The 6th edition of the Guides attempts to standardize how impairment value is assigned to a specific condition. Most impairments are based on the Diagnosis-based Impairments (DBI) where Impairment Class is determined by the diagnosis and/or other specific criteria; this is then adjusted by "non-key" factors (grade modifiers) that may include Functional History/Assessment, Physical Examination/Findings, and Clinical Studies/Objective Test Results.
The calculation of a single impairment is based on the following pieces of information:
Impairment class: 5 classes whenever possible. Classes 0-4 have been chosen to be consistent with ICF taxonomy.
- Impairment percentage: a range within each impairment class. For example, class 1 may have a range of 5% to 9%, class 2 may be from 10% to 14%, etc.
- Impairment criterion 1: History of Clinical Presentation – historical data to support the diagnosis-based or regional nature of the impairment class.
- Impairment criterion 2: Physical Findings – examination findings for each impairment class.
- Impairment criterion 3: Clinical Studies or Objective Test Results – specified where applicable for each impairment class.
- Impairment criterion 4: Functional History or Assessment – evidence of symptomatic dysfunction and functional loss due to impairment
An Impairment value is calculated using the following steps:
- Select which impairment criterion will be considered the "key factor". For most organ systems or disease processes (eg neurologic and musculoskeletal organ systems), this will be the History of Clinical Presentation. However, for certain organs such as the cardiac, pulmonary, and renal systems, well-validated organ-specific functional test measures exist that correlate well with levels of impairment. For these systems it appears more appropriate to choose Objective Test Results as the key factor.
- Select the appropriate class (aka severity). The class will have a range of impairments. For example, a class may have values 5%, 6%, 7%, 8%, and 9%. These are referred to as classes "A", "B", "C", "D", and "E" respectively (class A is 5% and class E is 9%). The middle class, class C, is the default impairment value.
- Optionally use the physical examination adjustment grid to adjust the impairment either up or down within the class range.
- Optionally use the clinical studies adjustment grid to adjust the impairment either up or down within the class range.
- Optionally use the functional history adjustment grid to adjust the impairment either up or down within the class range.
An impairment rating report contains a collection of individual impairments. Impairment values are often measured in terms of specific body regions. For example, an impairment rating relating to the forearm will be represented as Upper Extremity Impairment, whereas impairment of the spine will be Whole Person. In order to represent the "Whole Person Impairment" for a given report, each area must be converted to whole person impairment, then combined together to assure that the value never exceeds 100% (100% impairment essentially means the patient is dead).