The International Classification of Functioning, Disability and Health (ICF) classification uses five impairment classes, which permits rating of patients who range from having no problems to having significant problems. In the Sixth Edition "diagnosis-based grids" were developed for each organ system. These grids use commonly accepted consensus-based criteria to classify most diagnoses relevant to a particular organ or body part into five classes of impairment severity ranging from Class 0, normal, to Class 5, very severe. The final impairment is determined by adjusting the initial impairment rating given by factors that may include physical findings, the results of clinical tests, and functional reports by the patient. The basic template of the diagnosis-based grid is common to each organ system and chapter; therefore although there is variation in the ancillary factors used to develop the impairment rating (depending on the body part), there is greater internal consistency between chapters than was seen formerly.

This uniform diagnosis-based approach is a significant change from the anatomical approach that was the primary approach with many previous musculoskeletal assessments. However, there are similarities to other approaches used in the Fourth and Fifth Editions. For example, as mentioned previously, spinal impairment assessments have typically been based on the Diagnosis-Related Estimates Method, with specific findings or diagnoses used to assign the patient to a category. In the Fifth Edition the patient is assigned to one of five categories, with the first category having no ratable impairment and the other four categories having four possible impairment values. Thus, a patient with a lumbar radiculopathy would be assigned to a DRE Lumbar Category III which would lead to a whole person impairment rating of between 10% and 13% using the Fifth Edition (choice of a level is based upon the examiner's judgment regarding limitations in activities of daily living (ADLs) as a result of the impairment). Although the Fourth Edition also used the DRE system, there was not an allowance for variation in the impact of a given diagnosis upon ADLs so the rating for Category III was fixed at 10% whole person permanent impairment. Likewise, although lower extremity impairments had been based on thirteen possible approaches in the Fifth Edition, the most commonly used approach is the Diagnosis-Based Estimates where specific impairment values are provided for diagnoses. For example, a patient with a partial medial meniscectomy is assigned 1% whole person permanent impairment. Rating systems previously used for the lower extremity likewise did not provide for adjustments based on functional difficulties, physical examination findings, or the results of clinical studies.

The Sixth Edition reflects improvement in the approach to defining impairment; it should be considered a step in the evolution of the Guides rather than as an end point in and of itself.

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