In the tradition of many,worthwhile and very well done clinical trials from Canada investigating various aspects of thromboembolic disease, we now have a head to head comparison between the venerable ventilation/perfusion (v/q) lung scan and the upstart CT pulmonary angio (CTPA). See here for an abstract from JAMA.
Out patients with a Wells Score of 4.5 or higher were randomized to either undergoing the CT angio or a v/q lung scan. There were 700 plus in each group and there was a three month followup.
After reading the article a few thoughts come to mind:
One can with good confidence rule out PE if both the CTPA and lower extremity ultra sound are normal.
It still seems to be true than a normal v/q rules out PE .In this study, 35% of of the patients had a normal study. The majority of v/q scans are non-diagnostic (54 % in this study) and in those patients there is a strong tendency for clinicians to order a CTPA.
More instances of PE are diagnosed by CTPA than v/q but the significant of peripheral defects remains a work in progress,particularly those in sub-segmental arteries. In this study 7% of the positive CTPAs involved the sub-segmental arteries alone.
The earlier concerns about published data indicating low sensitivity of the CTPA seems to have been only the case for the older technique as the new and now in general use multidetector CT has excellent (some might argue too much so) sensitivity.
Both tests have value but the clinical momentum (i.e. beliefs and tendencies of clinical docs) is to favor the CTPA, in part, because so many v/q s are non-diagnostic clinicians reason that it is better to go to the CTPA first. Also as pointed out by Dr. RW the v/q interpretive skills of the radiologists may be on the decline and their skills with angiographic interpretation are on the ascendancy. Even back in the day of flurry of interest in and lucrative nature of nuclear medicine the house officers called it "unclear medicine".