About half of all patients are now diagnosed before they are born. Differential diagnosis is when physicians diagnose between two or more conditions for a persons symptoms and this can include primary pulmonary causes of cyanosis, cyanotic heart lesions, pulmonary stenosis and transposed arterial trunks. Chest radiograph
Before more sophisticated techniques became available, chest X-ray was the definitive method of diagnosis. The abnormal "coeur-en-sabot" (boot-like) appearance of a heart with tetralogy of Fallot is classically visible via chest X-ray, although most infants with tetralogy may not show this finding. The boot like shape is due to the right ventricular hypertrophy present in TOF. Lung fields are often dark (absence of interstitial lung markings) due to decreased pulmonary blood flow. : 171–72

Electrocardiogram
An electrocardiogram (ECG) is one of the most basic procedures for assessing the heart. Tiny electrodes are applied to specific areas on the body, near the chest, arm, and neck. Lead cables connect the electrodes to an ECG machine. The hearts electrical activity is then measured. Natural electrical impulses help maintain blood flowing properly by coordinating contractions in different areas of the heart. These impulses are recorded by an ECG, which shows how fast, the rhythm, intensity and timing of the electrical impulses as they travel through the heart.Electrocardiography shows right ventricular hypertrophy (RVH), along with right axis deviation. RVH is noted on EKG as tall R-waves in lead V1 and deep S-waves in lead V5-V6.
Insufficient myocardial protection is considered one of the main causes of death in the correction of TOF. Comorbidities
There are many comorbid conditions that can occur with TOF that may exacerbate the condition. Often, TOF can present with low birth weight and prematurity. In both of these cases, mortality and morbidity were both seen to increase. Differences in right atrial and ventricular mechanics and liver stiffness was also observed in adults with repaired TOF, as well as pulmonary atresia and persistent pulmonary stenosis. In patients with pulmonary atresia, there is complete failure of forward flow from the right ventricle to the pulmonary arterial vasculature. As such, pulmonary blood flow is entirely dependent on shunting from the systemic circulation, typically through a patent ductus arteriosus. The pathophysiology of TOF together with pulmonary arteriosus is uniquely attributable to defects of the pulmonary arteries. Even after operative care, these patients remain at higher risk for pulmonary arterial stenoses and pulmonary hypertension.Danon disease, which is a rare genetic disorder, was also observed to complicate TOF. In particular, elongation of the QRS complex and a shortened PR interval. Genetic abnormalities found in TOF may lead to the earlier diagnosis of Danon disease, helping to improve prognostic outcomes. Epidemiology
The prevalence of tetralogy of Fallot is estimated to be 0.02–0.04%, which corresponds to approximately 200 to 400 cases per million live births. It accounts for 7–10% of all congenital heart abnormalities, making it the most common cyanotic heart defect.