The major findings of this study are that older age, lower education, and the presence of dementia are independently associated with differential performance of scales used as indicators of disease burden. While the two measures evaluated here were developed for different purposes and in different population contexts, both the CDS (and its later derivatives) and the CIRS-G have been considered generally acceptable measures of comorbidity. The CDS has the sizable advantage of automated scoring from administrative pharmacy data. However, here we show that the CIRS-G, which requires detailed clinical assessment, provides a very different picture of chronic disease burden and that the differences are magnified in the very old and persons with dementia.
Strengths of the present study are several. Both scales were completed during the same evaluation, and all prescription drugs used by subjects, and their duration of use (to ensure compatibility with the timeframe used in scoring the CDS), were confirmed by detailed inspection in the home environment supplemented by prescribing records. High inter-rater reliability for the CIRS-G was established empirically, and the sample size was nearly twice that of the largest CIRS-G study published to date (18
). A limitation of our study is the use of the original CDS (1
), the only version available at the outset of data collection. Subsequent refinements of the CDS (2
) might perform somewhat better (e.g. Putnam et al, 20) against the CIRS-G, but do not overcome the weakness of ignoring dementia. Another limitation of the present study is the absence of clinical and health care utilization and cost outcomes.
The Canadian Study of Health and Aging (CSHA) is the only prior investigation we found that presented data on the CDS in persons of varying cognitive status (normal, CIND, and demented) (21
), but analyses suggesting higher scores in more cognitively impaired groups (who were almost certainly older) were not age-adjusted, and statistical comparisons were not reported. The proportion of CSHA participants with zero scores on the CDS is also not reported. The CSHA used the 1995 CDS update (19
), which has one major substantive difference from the original version we used (1
), in that it includes medications used to treat psychiatric disorders (depression, psychosis, bipolar disorder, and anxiety). Because of the association of dementia with psychiatric symptoms, psychotropic drug use could therefore have contributed to the difference in CDS scores observed in the CSHA across cognitive strata, but this speculation cannot be confirmed from the published data.
Differential performance of comorbidity scales and risk adjusters in varying clinical and population groups is not trivial, and we have shown that conditions that are unscored on the CDS – particularly in persons with dementia – include important and common determinants of overall health such as cardiovascular disease and diabetes. With the rising prevalence of dementia in the population, understanding how, why, and to what extent affected persons use health care differently from non-demented older persons, and what value should be placed on these differences, has taken on new importance. Gauging the effect of comorbidity on utilization of health care is a crucial component of studies assessing patterns of care and associated costs for specific conditions.
Empirical comparisons of medical care costs for demented with non-demented persons diverge widely, even in very recent studies. For Medicare beneficiaries with Alzheimer’s disease, cost differentials range from essentially none (22
), through excess yearly costs of $2500–3500 per demented person (23
), topping out at a difference of more than $22,000 (25
). Which is the ‘real’ number that should drive policy considerations and health care design for persons with dementia? Although adequate explanations for these large cost disparities are not to be found in the current literature, one likely source lies in methodological treatment of comorbidity. In the high cost estimate (25
), comorbidity was ignored, while in the more modest estimate (24
), propensity score matching for comorbidity (based on all Medicare claims diagnoses in the index year) was used to predict costs in the subsequent year. Both studies tracked similar utilization outcomes (ambulatory care, pharmacy, and inpatient care), so cost differences cannot be attributed to discrepancies in choice of which health care services were used as outcomes. This example illustrates the critical role of accurate comorbidity measurement in developing scientifically valid estimates of the effect of dementia on patterns and costs of health care.
Here we provide evidence that dementia itself might confer a kind of ‘health disparity’, inasmuch as CDS scores – based on pharmacological treatments for specified chronic conditions – were less reflective of total burden of disease in demented as compared to non-demented persons. Does this mean that physicians tend to prescribe fewer medications for chronic diseases if an individual has dementia? Physicians may under-treat chronic conditions in demented persons out of a sense of futility, as suggested by some studies (26
), or might appropriately limit the complexity of medication regimens in patients with cognitive difficulties. Or might physicians under-recognize chronic conditions in demented persons, due to patients’ inability to report their own symptoms (12
) or caregivers’ difficulty in interpreting and reporting clinically meaningful changes (which has not been formally studied to our knowledge)? Or does dementia widen the gap between physicians’ intended therapies and actual patient behaviors with respect to medication use? In any of these scenarios, fewer medications would be available for scoring the CDS, yielding spuriously low scores relative to the CIRS-G. Whatever the reason(s), analysis and prediction of health care utilization and costs for persons with dementia is likely to be significantly affected by the measures used to parse comorbidity. By underestimating true comorbidity in older persons with dementia, the CDS would overestimate use and costs of health care attributed to dementia itself.