The impairment rating scale is a standardized medical tool used to quantify the permanent loss of function an individual experiences following an injury or illness. This assessment is performed by a physician and results in a specific percentage that reflects the degree of the patient’s functional loss. The percentage represents the extent to which the damage affects the person’s ability to perform common activities of daily living. This rating is a foundational element in many legal and administrative systems that determine compensation for permanent injuries.
The Standard: The AMA Guides
The term “impairment rating scale” is largely synonymous with the methodology established by the American Medical Association (AMA) in their publication, the Guides to the Evaluation of Permanent Impairment. These Guides provide standardized criteria and specific procedures that medical professionals must follow to ensure consistent and objective assessment across different patients and conditions. The criteria within the Guides cover numerous body systems, including the musculoskeletal, nervous, respiratory, and cardiovascular systems.
The AMA has released several editions of this document over the years, with the 5th and 6th Editions being the most commonly cited. Different legal jurisdictions officially adopt a specific edition of the Guides for use in their legal systems. This formal adoption dictates which version a physician must apply when performing an evaluation, granting the resulting rating its necessary legal weight.
Impairment vs. Disability
It is important to understand the difference between impairment and disability in this context. Impairment is defined as a purely medical assessment of a physical or mental loss of function, representing a deviation from a person’s normal health status. A doctor determines this percentage based on objective clinical findings, without considering the patient’s specific occupation or job duties.
Disability, conversely, is an administrative or legal determination regarding the impact the impairment has on a person’s ability to work and perform daily activities. This determination often incorporates non-medical factors like a person’s age, education, prior work history, and available job market. Therefore, the impairment rating is the medical input—a percentage of functional loss—while the disability determination is the legal output—a status that qualifies an individual for benefits based on their inability to work or earn a living.
How Impairment Ratings Are Calculated
The process of calculating an impairment rating begins only after the patient has reached Maximum Medical Improvement (MMI). MMI is the point at which the treating physician determines the patient’s condition has stabilized and is unlikely to improve substantially with or without further medical intervention. This stability ensures the rating reflects a permanent condition rather than a temporary recovery phase.
The physician uses the AMA Guides to conduct a detailed examination. This evidence includes specific measurements like joint range of motion, results from nerve conduction studies, muscle strength testing, and diagnostic imaging reports. These findings are then compared to the tables and formulas within the Guides, which assign a percentage to the loss of function in a specific body part, such as a hand or a segment of the spine.
If multiple body parts are impaired, the physician converts the localized percentages into a single Whole Person Impairment (WPI) percentage. This conversion is not a simple addition; instead, it uses a complex mathematical calculation known as the Combined Values Chart, which accounts for the interconnected nature of the body’s functions. The final WPI percentage represents the permanent functional loss to the entire person.
Applying the Rating in Legal and Administrative Contexts
Once the physician issues the final Whole Person Impairment rating, that percentage becomes the basis for determining financial compensation in various legal settings. Its most frequent application is within Workers’ Compensation systems, where the WPI percentage is often directly tied to a specific dollar amount. This payment is typically referred to as a Permanent Partial Disability (PPD) award, intended to compensate the injured worker for their lasting physical loss.
The rating also plays an important role in personal injury litigation, where it serves as a quantifiable measure of damages. Attorneys use the WPI percentage to help establish the financial value of the permanent harm suffered by the plaintiff, including compensation for pain and suffering and future loss of earning capacity. While the Social Security Administration (SSA) uses a different set of rules for its Disability Insurance (SSDI) claims, the impairment rating still provides medical evidence that supports the severity of the claim.