When defendants seek to apportion an applicant's permanent disability, most often they look to apply Labor Code § 4663, which directs that apportionment be based on causation. LC 4663 requires a physician to consider factors both before and subsequent to the industrial injury, and the courts have not limited what can be apportioned under § 4663. They have allowed defendants to apportion to asymptomatic previous conditions, risk factors and even genetic factors. The only limitation has been that to apportion under § 4663, the physician's opinion about apportionment must be substantial evidence.
Labor Code § 4664 is the other statute that addresses apportionment in the California workers' compensation system, and it allows apportionment to a previous award of permanent disability benefits. LC 4664(b) states, "If the applicant has received a prior award of permanent disability, it shall be conclusively presumed that the prior permanent disability exists at the time of any subsequent industrial injury." If § 4664 applies, the percentage of permanent disability found under the previous award will be subtracted from the current overall percentage of disability.