When a laboratory process is conducted by someone other than the treatment or reporting physician or another credible health care professional, the process is recognized by incorporating modifier 90 to the usual procedure number. When lab tests for a patient are conducted by an outside or reference laboratory, a physician or clinic will use modifier 90. This modifier indicates that, whereas the physician is reporting the facts of a laboratory test, the actual testing component was a laboratory service. When lab procedures are performed by someone other than the treating or trying to report health professional and the laboratory bills the physician for the service, modifier 90 is used.
