What is being tested?

This test measures the amount of small molecules called serum mesothelin in the blood. These peptides are breakdown products from proteins found in the membranes lining the cavities that surround the lungs, heart, and abdomen. High amounts of mesothelin are often seen in the blood of patients suffering from mesothelioma, and the amount of mesothelin in the blood is thought to be related to the extent of disease.

Malignant mesothelioma is a rare cancer of the membranes that cover the outside of internal organs and line body cavities, including the chest (pleural mesothelioma), abdominal cavity (peritoneal mesothelioma), and the heart (pericardial mesothelioma). Pleural mesothelioma is the most common type, accounting for 90% of all cases. Most cases of pleural mesothelioma — about 70% to 80% — arise in patients with a history of working with asbestos, especially in the shipbuilding, construction, automotive, and fireproofing industries. The disease has a long latency period, meaning that patients usually develop mesothelioma 20 to 50 years after asbestos exposure.

How is it used?

This test is limited to patients who have already been diagnosed with malignant mesothelioma. It is not used to diagnose the disease. This test is most often used in conjunction with imaging tests such as positron emission tomography (PET) and computed tomography (CT) to:

• determine if you are responding to treatment,
• see if the disease has spread,
• determine if the disease has returned after you have been treated.

However, if mesothelioma is a possible, but not yet a definite diagnosis, the test is not used to diagnose the disease but can also be used to help plan further investigations. For example, if mesothelin is significantly elevated in a patient in which mesothelioma is a possible diagnosis a high resolution CT scan investigation may expedited. This can be particularly useful in a rural setting where access to such technology involves a decision to transport the patient a significant distance.

When is it requested?

A physician might order a series of mesothelin tests to monitor progression of mesothelioma or its recurrence:

• after diagnosis,
• after surgery to remove tumour tissue in the chest cavity,
• during and/or after chemotherapy or radiation therapy.

Mesothelin might be requested when a physician is planning the investigations for a patient in which mesothelioma is a possible diagnosis.

What does the result mean?

If a series of mesothelin tests show mesothelin levels are increasing, it may indicate that mesothelioma has progressed. If the series of tests indicates that mesothelin levels are decreasing, you may be responding to therapy. If the levels are unchanged, it may mean that your disease has stabilised.

The interpretation of your mesothelin test results will likely be done in conjunction with results from imaging studies. That is because:

• the test does not always provide physicians with useful information. Patients with other cancers —including lung, ovarian, endometrial, and pancreatic cancers - can have apparent elevated test levels as a result of their cancers producing substances that interfere with the test.
• some patients with epithelial mesothelioma, a form of the disease that affects the cavity lining or membranes, do not produce mesothelin.

When used in investigation planning for someone with a possible diagnosis of mesothelioma but when high resolution CT scanning is difficult to access, if mesothelin is elevated then referral for such scanning may be expedited.

Is there anything else I should know?

This test is not used to diagnose mesothelioma but is usually used to monitor progression or recurrence. The diagnosis of this disease is often difficult and involves many steps. It usually begins with a review of the patient’s medical history, including a history of exposure to asbestos, and with a complete physical examination. These may be followed by imaging studies of the chest and abdomen, including X-rays and computed tomography (CT) scans, and with lung function tests. Diagnosis may be confirmed by examining tissue from a biopsy or fluid from the affected area for malignant cells.

Common questions

Asbestos is a heat- and fire-resistant insulating material that was widely used in construction, shipbuilding, and the automotive industry after 1945 through the 1970s. Workers breathed in the tiny fibres that comprise asbestos. Those fibres entered their pleura, the lining of the chest cavity and lung, and damaged its cells. Mesothelioma develops over a long period of between 15 and 40 years. Research has also shown that family members and others living with workers exposed to asbestos are at an increased risk of developing mesothelioma.