Many people who have clinical symptoms of mesothelioma have advanced disease at the time of diagnosis. This is because symptoms tend to present late in most cases. The majority of people have symptoms for only two or three months before a diagnosis of malignant mesothelioma is made. About one quarter of all patients with malignant mesothelioma have symptoms for at least six months before diagnosis.

A high index of suspicion is needed to make a timely diagnosis of malignant mesothelioma. This is because many of the clinical symptoms are seen in other conditions. Symptoms of pleural mesothelioma, such as cough, chest pain and breathlessness, are also seen in a number of other chest conditions such as infections, emphysema and lung cancer. Similarly, symptoms of peritoneal mesothelioma, such as nausea, vomiting and appetite loss, may be seen in a number of abdominal and systemic conditions. Any person who has these symptoms, accompanied by a history of exposure to asbestos, should see a doctor to be evaluated for malignant mesothelioma.

Physicians use the symptoms that patients report to determine what tests are required to confirm a suspected diagnosis of malignant mesothelioma. If the patient has primarily chest-related symptoms, a chest x-ray, CT scan, MRI, and other imaging techniques may be used to locate any abnormalities that suggest malignant mesothelioma. Next, a pleural biopsy may be performed. There are some relatively new tests that can help differentiate pleural mesothelioma from lung cancer; this differentiation is sometimes difficult because the symptoms are so similar. If a patient presents with mostly abdominal symptoms, abdominal imaging techniques and biopsies may be used to confirm a diagnosis of peritoneal mesothelioma.