Table 1 presents the sociodemographic and health profile of the study participants with linked Medicare records compared with the original sample of NHEFS participants born in 1935 or earlier and with those participants who were not linked to Medicare records.
A study participant was identified as having a chronic condition in the NHEFS data if either of the following two conditions were met:
An appropriate International Classification of Diseases (ICD) code, matching the CCW algorithm codes, was found in the medical records in the NHEFS for an overnight facility stay or a death record (ICD-9-CM for diagnoses and ICD-9 or ICD-10 for deaths).
For those study participants who were identified with a chronic condition based on the NHEFS data and who also had Medicare claims records, we calculated the percent of cases identified by claims during the CCW reference period and also the time needed by the CCW algorithm to identify the preexisting condition.
To estimate the number of years of claims data needed to identify preexisting conditions, we selected those study participants with linked Medicare claims records at some point during 1991-2000 who were identified in NHEFS with selected chronic conditions before the first available Medicare claim.
A condition was considered preexisting if it was identified by NHEFS before the participant's first available claim; otherwise it was considered newly diagnosed by CCW.
Only about 30 percent of dementia cases identified by the CCW were previously diagnosed in NHEFS.
Because only about half of the original NHEFS participants born in 1935 or earlier survived until 1991 and could be linked to Medicare claims, our analysis was conducted on a subsample that is not fully representative of the original NHEFS participants.
Even though NHEFS covers approximately 20 years, the data may not be complete.
In contrast to NHEFS data, claims data (with the CCW algorithm) identify chronic conditions based only on utilization of health care for that particular condition, such as a doctor visit or hospitalization.
The identification of dementia in NHEFS is confounded by a variety of issues.
Appendix SA3: Means Years of Claims Available for Preexisting NHEFS Cases from 1991 to 2000.