases assigned code 785.52 (septic shock) rose from 28.6 to 46.5 cases per 100 000 inhabitants, while cases identified using 995.92 (severe sepsis) rose from 7.4 to 27.3 cases per 100 000 inhabitants

Fig. 3

Age-adjusted in-hospital mortality rate for severe sepsis according to discharge ICD-9-CM codes. Values are adjusted annual mortality rates. From 2006 to 2011, in-hospital mortality had a significantly decreasing trend in both cohorts. In the combination code cohort in-hospital mortality fell from 37 % in 2006 to 25 % in 2011. In the explicit code cohort, in-hospital mortality fell from 60 % in 2006 to 50 % in 2011. Within the explicit cohort, rates over the same period diminished from 60 % to 50.4 % in the code 785.52 group (septic shock) and from 59 % to 49.6 % in the code 995.92 group (severe sepsis)

Fig. 4

Trends in the number of organ dysfunctions recorded in the two cohorts examined. Values are percentages. In the explicit code cohort, 34 % of cases involved single organ dysfunction, 32.7 % two organs and 29.4 % more than two affected organs in 2006 versus 30.7 %, 30.2 % and 31.3 %, respectively in 2011. In 2006, the number of dysfunctional organs was not specified in 3.9 % compared with 8 % in 2011. In the combination code cohort, 70.3 % of cases involved single organ dysfunction, 21.6 % two affected organs and 8.4 % more than two affected organs in 2006 versus 73.1 %, 20.8 % and 6.1 %, respectively in 2011