Diagnosing mesothelioma is difficult because the symptoms are similar to those of other illnesses and conditions. Diagnosing mesothelioma includes any or all of the following: review of the your medical history and asbestos exposure, a complete physical examination, with x-rays of the chest or abdomen and lung function test. A CAT scan or MRI of the chest or abdomen may also be ordered by the doctor.

Diagnosis of mesothelioma is often obtained with assessment of clinical and radiological findings and a biopsy is needed to confirm diagnosis of mesothelioma. Depending on where the abnormality exists a biopsy will be done in different ways.

For pleural mesothelioma a thoracoscopy may be performed where the doctor looks inside the chest cavity with an instrument called a thoracoscope that is put into the chest between two ribs. The thoracoscope allows the doctor to gather tissue samples and look inside the chest. For cancer in the abdomen, peritoneal mesothelioma, the doctor may perform a peritoneoscopy where a small opening is made in the abdomen and an instrument called a peritoneoscope is inserted in the abdomen cavity to collect tissue.

If mesothelioma is diagnosed the doctor will want to know the stage of the disease. The staging will help determine which part of the body the cancer has spread to. According to the stage the doctor can plan treatment. If the cancer is found only on the membrane surface where it originated it is called localized. If the has spread beyond the original membrane surface to other parts of the body it is called advanced.