Diagnosis of mesothelioma is considered to be one of the difficult procedures for analyzing the symptoms in the initial stages. The symptoms of this rarest disease make the diagnosis very difficult even to a doctor, since the symptoms would be very similar to other common diseases in the early stages. Delaying the diagnosis is very common in most of cases; the symptoms can appear as non-specific to both the patient and their doctors.

In general, a diagnosis of mesothelioma is most often done with a careful assessment of factors relating to clinical and radiological findings. The tissue biopsy is also confirmed carefully along with the above factors. The reviewing of the patients medical history can be very helpful to diagnose mesothelioma. A careful observation of the history includes the patients asbestos exposure, the complete physical examination, x-ray results of chest and abdomen and the lung function tests.

A Computerized Tomography (CT) scan or Magnetic Resonance Imaging (MRI) scan may also be taken at this level. If the preliminary test results prove suspicious, then a biopsy is performed for confirming the diagnosis. The location of the cancer can be determined with the help of performing the biopsy. If biopsy confirms the presence of mesothelioma, the stage of the cancer is then determined. The amount of the cancerous cells that has spread is then determined by conducting a series of tests. The disease is considered to be localized, if the test shows that the cancerous cells are not developed beyond the membrane.

During the diagnosis, various tests are performed, such as the imaging tests, which include: the use of x-rays, CT (Computed Tomography) scans, MRI (Magnetic Resonance Imaging), PET (Positron Emission Tomography), bronchoscopy, or a mediastinoscopy is used to look into the affected area. Cytology is used to test the pleural fluid after it is removed by a needle, thoracoscopy is used to look directly into the tumor, needle biopsy and open biopsy are the major tests that are performed during the mesothelioma diagnosis.