What is new and what should change now?
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Presence of comorbidity, irrespective of method used to measure it, was independently associated with an increased risk of 1-year and 5-year mortality.
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The Enhanced-Charlson Index matched the original Deyo–Charlson Index in predicting mortality.
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Retrieving information on comorbidity from previous hospital admissions significantly increased the prevalence of patients' comorbidities but did not result in any improvement in model discrimination. Thus information from any index hospitalization could be sufficient to measure and adjust for comorbidity.
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The Western Australia hospital morbidity data (HMD) system is a valid tool to assess major outcomes.
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A dose-response effect of repeated conditions (e.g., myocardial infarction and cerebrovascular accident) as reported in the administrative HMD was demonstrated on the risk of 5-year mortality.
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The findings suggest that comorbid conditions diagnosed at different points in time may have different associations with the risk of dying.
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More research is required to integrate the effect of repeated episodes in currently used methods that measure and adjust for comorbidity.
Monitoring systems often use administrative data (e.g., hospital morbidity data [HMD]) to predict, at the time of hospital admission, each patient's probability of developing an adverse outcome if average care were given [1], [2]. Differences in outcome among patients may or may not indicate differences in the quality of care that the patients received because these differences may be attributed to many factors, including differences in age, severity of illness, comorbid conditions, and also differences in methods of data collection and data quality [3], [4], [5], [6], [7], [8].
The power of any model to predict an adverse outcome depends on the extent and accuracy of the data on each patient's clinical condition when care began [9]. Although clinical data retrieved from patients' files (or chart review) are considered the gold standard for accurate clinical information, these are costly and time consuming to obtain, and often large clinical databases for comparative purposes are not easily available. Therefore, administrative data or claims data are being increasingly used to assess clinical outcomes and monitor, evaluate, and improve the quality of care [10], [11]. These data have many advantages over the clinical data derived from chart review, including their availability and coverage of large populations. However, research based on administrative data, such as HMD, requires a rigorous assessment of their quality, so considerable effort must be invested in data validation [4], [12], [13], [14]. This is particularly true given that the HMD were not originally collected for the purpose of health research. Another major disadvantage of HMD is the difficulty of differentiating complications from coexisting conditions [15], particularly if a single hospital admission (or index admission) rather than all previous hospital admissions were used to derive the medical information. Extraction of comorbidity information that relies on a single hospital admission may underestimate a patient's comorbid status [16], [17]. This becomes evident when an attending physician who summarizes the discharge document chooses to document more serious clinical conditions, while leaving out “less serious” diagnoses, such as chronic conditions [9]. This is especially true in administrative data sets that have limited coding spaces [18]. Incomplete coding of secondary diagnoses may result in inadequate adjustment for illness severity because of differences in case-mix reporting, which may bias assessments of patient risks of poor outcomes, as demonstrated in a large study on in-hospital mortality among elderly patients [9]. Iezzoni et al. found that chronic conditions, such as adult-onset diabetes mellitus (DM), hypertension, angina pectoris, heart valve disease, and previous myocardial infarction (MI) significantly lowered the risk of in-hospital mortality. Because on a clinical basis, these conditions were expected to increase the risk of death, the researchers concluded that their findings were probably because of undercoding of chronic conditions for more severely ill patients [9].
In hospitalized populations, comorbidity is one of the main factors associated with adverse outcomes [5], [7], [19], [20], [21], [22]. Studies have consistently shown that comorbidity (either acute or chronic illness) predicts higher in-hospital mortality, longer length of hospital stay, higher hospital costs, and readmissions [19], [20], [21], [22]. Thus, many researchers have developed coding algorithms that suit administrative data to account for patients' comorbidity conditions [23], [24], [25], [26], [27], [28], [29], [30]. Among the International Classification of Diseases (ICD) coding algorithms, Charlson Comorbidity Index (CCI) [30] with its many adaptations [24], [25], [26], [27], [28] and Elixhauser's comorbidities [29] are the most widely used in administrative data sets to measure and control for the effects of comorbid illness. Initially developed in 1987 [30], the CCI predicts major outcomes (mainly 1-year all-cause mortality) for patients who may have up to 19 comorbid conditions. Each condition is assigned a score of 1, 2, 3, or 6 depending on the risk of dying associated with the condition. The scores are summed to give a total score that predicts the event of interest. In contrast, Elixhauser's 30 comorbidities do not sum to an index but are retained as separate and independent measures, thus allowing the assessment of the association of different comorbid conditions with different outcomes [29].
The objectives of this study that integrated longitudinal data from a large population-based cohort of men with Western Australia (WA) HMD, Cancer Registry, Mental Health Services System (MHSS), and mortality records were the following: (1) to validate the HMD of WA, (2) to compare the performance of three comorbidity adjusting methods in predicting 1-year and 5-year all-cause mortality in a male elderly general hospital population in WA, (3) to compare levels of model discrimination using information derived from a single index hospitalization with that derived from all past hospital admissions, (4) to assess the association of comorbidity with 1-year and 5-year mortality and with any hospitalization after baseline screening, and (5) to assess the associations of past repeated conditions with mortality.