Cigna, a large health insurance company, has a system that allows its doctors to instantly reject claims without reviewing patient files, resulting in unexpected bills for patients. This system, known as PXDX, uses an algorithm to flag mismatches between diagnoses and acceptable tests and procedures, and medical directors then sign off on denials in batches. The system has been criticized for being unfair to patients and potentially violating insurance laws and regulations. Cigna has defended the system, saying it is designed to accelerate payment of claims and prevent unnecessary care.