Original ArticleLength of comorbidity lookback period affected regression model performance of administrative health data
Introduction
It is well recognized that patient comorbidity plays an important role in determining mortality and morbidity outcomes [1], [2]. As a result, it has become common practice to adjust for comorbidity in analyses of health outcome data [3], [4], [5].
With the emergence of administrative databases, the ability to access longitudinal patient data to adjust for comorbidity has improved considerably. This raises the issue of the most appropriate lookback period to determine patients' disease status for risk estimation, given that the effects of different comorbid conditions may vary depending on their recency and duration. Most research employing comorbidity adjustment has used relatively short lookback durations (<1 year), based primarily on practical considerations such as convenience, clinical judgement, and availability of data, rather than empirical evidence [6]. Longer lookback periods are likely to capture more conditions per patient, as well as assign comorbidities to a greater proportion of patients. However, this may come at a cost of weakening the effect of comorbid conditions deemed to be present, and degrade the fit of the risk adjustment model.
The influence of lookback period on risk estimation has been demonstrated with certain primary diagnoses such as ischaemic stroke [7] and cancer [8]. However, appropriate lookback period length for ascertainment of comorbidity for risk adjustment has received limited attention. To date, research has mostly ascertained comorbid disease status either at index admission only or within a relatively short period of 1 year [2], [3], [9]. Isolated research has employed lookback periods as long as 2 years for evaluating posthospitalization mortality [6]. However, it is unclear what effect longer lookback periods would have for such outcomes in terms of statistical modeling and predictive ability. Further, the appropriateness of different lookback durations on health outcomes such as hospital readmission is yet to be investigated. Also, it remains unclear whether the suitability of different lookback periods differs depending on the clinical population investigated.
Our aim was to determine, using routinely collected administrative health data, the effects of comorbidity ascertainment with different lookback durations for modeling (1) death within 1 year of hospital admission; and (2) readmission within 30 days of hospital separation, for two clinically distinct populations.
Section snippets
Western Australian Data Linkage System (WADLS)
The WADLS systematically links administrative health data within a single Australian State (population 1.95 million) [10], [11]. It combines seven core databases; births, midwives' notifications, cancer registrations, inpatient hospital morbidity, mental health services, deaths, and electoral registrations, variously dating back to 1966. The system is updated on a continuous basis, and includes working links with >30 external health service and research databases. The Western Australian (WA)
Results
Longer lookback resulted in more comorbidity being identified. For the entire sample (n = 676,142), 46.8% of comorbidity observed across the 5-year lookback period was recorded at index hospitalization. This increased to 68.6, 79.1, and 89.5% with the addition of 1, 2, and 3 years of lookback, respectively. Similar 5-year trends were seen for both patient groups, although a greater occurrence of comorbidity was present in the medical group (163.0% of procedural records).
Discussion
This study is the first to investigate effect of lookback durations >2 years for comorbidity ascertainment on model fit and predictive ability. Further, it is the only investigation to focus on this area of modeling methodology for both mortality and readmission, and is the first to report differences in lookback period performance between clinical groups.
Our results demonstrated that <50% of comorbidity recorded in the preceding 5 years was present at index hospitalization. As anticipated,
Conclusion
Comorbidity adjustment with health outcome modeling is influenced by lookback period length, outcome of interest, and patient type. For posthospitalization mortality, while the addition of comorbidity up to 5 years preindex admission does not degrade model fit, little improvements are seen above that with approximately 1 year of lookback. Further, ascertainment of comorbidity (for adjustment purposes) at index admission only appears appropriate for procedural patients but not for medical
Acknowledgments
The initial construction of the Western Australian Data Linkage System (WADLS) was funded by the WA Lotteries Commission. This study was supported by a project grant from the Australian National Health and Medical Research Council.
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