Definition of permanent impairment
A prerequisite to determining the level of permanent impairment is the understanding that impairment should not be determined until the claimant has reached a point of maximum medical improvement. This is the point at which the impairment has become stable, or is not likely to improve despite medical treatment.
In addition to the principles of assessment contained in the American Medical Association (AMA) Guides, scheme legislation also provides substantial guidance on how to determine whether or not impairment is permanent. Table 4.14 lists the legislative definitions of permanent impairment and also the criteria by which an injury is judged to be permanent.
Permanent impairment guidelines
Each of the schemes substitutes or removes sections of their respective editions of the AMA Guide. The necessity for these modifications is primarily due to differences in Australian and US clinical practice, but these are sometimes the result of differences in legislative processes. Table 4.15 illustrates the particular approach taken by the various schemes to substitute or remove assessment criteria from the AMA Guide.
Discounting of prior impairments
Most schemes require that where a pre-existing non-compensable impairment exists, the assessing doctor must discount this pre-existing impairment before making a final assessment of impairment. However, if the deductible portion is difficult or costly to determine, schemes may designate a nominal amount for this purpose or in some instances, accept complete liability for the injury. Table 4.16 lists the discounting provisions under each scheme.