Most reliable investigation in the diagnosis of pulmonary embolism is

The first line diagnostic test for pulmonary embolism is CT pulmonary angiography(CTPA).It has the advantages of visualising the distribution and extent of the emboli,or highlighting an alternative diagnosis,such as consolidation,pneumothorax or aoic dissection. Chest X-ray is most useful in excluding key differential diagnosis such as pneumonia, pneumothorax etc. ECG changes include sinus tachycardia, anterior T-wave inversion and S1Q3T3 pattern. The most common finding in ECG of pulmonary embolism is sinus tachycardia whereas specific finding is S1Q3T3 pattern. Reference:Davidson's Medicine:22nd edition,page no:722.