No simple test can definitively prove that an individual has sarcoidosis. The diagnosis is usually reached by putting together the results from a number of different tests. 

Confirming the diagnosis

Often the diagnosis of sarcoidosis can be made confidently on the basis of the appearances seen on the chest X-ray or the chest CT (computed tomography). Sometimes, the diagnosis is less certain and it is necessary to perform a bronchoscopy or another procedure, called a mediastinoscopy, so as to get a small piece of tissue (a biopsy) to look at under the microscope. This is especially important when there is the need to distinguish sarcoid from diseases such as tuberculosis and lymphoma. 

Assessing dangerous disease

Although rarely, sarcoidosis can involve vital organs such as the heart or the brain. If we think this is a possibility, specific tests will be arranged. An MRI (magnetic resonance imaging) scan is a very useful, non-invasive, test to look for heart or brain sarcoid. An additional test to look for cardiac sarcoidosis includes a 24-hour ECG (electrocardiogram) tape to look for abnormalities in the rhythm of the heart. Sarcoidosis to the eye can also be dangerous, as it can cause permanent damage, and in very rare cases blindness, if unrecognised and not treated. Symptoms that should prompt a referral to a specialist eye doctor include pain to the eye or blurry vision. 

Measuring activity

As the lungs are the most commonly involved organ, repeated lung function tests over time are often the most useful way of assessing activity of sarcoidosis. A significant persistent worsening of lung function tests over time usually means that treatment needs to be introduced or intensified. Depending on the pattern of organ involvement, other tests will be used to assess ongoing activity, such as measuring calcium in the urine or blood tests for liver function, among others.