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

Adult severe sepsis is associated with high mortality, morbidity and hospital costs [1‐5] and remains a real challenge for clinicians and health care systems [5‐8]. Population estimates of severe sepsis and their trends are essential to assess disease burden and estimate healthcare resource requirements [3, 8‐10].

Owing to the difficulty in prospectively identifying cases at the population scale [8‐11], population-based studies of severe sepsis have been based on the International Classification of Diseases, Ninth Edition, Clinical Modification (ICD-9-CM). However, reported incidence estimates of severe sepsis and related hospital mortality vary widely from 13 to 300 cases per 100,000 inhabitants and from 28 % to 50 %, respectively [1, 2, 4, 9]. Among other factors, this disparity seems to be determined by biases introduced by the different strategies used to identify cases [9, 12, 13].

Following the definition in 1991 of severe sepsis as sepsis associated with acute organ dysfunction [14], population estimates of the incidence of severe sepsis and its associated mortality have been based on combination algorithms of ICD-9-CM infection and organ dysfunction codes [15, 16]. Recognizing the limitations of such algorithms, sometimes described as excessively inclusive and with scarce specificity to represent real cases of severe sepsis [10, 16, 17], in 2002, a set of explicit ICD-9-CM codes was issued [18]. However, despite this coding system having been developed more than 10 years ago, and the specificity being close to 100 % [19], studies conducted in the USA indicate that the use of this strategy is scarce and is restricted to patients with more severe sepsis [13]. In effect, both for population-based and other research studies, combination codes are most widely used [9, 13]. However, it is not known if this practice is generalized because no European population-based studies have addressed the use of explicit severe sepsis codes, and the impacts of the given coding system used on epidemiological incidence and mortality estimates are unknown.

The present study sought to identify cases of severe sepsis captured by explicit and combination codes from a national registry, and compare these two coding strategies in terms of: (1) their implementation to identify severe sepsis in adults in Spain and their impacts on incidence estimates and (2) their definition of a given profile of patient demographics, clinical characteristics and hospital outcomes.

Over the six-year period, there were 222 846 adult hospitalizations for severe sepsis. The codes used for these hospitalizations were combination in 84 329 (37.8 %) and explicit in 138 517 (62.2 %). Among the latter, 93 380 cases (67 %) were coded 785.52 and 45 137 (33 %) were coded 995.92.

In 2006, 51 % and 49 % of cases were coded using explicit and combination codes respectively. However, in the last year of the study period (2011), these figures were 64.2 % and 35.8 % respectively. Trends in explicit coding practices (Fig. 1) indicate the fairly stable use of code 785.52 (from 40.5 % to 40.2 % across the period) and a notable increase in cases coded 995.92 (from 10.4 % in 2006 to 24 % in 2011).

The episodes of severe sepsis identified amounted to 1.1 % of all adult hospitalizations over the 6-year period and an overall incidence of 97.5 cases per 100,000 inhabitants. However, crude incidence rates were 36.9 cases per 100 000 inhabitants for the combination codes and 60.6 cases per 100,000 inhabitants for the explicit codes. In this cohort, incidence rates were 19.75 cases per 100,000 inhabitants for code 995.92 (severe sepsis) and 40.85 cases per 100 000 inhabitants for code 785.52 (septic shock).

From 2006 to 2011, the overall number of captured cases increased from 25 808 to 46 774, representing an annual increase of 13.5 %. Figure 1 shows the number of cases of severe sepsis identified using explicit and combination codes across the study interval. In the explicit code cohort, adjusted incidence rates (Fig. 2) went from 36 cases per 100 000 inhabitants in 2006 to 73.6 per 100 000 in 2011, giving an APC of 12.3 % (95 % CI 4.4, 20.8) and from 34.6 to 40.9 cases per 100 000 inhabitants in the combination code cohort, giving an APC of 3.8 % (95 % CI 1.3, 6.3). Within the cohort of explicit codes, the adjusted incidence rate of cases assigned code 785.52 increased from 28.6 to 46.5 cases per 100 000 inhabitants, giving an APC of 8.1 % (95 % CI 2.2, 14.3), while cases identified using 995.92 changed from 7.4 to 27.3 cases per 100 000 giving an APC of 21.7 % (95 % CI 7.2, 38.1).

The results of this study reveal the elevated use of explicit codes to define severe sepsis in Spain and an upward trend in this practice over the period 2006 to 2011. Compared to combination severe sepsis codes, these ICD-9-CM codes captured a case profile featuring greater organ dysfunction, healthcare effort and in-hospital mortality. These differences will have an enormous impact on estimates of disease burden. Our findings also indicate that cases coded with explicit 785.52 (septic shock) or 995.92 (severe sepsis) codes, though having different characteristics, had similar outcomes and practically the same in-hospital mortality.

Our findings are inconsistent with recent reports from the USA such as that by Gaieski et al. [9], who in a retrospective study based on nationwide in-patient data for 2004 to 2009, found that only a minority of cases of severe sepsis (between 14 % and 36.9 % according to the capture algorithm used) were assigned explicit discharge codes. In another retrospective study by Whittaker et al. conducted at a single tertiary hospital [13], it was observed that among 1735 cases of severe sepsis between 2005 and 2009, only 21.5 % of cases had explicit severe sepsis/septic shock discharge codes (995.92, 785.52) and that this trend remained stable over the period examined. In contrast, in our study over 60 % of cases in the national health network were documented with such codes. In addition, the implementation of these explicit codes, especially code 995.92, had an increasing trend over the six-year study period. Although the reasons for this difference are unknown, given Spain's universal health system, we can assume that coding practices will not be related to financial incentives. With regards to other factors, it is likely that the efforts to improve coding strategies for hospital discharge registries and the education programmes and campaigns carried out in our country in recent years, such as the Edusepsis campaign [32], will have improved the awareness and training of healthcare professionals in the identification and diagnosis of severe sepsis, leading to the observed increased use of explicit codes.

