WHEN DOES A NEGATIVE CT PULMONARY ANGIOGRAM EXCLUDE PULMONARY EMBOLISM? A DECISION ANALYSIS

Background. CT pulmonary angiography (CTPA) is often used to exclude the diagnosis of pulmonary embolism (PE), although there is disagreement about if and what additional testing should be performed in patients whose results on CTPA are negative.

Purpose. To determine optimal management strategies for patients with suspected PE and negative results on CTPA, depending on the pre-test probability of PE, the risk of major or fatal bleeding complications, and the performance characteristics of the current generation of diagnostic tests.

Methods. We used decision analysis to model 3 alternative management strategies in patients with suspected PE who had negative results on CTPA: anticoagulation, further testing and withholding anticoagulation. The payoffs of the 3 strategies were expressed in quality adjusted life-years after 6 months from the encounter. We used values from the literature to estimate the performance characteristics of the diagnostic tests, and developed Markov models to estimate the outcomes of each strategy for patients with and without PE who had or had not received anticoagulation. Expected value calculations as well as deterministic and probabilistic (second order microsimulations) sensitivity analyses were performed over a broad range of pretest probabilities of PE, risks of bleeding complications, and diagnostic test performances.

Results. In patients with low pretest probability of PE, the workup can be stopped after a negative CT pulmonary angiogram, unless the specificity of the second test (Spec2) is very high. For high pretest probability patients, further testing is associated with the best outcome. The optimal approach for patients with intermediate probability of PE depends on their bleeding risk: with a low risk, further testing is beneficial; for patients with intermediate to high bleeding risk, further testing is preferred only when the specificity of the test is sufficiently high (Table).

			Pre-test Probability of PE
Bleeding risk			Low (9%)	Intermediate (28%)	High (72%)
	% / yr		Test if Spec2 >	Test if Spec2 >
Low	1.9		0.80	0.24
	2.5		0.83	0.35
Intermediate	5.3		0.90	0.62	Test
High	8.4		0.93	0.74
	10.4		0.95	0.80

Conclusion. In patients with suspected PE and negative CTPA results, subsequent testing should be performed when the pre-test probability is high, and when the pre-test probability is intermediate and the risk of bleeding complications is low.