Although the overall incidence of PD among this 40–65-year-old Medicaid-enrolled sample was similar to that found in other studies,11
African-Americans were diagnosed with PD at half the rate of whites, even when controlling for age, sex, location of care, healthcare use, and reason for Medicaid eligibility.
Older age was associated with increased PD risk as in multiple other studies; however, our study did not show an increased PD risk among men.5,6,11
The association between male sex and PD diagnosis was marginally significant (RR 1.2, P
-value = 0.09). The lack of statistical significance in our findings may have been due to at least three reasons: (1) Gender differences may only be apparent in patients over the age of 65. Although there was no statistically significant interaction between age and sex in our sample, we only studied individuals younger than 65. An earlier study has shown that differences in PD diagnosis between men and women varied by age, and these differences were observed at ages greater than 65-years;6
(2) As gender inequities in healthcare are addressed gender differences will narrow; and (3) Chance may have resulted in a disproportionate sample of women with PD in our sample thus inflating the risk of PD compared with the general population.
This study also found that greater use of healthcare, as measured by annual physician visits, increased the probability of PD diagnosis, suggesting the importance of healthcare access. The finding that individuals who qualified for Medicaid due to disability had a decreased probability of PD is likely related to our investigation of only incident PD, when functional disability is minimal.
There are some potential limitations to this study. The accuracy of the ICD-9 codes associated with PD in Medicaid claims is unknown. A recent study using Medicare data found the highest specificity (>99%) and positive predictive value (73%) when using ICD-9 332.0 alone.12
To minimize the number of false positives included as cases, we used only this ICD-9 code. Additionally, we excluded those individuals who were at increased risk for secondary parkinsonism based on a history of stroke, bipolar disorder, or schizophrenia. There is still the potential for additional unmeasured factors leading to misclassification bias. However, this misclassification would need to differ between African-Americans and whites to confound any observed differences.
A second limitation is that the generalizability of results may be limited. The sample was relatively young, poor and disabled. However, having a large, diverse cohort with a complete record of health service use offers a valuable resource to approach questions related to the epidemiology of less common disorders and the study of healthcare access. Furthermore, this sample represents a particularly vulnerable group of adults whose needs are important to understand for intervention and policy development.
Lastly, there was unequal dropout because of either death or change in Medicaid eligibility from the study. Although African-Americans were more likely to be censored from the analysis than whites, this is unlikely to completely explain the observed racial differences in the probability of PD diagnosis. For the incidence of PD in African-Americans to equal whites, 2% of censored African-American subjects would have to develop PD which is 10 times greater than the risk we found in our study.
Despite these limitations, there are important implications of these findings. This study is among the few that have examined racial differences in PD incidence.5,6
Our findings of a lower diagnosed incidence of PD among African-Americans than whites are similar to the only other study of PD incidence among a population of adults with the same insurance access.6
Our study had an even lower relative risk of PD among African Americans (RR 0.45) than Van Den Eaden et al. (RR 0.75). This may be explained by differences in our samples (e.g. lower income and lower education in a Medicaid population) and case ascertainment methods, and may suggest that among lower income adults, racial disparities are exacerbated. We cannot compare our results to the second PD incidence study because that study identified no cases among white men under the age of 65.5
To evaluate the association between healthcare access and PD risk we assessed the average number of physician visits per year. Although average yearly visits were independently associated with the identification of PD, they did not affect the relationship between race and PD risk. The number of visits may reflect the level of co-morbid conditions or the ability to negotiate medical bureaucracy to arrange visits. One other study found no contribution of co-morbid conditions in racial differences in rates of PD diagnosis.5
Although we similarly observed lower service use among African-Americans than whites,13
this did not correlate with differences in probability of PD diagnosis.
Some researchers have argued that geographic variations in healthcare may be an important explanation for racial differences in healthcare.14-16
Variations in health systems and healthcare providers can affect the quality of care that patients receive. As African-Americans and Latinos tend to live in different areas than whites, these geographic differences in quality of care may exacerbate racial/ethnic differences. Our study did not find that location of care affected racial differences in PD identification; however, our measure of urbanization may not have captured the geographical variation in quality of care that may be better seen when comparing private, teaching or high-volume centers.
The observed racial differences in diagnosis of PD were not explained by insurance, income, location of care, or healthcare utilization. Biological factors may contribute to observed racial differences in PD. However, in the only US population-based study of PD prevalence, there were no significant racial differences when both post-encephalitic and idiopathic PD cases were counted and less stringent diagnostic criteria than used today were applied to cases.17
Furthermore, this door-to-door study found that almost twice as many African-Americans than whites with PD had been undiagnosed before the study. This finding substantiates the conclusion that the racial differences presented in the current study are due to racial disparities, that is, differences that are not due to clinical needs or appropriateness of care, but may be due to discrimination or bias.
Many other causes of the observed racial disparities should be considered. Patient, physician, and system-level factors may all contribute. Patient-level factors may include education, culture, aging beliefs, trust, and stigma. For example, research on dementia has found that expectations regarding aging vary among people of different racial/ethnic backgrounds and these expectations delay healthcare seeking for symptoms of cognitive decline in ethnic minorities.18
Provider-level factors such as clinician biases, stereotyping and medical uncertainty also play a role in racial disparities. This is evident in studies of depression care in which researchers have shown racial differences in the quality of physician–patient communication.19
Healthcare system factors include financing, accessibility, fragmentation of health coverage and legal policy can all influence racial disparities as well.20
Alternatively, despite a similar burden of PD between racial/ ethnic groups, different clinical phenotypes by race/ethnicity may prevent the appropriate identification of PD in non-white groups because consensus diagnostic criteria are based on evaluation of only white patients.21
It also possible that genetic or environmental risk factors may differ by race/ethnicity and affect the true incidence of PD in non-white populations.
What has been learned in other fields of medicine can be applied to PD. It is possible that PD is under-recognized in African-Americans based on this data, but it remains uncertain why. To develop targeted interventions to improve the appropriate diagnosis of PD among underserved minorities, we will need to use a population-based approach in our research and gain a better understanding of the complex biological and social interactions in creating disparities in neurodegenerative diseases.