In line with prior findings [15, 16, 22], we detected a marked increase in the incidence of severe sepsis though, notably, this increase was mainly accounted for by the cohort of explicit codes, and especially, of code 995.92. Even if the interpretation of this increase may be confounded in part by factors such as better diagnosis of sepsis, improvement in coding practices or other methodological issues [33], the specificity of these codes [9] suggests that the rising trend in the incidence of severe sepsis in adults in our country may be real and not the consequence of excessive coding of infection and organ dysfunction [23, 34].

In addition, although the populations included in both our cohorts should have been similar as the codes assigned define the same disease, the patients in each cohort had different outcomes and were only well-matched in terms of age and sex.

In agreement with another report [13], our data indicate that it was the cases of greater severity that were assigned explicit codes, and that these codes defined a cohort of patients who, despite having fewer comorbidities, had a higher rate of infection of pulmonary and abdominal origin, more Gram-negative pathogens, and above all, more affected organs and a higher mortality rate. In-patient mortality for the cohort captured using explicit codes was 54 % and practically doubled the rate recorded for the cohort identified through combination codes. Notably, mortality in this latter cohort was similar to the rates reported in other population studies, such as those of Angus [15] and Martin [16], in which combination strategies were exclusively used to identify cases, while our rate for the explicit code cohort was consistent with the mortality rates cited for intensive care units [35, 36]. Furthermore, in this cohort we recorded practically identical mortality rates among cases coded 995.92 or 785.52. While there are appreciable differences in the demographics, comorbidities and potential infection sources in these cases, both groups featured similar multiple organ dysfunction and this factor likely accounts for the high mortality observed in both sub-cohorts [37].

Recent data indicate there is great variability in mortality due to severe sepsis and septic shock which, among other factors, seems to be related to the different definitions used in each study [38]. In addition, studies assessing the use of codes 785.52 (septic shock) and 995.92 (severe sepsis) have been scarce. On reanalysis, Gaiesky [9] observed that hospital mortality among adults with explicit codes was 36.9 % for those coded 995.92 and 42.2 % for those coded 785.52. In 373 patients coded as having severe sepsis/septic shock (995.92, 785.52), Whittaker [13] observed 28-day mortality of 41 %. However, no study has compared the epidemiological characteristics and progression among cases coded 995.92 and 785.52. We feel the novelty of our work is that this type of comparison was performed using a national population database including a large number of cases and a representative case-mix. Very probably, because of the new sepsis definitions [39] in which the use of explicit codes is recommended, new studies with which to compare our findings will soon emerge.

The high percentage of cases assigned explicit codes and their high mortality rate, in large measure explains the differences observed in in-hospital mortality between the present and other population studies including a lower percentage of cases captured by explicit severe sepsis codes. Further, although mortality in our study had an overall declining trend from 2006 to 2011, this decline was significantly lower in the explicit code cohort. Thus, although our data reflect the decreasing trend in hospital mortality due to severe sepsis in adult patients observed in other studies [9, 16, 40, 41], it remains clear that this trend is more limited in cases of greater disease severity. Interestingly, in both our cohorts a stable trend in the extent of organ dysfunction was produced, which despite not confirming recently published data from the USA [34], could nevertheless explain, at least partly, the observed downward trend in mortality.

The elevated proportion of hospitalizations assigned explicit codes observed in our study is perfectly in line with the new definitions of sepsis and recommendations for the use of ICD-9 codes 995.92 and 785.52 [39] for such cases. In effect, although the implementation of these explicit codes is not yet complete in Spain, it is still high and shows an increasing trend. Consistent with these recommendations [39], we predict that better understanding of the concept and definition of severe sepsis through continued education programmes will improve its description in clinical records and thus allow for a more consistent measure of the burden of severe sepsis and its trends.

The limitations of our study are those inherent to investigations based on retrospectively collected clinical-administrative data. Although there are national directives for the use of the ICD-9-CM coding system, this may not have been uniform across all hospitals of the national health network and we cannot rule out coding errors despite regular audits making major errors unlikely. We are also aware that, because of its confidential nature, the database used lacks sufficient information and the data do not allow for causal inferences. However, the use of such databases is well-established in severe sepsis epidemiology and the results of a recent meta-analysis clearly support the use of administrative data to monitor mortality trends in severe sepsis [41], confirming the essential role of the consistent use of national administrative data for epidemiological monitoring of incidence and outcomes [9, 10].

Our country has a large national population-based database covering over 90 % of all hospitalizations produced annually in the country. There are potential benefits of this system including representativeness, identification of systemic problems, and precision of estimation in statistical analysis. Additionally, this study followed the publication guidelines for observational studies laid down in the strengthening the reporting of observational studies in epidemiology (STROBE) initiative [42].

A limitation of our study was that non-hospitalized patients with severe sepsis were not included, meaning that incidence was really an estimate of treated sepsis [43]. Similarly, our mortality estimates were conservative in that we did not include mortality after hospital discharge [3, 5, 44].

Our study reveals an elevated and increasing use of explicit coding practices for adult severe sepsis in Spain. This trend will have substantial impacts on epidemiological and disease burden estimates, because cases are registered of greater severity, care intensity and in-hospital mortality. The variation detected in severe sepsis coding and its effects on population estimates calls for improved continuous education for physicians and the introduction of standardized measures targeted at reducing heterogeneity in coding practices.