The diagnosis for Peritoneal Mesothelioma begins with the thorough review of the patients medical history to determine symptoms and any past asbestos exposure, as well as a complete physical. In addition, sometimes physicians might request an MRI (magnetic resonance imaging) scan or an X-ray of the abdomen or chest region to help determine the location and size of the tumor. In an MRI, a powerful magnet linked to a computer is used to make detailed pictures of areas inside the body. These pictures are viewed on a monitor and can also be printed, clearly depicting the areas affected with the tumor.

In cases of pleural and peritoneal mesothelioma, fluid (effusion) may collect in the chest or abdominal region. In such cases, physicians may use a procedure known as fine needle aspiration to obtain a sample of this fluid for further testing. In addition, this procedure can be used to drain the effusion to temporarily relieve chest pain or other painful symptoms.

Usually, a biopsy of the tumor is required to confirm a diagnosis of mesothelioma, however, sometimes only an effusion sample is needed. Some of the procedures commonly used for the diagnosis of the peritoneal mesothelioma include peritoneoscopy, thoracoscopy, and biopsy. With peritoneoscopy, the doctor will use a tool called a peritoneoscope, which is placed inside an opening made in the patient's abdomen. Fluid, if found, is then drained; the process of draining the fluid is called paracentesis, and it requires a needle to be placed inside the abdomen in order to extract the fluid.

In thoracoscopy a doctor uses a special instrument called a thoracoscope in order to examine the patient's chest cavity. An incision is made in the chest wall, and the thoracoscope is placed between the patient's ribs. If the fluid is found in the chest, the doctor drains it from the chest cavity through a needle. A thoracoscopy commonly requires an anesthetic or some other form of pain suppression.

In a biopsy, if cancerous tissue is found in the patient's lungs or abdomen it will be extracted and examined by the doctor. This process helps the doctor determine potential ways of treatment and is usually done during one of the other two processes or may require additional surgery.