The VEHSS analysis of Medicare claims is subject to several potential limitations:
- Medicare claims are intended for billing purposes only. Diagnosis information included on claims is intended to justify payment. Therefore, diagnosis data on claims may suffer from bias or limited detail.
- Medicare FFS does not cover all healthcare services, such as routine eye exams or optometry care. Medicare patients may receive these services using a different payer, and thus these services are not captured in Medicare FFS claims.
- Patients may be insured by multiple insurers, such as a supplemental, medigap or Medicare managed care plans. Thus, even normally covered ophthalmology services may not be indicated in FFS claims if services were reimbursed by another plan.
- Many Medicare patients move seasonally. We report patient residence using the last location of observation. This may skew patient numbers towards “sun belt” states.
- Medicare is not nationally representative for individuals younger than 65 years and includes only persons who are eligible for the reasons listed above. These patients may obtain Medicare coverage due to medical need, such as legally recognized disability, or other specially defined benefit, such as end-stage renal disease. Individuals who become disabled for reasons of blindness cannot be distinguished from others who are disabled unless their visual condition of low vision and blindness is diagnosed in their medical claims.
- Medicare FFS is a complete population of Medicare FFS beneficiaries, including all FFS beneficiaries younger than 65 and those 65 and older. For those ages 65 and older, the data exclude beneficiaries with MA plans, people who defer Medicare coverage, or services paid out of pocket.
 US Census Bureau. Profile America facts for features: CB17-FF.08. Washington, DC: Economics and Statistics Administration; 2017. https://www.census.gov/content/dam/Census/newsroom/facts-for-features/2017/cb17-ff08.pdf. Accessed March 1, 2018.
 State Health Facts: Menlo Park, CA: Kaiser Family Foundation. https://www.kff.org/statedata/. Accessed March 1, 2018.