A permanent impairment rating is a medical assessment that quantifies the loss of normal physical or mental function due to an injury or illness. Expressed as a percentage, this rating represents the degree of permanent functional loss after medical treatment has concluded. This numerical value is used primarily in legal and administrative contexts, such as workers’ compensation and disability claims, to determine compensation.
Understanding Permanent Impairment Ratings
The impairment rating provides an objective, medical measure of a person’s physical condition following an injury. It focuses on the functional loss of a specific body part or the body as a whole, independent of economic or vocational factors. This measurement is distinct from “disability,” which is a legal determination of a person’s inability to earn a living.
A rating can only be assigned after the injured person has reached Maximum Medical Improvement (MMI). MMI is the point where a physician determines the medical condition has stabilized and is unlikely to improve substantially with further treatment. Reaching MMI signifies that the recovery process has plateaued, allowing the permanent residual effects of the injury to be assessed.
The Foundational Medical Standard
Calculating a permanent impairment rating relies on a standardized guide to ensure consistency and objectivity across medical practitioners. The primary standard used across the United States is the AMA Guides to the Evaluation of Permanent Impairment. This comprehensive manual provides physicians with detailed, evidence-based criteria for evaluating functional loss.
The uniform guide translates a complex medical condition into a single, reliable percentage for legal settings. Without this standard, ratings would vary widely based on individual physician opinion, leading to inconsistent compensation results. The AMA Guides have undergone multiple revisions, with the 5th and 6th Editions being the most commonly referenced versions today.
The specific edition used is determined by jurisdiction, as state workers’ compensation systems often mandate a particular version. Different editions employ different methodologies for calculating impairment, which can lead to different final percentage ratings for the same injury. The guide’s authority rests on its widespread adoption by regulatory bodies to provide a sound framework for determining benefits.
Step-by-Step Calculation Methodology
The calculation process begins with the physician confirming MMI and performing a thorough clinical evaluation. This involves a detailed review of the patient’s medical history, physical examination findings, and diagnostic results. A clear diagnosis of the permanent condition is foundational to selecting the correct chapter and criteria within the AMA Guides.
Once the condition is confirmed, the physician uses objective clinical findings, such as goniometric measurements or manual muscle strength testing, to determine functional loss. The AMA Guides are structured by body system, requiring the physician to locate the specific chapter for the injured body part (e.g., spine or extremity). The physician then applies the corresponding tables or criteria to assign an impairment percentage to that specific body part or region.
For extremity injuries, the physician first calculates a regional impairment rating (e.g., percentage of the hand). This regional value is then converted into a Whole Person Impairment (WPI) percentage, representing the functional loss relative to the entire body. For example, a 100% impairment of the hand may convert to a 60% WPI, based on the conversion tables in the Guides.
When a patient has multiple impairments, such as a shoulder and knee injury, the physician must use the Combined Values Chart (CVC) to arrive at a single, total WPI rating. The CVC is a non-additive formula that accounts for the fact that multiple losses overlap in their effect on overall function. The formula, represented as A + B(1 – A), where A and B are the decimal equivalents of the two highest impairments, prevents the total rating from exceeding 100%.
Apportionment
Apportionment involves assessing whether a pre-existing condition contributed to the final impairment. This process identifies the percentage of the current permanent impairment due to non-work-related factors, such as a prior injury or age-related degeneration. The physician must provide a medical opinion, based on historical records, to reduce the final impairment rating by the percentage attributed to the pre-existing cause.
Translating the Rating into Disability
After the physician finalizes the Whole Person Impairment percentage, this medical rating serves as the anchor for the subsequent legal determination of disability. The impairment rating itself does not equal the monetary compensation; it is a medical input used by legal and administrative bodies. Impairment is a loss of body function, while disability is the impact of that loss on a person’s ability to work and earn a living.
The final impairment percentage is converted into a disability rating through a jurisdiction-specific formula, particularly in workers’ compensation systems. These state formulas integrate the medical WPI with non-medical factors to account for the real-world impact of the injury. Factors such as the injured person’s occupation, age, and pre-injury wages are applied as modifiers to the medical impairment rating.
The resulting permanent disability rating is then used to calculate the duration and amount of permanent disability benefits. For example, a state’s schedule may assign a specific number of weeks of compensation for each percentage point of disability. This process ensures the medical assessment of functional loss is translated into a financial benefit reflecting the unique circumstances of the injured worker.