Myocarditis is an inflammatory disease of the myocardium, which can be confirmed by the following tests:

Chest X-rays can show an enlarged cardiac silhouette. Electrocardiograms can reveal changes in the ST-T segments, and various arrhythmias may also occur, especially ventricular arrhythmias and atrioventricular conduction blocks. Echocardiography might be normal, or it might show enlargement of the left ventricle and weakened wall motion.

Cardiac MRI is of significant importance for the diagnosis of myocarditis, showing patchy enhancement of the myocardium. Biochemical blood tests can show elevated non-specific inflammatory markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein, and elevated levels of myocardial enzymes and troponin. Serological testing for viruses can suggest a cause but is not definitive for diagnosis.

Finally, endomyocardial biopsy, besides diagnosing, can also aid in assessing the condition and prognosis. However, it is invasive, so it is generally used only in urgent and severe cases, cases with poor treatment response, or in patients with undiagnosed causes. It is not commonly performed in patients with mild conditions.