Diagnosing sarcoidosis can be challenging. Currently, there is no blood test to diagnose sarcoidosis. For pulmonary sarcoidosis, imaging tests with chest X-rays or chest CT scans may reveal findings typical for sarcoidosis, but such imaging tests usually aren’t sufficient to confirm the diagnosis. Tests to mention lung function called “pulmonary function tests” can be useful to measure the severity of the disease and to track response to treatment, but also will not separate out sarcoidosis from other lung disease. Sometimes, diagnosis is made by performing a biopsy of a site of disease, such as a nodule in the lung, an enlarged lymph node, or a rash on the skin. Other times, a diagnosis is made by a combination of symptoms, physical exam, and imaging findings.
In the case of cardiac sarcoidosis, biopsies are rarely used because cardiac sarcoidosis involvement of the heart is “patchy” –not uniformly affecting all of the heart muscle. Even in a patient with active inflammation, a biopsy showing normal heart muscle can be seen, and does not exclude the diagnosis. For that reason, most of the time a diagnosis of cardiac sarcoidosis is based on a combination of symptoms, imaging, and evaluation of the heart’s electrical rhythm. Imaging will almost always include an ultrasound of the heart (called an “echocardiogram”) and a nuclear medicine scan called Positron Emission Tomography (PET).
A PET scan is a nuclear medicine scan that takes place in a special scanner. The main principle of a PET scan is that areas of inflammation use more energy stores than normal tissues. For 24 hours prior to the test you will be asked to fast from any form of carbohydrates to cause the normal heart muscle to switch its primary energy source use from sugar (called “glucose”) to fats. Because areas with active inflammation need more energy, they will continue to use glucose as well. During the test, a special form of sugar called FDG (a form of sugar which can be tracked by the scanner) is administered. The scanner then detects how the FDG is used in the body, with areas of inflammation in the heart appearing “bright” on the PET scan. The PET scan may also identify other areas of the body where there is active inflammation from sarcoidosis (such as lymph nodes and the lungs), which can be helpful for confirming the diagnosis and/or tracking response to treatment. For cardiac sarcoidosis, serial PET scans are often used to determine the right dose of treatment for you, particularly in the first 6-12 months after diagnosis.
In some cases, an magnetic resonance imaging (MRI) scan of the heart can be useful, both because it gives high-resolution pictures of heart function, and because it can demonstrate areas of scar/inflammation. However, because it cannot differentiate well between areas of active inflammation and past inflammation, it is often less useful than PET scans for diagnosing cardiac sarcoidosis and for guiding ongoing therapy decisions.