We conducted a cross-sectional study using data from fiscal year 2015 national Medicare 100% Inpatient Limited Dataset (LDS) (October 1, 2014–September 30, 2015) and identified patients with ocular hospitalization from all causes, including chronic conditions and traumatic injuries, who were then merged with the 2015 Robert Wood Johnson Foundation (RWJF) County Health Rankings [21,22,23]. Fiscal year was chosen over calendar year due to transitions in billing from ICD-9 to ICD-10 during the last few months of 2015. This study abides by the Dataset Use Agreement (DUA) and the Northwestern University Institutional Review Board, which granted a study exemption. All methods adhered to the tenets of the Declaration of Helsinki. All authors declare no conflicts of interest. Under Data Use Agreement (CMS-R-0235L) section 8a no cell less than 11 may be displayed. We employed the national Medicare Inpatient LDS for 2 years (2014, 2015) to create FY 2015, to identify patients who had either an admitting diagnosis or a principal diagnosis of an ophthalmic condition as previously described [21].
The RWJF County Health Rankings, publicly available datasets (annually from 2011 to 2018), reported an aggregate of 35 health measures and incorporated health rankings for almost every county in the US [24]. Each county’s health measures were determined using data from the National Center for Health Statistics, Center Disease Control’s Behavioral Risk Factor Surveillance System, the American Community Survey, and the United Stated Department of and Agriculture Food Environment Atlas. The RWJF data provide general public access and can be obtained annually and across 3191 US counties.
Merging the data at the county level between Medicare LDS and the RWJ data required linkage of Medicare’s LDS two-digit state and three-digit county codes to create the Social Security Administration’s (SSA) five-digit code. These codes were based on where the patient resides and not the hospital location in order to capture their residential SDH characteristics. The RWJF data sets contain the Federal Information Processing Standards (FIPS) county codes, another format of combined state and county codes, thereby requiring a crosswalk to merge the two datasets. Using the National Bureau of Economic Research data, we cross-referenced the Medicare SSA codes to FIPS county codes and then merged hospitalizations within a county to RWJF county health rankings data [25]. Among all 6,673,799 Medicare inpatients, there was a data loss of 30,853 patients due to minor limitations with the RWJF data of including all patients living in or receiving care in US territories. Henceforth, among all Medicare patients (N = 6,642,946), 17,871 Medicare patients had ophthalmic hospitalizations compared to those without ophthalmic hospitalizations (N = 6,625,075). The key covariates from the Medicare data included patient-specific adjustments by age cohorts, gender and black race.
The RWJF data were used to create covariates that captured the six key domains of the SDH (economic stability, neighborhood and physical environment, education status, food access, social and community context, healthcare). Variable selection within domains were based on a literature search among the 35 measures used in the RWJF community health rankings [26]. Due to issues of multicollinearity among these measures, 13 of the 35 measures that encompassed all 6 SDH domains were examined in this study [27]. For interpretation, we converted RWJF measures to binary variables, where ‘1’ represents a county above the median (upper 50%) for a selected measure and ‘0’ represents counties below the median (lower 50%).
Across all six domains, we used standardized measures established by Healthy People 2020, along with other literature, to ensure representation among each domain for analysis [28]. For economic stability, we included measures of unemployment and income inequality because of their known relations to economics and their associations with poor health outcomes [29, 30]. For neighborhood and physical environment, we included measures of air pollution and severe housing problems because of their associations with residential isolation and poor environments [31,32,33]. We measured education status by level of high school education since it is a well-established measure associated with health [34]. We measured food access by food insecurity, as this measure has been associated with chronic disease and poor health [35, 36]. We measured social and community context by the number of children in single-parent households and the amount of violent crime, as these have been linked to less social cohesion and poor health [37, 38]. Lastly, we measured health care by rates of diabetes, smoking status, injury death, drug poisoning deaths, and sexually transmitted diseases as they are related to poor health [39,40,41,42,43,44].
For the analysis, we performed nested logistic regression using Proc Gen Mod in SAS®, nesting Medicare patients in their respective counties. To capture SDH impacts on patients, we nested Medicare patients in their respective counties (where they reside) accounting for the potential effects of community level characteristics, while also capturing specific patient characteristics such as age, gender and race. Nested logistic regression was chosen as the primary method of analysis as it integrates unobserved attributes of these counties. Using a non-nested logistic regression would be insufficient and unable to address potential endogeneity within each county. For our outcomes, in order to understand the effects of SDH on ocular hospitalizations, we compared patients with ocular hospitalizations (a binary measure of “1”) to those hospitalized without ocular hospitalizations (a binary measure of “0”).
As a secondary analysis we examined regional variation of ocular hospitalizations among Medicare beneficiaries expressed as a rate per 10,000 using a national map at the county level, adjusted with U.S. Census data. The data management and statistical analysis were conducted in SAS®, version 9.4 Cary, NC, and the map was developed using ArcGIS ArcMap, version 10.5 ESRI.