Chest X-Ray Cannot Definitively Diagnose Carditis
A chest X-ray is generally normal in patients with acute pericarditis and myocarditis, and cannot definitively diagnose carditis. The European Society of Cardiology explicitly states that chest X-ray is generally normal in patients with acute pericarditis since an increased cardiothoracic ratio only occurs with pericardial effusions exceeding 300 ml 1. While chest X-ray is recommended as part of the initial workup (Class I recommendation), it serves primarily to identify alternative diagnoses rather than to confirm carditis 1.
Why Chest X-Ray Has Limited Diagnostic Value for Carditis
Chest X-ray lacks the sensitivity and specificity to detect myocardial or pericardial inflammation directly. The pericardium itself is electrically and radiographically inert, making inflammatory changes invisible on plain radiography 1.
Key Limitations:
- Poor sensitivity for cardiomegaly: In post-NSTEMI patients, chest X-ray had only 40% sensitivity and 56% positive predictive value for detecting true cardiomegaly when compared to echocardiography as the gold standard 2
- High false positive rate: 44% of patients reported to have cardiomegaly on chest X-ray did not have true cardiomegaly on echocardiography 2
- Misses 60% of cases: Among patients with confirmed cardiomegaly on echocardiography, 60% did not have cardiomegaly identified on chest X-ray 2
The Appropriate Diagnostic Algorithm for Carditis
When carditis (myocarditis or pericarditis) is suspected, the diagnostic pathway should proceed as follows:
Initial Assessment (All Patients):
- 12-lead ECG (Class I recommendation) to identify ST-segment elevation, PR depression, or conduction abnormalities 1, 3
- Cardiac biomarkers including troponin, CK, CRP, and ESR (Class I recommendation) 1, 3
- Transthoracic echocardiography (Class I recommendation) to assess ventricular function, wall motion abnormalities, pericardial effusion, and wall thickening 1, 4, 3
- Chest X-ray (Class I recommendation) primarily to exclude alternative pulmonary diagnoses, not to diagnose carditis 1
Definitive Diagnosis:
Cardiac MRI is the definitive noninvasive diagnostic test for myocarditis, with 78-94% sensitivity and 88-100% specificity 1, 4, 3. The American College of Cardiology and American College of Radiology recommend CMR as the gold standard for tissue characterization, detecting myocardial inflammation, edema, and necrosis 4. CMR uses the updated 2018 Lake Louise Criteria requiring at least one T2-based criterion (edema) plus at least one T1-based criterion (necrosis/fibrosis) for diagnosis 1, 4, 3.
Critical Clinical Pitfall
Do not rely on chest X-ray findings to rule in or rule out carditis. The specificity of 91% means that while a normal chest X-ray somewhat reduces likelihood, it cannot exclude disease 2. Conversely, cardiomegaly on chest X-ray has such poor positive predictive value (56%) that it should prompt further investigation with echocardiography or CMR rather than being interpreted as diagnostic 2.
When to Proceed Directly to Advanced Imaging:
Patients with any of the following require immediate echocardiography and consideration for CMR 1, 3:
- Elevated troponin levels 1, 3
- ECG abnormalities (ST changes, conduction delays, arrhythmias) 1, 3
- Hemodynamic instability or heart failure symptoms 3, 5
- Ventricular arrhythmias or heart block 3, 5
The role of chest X-ray in suspected carditis is limited to identifying pleuro-pulmonary diseases or alternative diagnoses, not confirming cardiac inflammation 1.