[Diagnosis of lung embolism]

Although lung scanning has been performed for nearly 3 decades, the diagnosis of pulmonary embolism (PE) still remains problematic. Only about 20% of all autopsy proven cases of PE have been diagnosed during life time. Biello, and later authors of the PIOPED study defined 3 probability levels for perfusion/ventilation (inhalation) scan interpretation in comparison with pulmonary angiography: High probability (about 90% PE), indeterminate (27 to 50% PE) and low probability (about 5% PE). If the scan results are in the high and low probability group, the diagnostic procedure can be stopped. However, in the indeterminate group including 36 to 52% of all patients, diagnosis should be confirmed by pulmonary angiography. This diagnostic procedure is not well accepted by clinicians, only 0 to 15% of all patients suspected of PE undergo angiography. The diagnostic problem is more complicated due to the large amount of clinically silent PE (39 to 59% of all PE). The inclusion of the diagnosis of thrombosis ("thromboembolism") into the diagnosis of PE increases the pre-test probability for PE to 55 to 60% (= prevalence of PE in patients with deep vein thrombosis). According to Bayes' theorem, the post test probability then exceeds 95%. Without considering thrombosis, the pre-test probability is very low and therefore the post-test probability of a pathologic scan is markedly lower than 50%.