CT angiography is increasingly maligned for its overuse, its likely overdiagnosis of pulmonary emboli, and its radiation risk. The problem is, with >90% sensitivity and specificity, it’s just so damn good. And alternatives like V/Q, even when combined with clinical decision rules, haven’t been seen as providing sufficient accuracy.
In a phase II clinical trial, Morris et al enrolled 52 patients with moderate to high clinical probability of PE. All got CT-angiography (the reference standard). 42 of them also got “Thromboview” (by Agen), a monoclonal antibody against D-dimer tagged with 99-technetium. A SPECT scan then shows where clot is hiding. Pretty clever.
Of the patients who got both scans, 21 patients had PE, 21 did not. With CT-A as the reference standard, Thromboview (or 99mTc-DI-80B3/SPECT if you like) was 76% sensitive and 90.5% specific for diagnosing pulmonary embolism. There were no adverse events.
Besides eliminating contrast toxicity, there’s no need to time a contrast bolus with this test. Authors point out 4 of the 5 PEs it “missed” were segmental/subsegmental on CT-A — so were they even real? On the other hand, inter-reader variability was high (kappa 0.5). Authors keep saying Thromboview is as sensitive as CT, but isn’t CT more sensitive than 76%? It’s too sensitive — that’s one of the problems with it. Until we answer the very hard question of whether subsegmental PEs matter, it’s hard to say how sensitive the perfect test for PE should be.
Morris TA et al. Detection of Pulmonary Emboli With 99mTc-Labeled Anti–D-dimer (DI-80B3)Fab’ Fragments (ThromboView). Am J Resp Crit Care Med 2011;184:708-714.