Episode of Care

The four continuity measures - the Bice-Boxerman Continuity of Care (COC) Index,^{14 (link)} Herfindahl Index (HI), usual provider of care (UPC),^{15 (link)} and Sequential Continuity of Care Index (SECON)^{16 (link)} —are described in Table 1.
The COC index reflects “the extent to which a given individual’s total number of visits for an episode of illness or a specific time period are with a single or group of referred providers.”^{14 (link)} The HI, which is most commonly used in economic analyses of market concentration, is similar to the COC index in that it reflects the extent to which an individual’s visits during an episode of care are concentrated with a single or group of providers. Although conceptually similar to the COC index, it is calculated using a different mathematical formula. Both measures sum the squared number of visits to a given providers. UPC reflects the “density” of care, or the extent to which visits are concentrated with a single usual provider or group of providers during an episode.^{11 (link)} It equals the number of visits to the provider or practice group with the highest number of visits divided by the total number of visits. SECON varies from the others in that it considers the order of visits, not just their concentration or dispersion among providers. It equals the fraction of sequential visits pairs at which the same provider is seen, i.e. same provider being seen at both the previous and current visits.
We limited the calculation of these measures to outpatient evaluation and management visits defined as Berenson-Eggers Type of Service codes M1A, M1B, M4A, M4B, M5C, M5D, and M6. Only a single E&M visit per day for each patient-provider dyad was counted, where providers were determined using the National Provider Identifier. Visits that were related to complications, hospitalizations, or emergency department visits were excluded from our calculation of the COC index. In addition, we counted only visits to those clinicians that were most likely to be involved in outpatient management for each of the three conditions. For CHF, this included primary care providers (PCPs - general practitioners, family practitioners, internal medicine without subspecialty training, and nurse practitioners), cardiologists, and pulmonologists. For COPD, we included PCPs and pulmonologists; for DM, we included PCPs, cardiologists, endocrinologists, podiatrists, and ophthalmologists. Physician specialty was determined using the specialty code from the Carrier file. With the exception of general practitioners, each specialty class of provider accounted for more than 2% of outpatient E&M visits, and the included providers accounted for 90.6% of total outpatient E&M visits for CHF, 89.6% for COPD, and 86.0% for DM. Practice groups were defined using the tax identification number assigned to each outpatient evaluation and management claim for the above provider types. Each measure was constructed separately using visits to providers and to practice groups.

To qualify for inclusion in this study cohort, patients with AMI must have had a principal hospital discharge diagnosis of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code 410.xx, excluding those with 410.x2 (AMI, subsequent episode of care) at the discharging hospital. We defined an index admission as an admission in which we evaluated the subsequent 30 days after discharge for a readmission. A single patient may have >1 index admission in the study year. We excluded patients aged <65 years, those who died during their index hospitalization, and those admitted and discharged on the same day. If a patient had ≥1 AMI admission within 30 days of discharge from an index AMI admission, we did not consider the additional AMI admissions as index admissions; they were considered as readmissions. Thus, any AMI admission was either an index admission or a readmission but not both. We considered admissions with transfer as a single episode of care, with the codes to define the variables in the model derived from the discharging hospital. The readmission was always attributed to the discharging hospital. To maximize our ability to risk adjust and to identify readmissions, we restricted the cohort to patients enrolled in fee-for-service Medicare Parts A and B for 12 months before their AMI hospitalization who continued in the fee-for-service plan for at least 30 days after discharge.