Scanning imaging techniques for the exclusion of pulmonary embolism during pregnancy

Pulmonary embolism is a blood clot that blocks blood flow to a portion of the lungs. Pregnant women are at high risk of pulmonary embolism, and it is a leading cause of death during pregnancy. Women at risk are treated with blood thinning medication. It is important that no cases are missed, and that treatment is prevented in women without the disease. Pulmonary embolism can be diagnosed through different scanning techniques. Little is known about the performance of these tests during pregnancy, which might be different from their performance outside pregnancy. We performed this review to establish the accuracy of the following imaging tests for diagnosing pulmonary embolism during pregnancy: computed tomography pulmonary angiography, lung scintigraphy and magnetic resonance angiography.

We found 11 studies (current until July 2015) that described 695 computed tomography pulmonary angiography results, 665 lung scintigraphy results and no magnetic resonance angiography results. Studies on lung scintigraphy used varying techniques. Overall, these studies were of poor quality; therefore, we could not analyse results together to obtain a single estimate of their accuracy. The identified studies followed-up patients clinically to confirm the absence of pulmonary embolism as revealed on the initial scan, so information could be used to draw conclusions only on the ability of these imaging tests to exclude pulmonary embolism, not on their ability to establish the diagnosis.

Both computed tomography pulmonary angiography and lung scintigraphy appear appropriate for excluding pulmonary embolism in pregnancy. Almost no cases were missed, especially when the imaging test indicated the absence of disease without a doubt. However, this result should be interpreted with care because of the low quality of and variation between identified studies. Around 5% of the scans were unclear, but this percentage was as high as 36% in one study. About 3% of all women included in the studies had pulmonary embolism. We do not know which of the tests is better because tests were not directly compared in the same patients, and because aspects besides test accuracy need to be taken into account. Major limitations of this review include the use of clinical follow-up within studies to confirm the absence of disease, unclear test results and the inability of studies to provide information on the accuracy of these tests in establishing rather than rejecting the diagnosis. High-quality research is needed to investigate the use of computed tomography pulmonary angiography, lung scintigraphy and magnetic pulmonary angiography in the same patient groups.

Authors' conclusions: 

Both CTPA and lung scintigraphy seem appropriate for exclusion of pulmonary embolism during pregnancy. However, the quality of the evidence mandates cautious adoption of this conclusion. Important limitations included poor reference standards, necessary assumptions in the analysis regarding inconclusive test results and the inherent inability of included studies to identify false positives. It is unclear which test has the highest accuracy. There is a need for direct comparisons between diagnostic methods, including MR, in prospective randomized diagnostic studies.

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Background: 

Pulmonary embolism is a leading cause of pregnancy-related death. An accurate diagnosis in pregnant patients is crucial to prevent untreated pulmonary embolism as well as unnecessary anticoagulant treatment and future preventive measures. Applied imaging techniques might perform differently in these younger patients with less comorbidity and altered physiology, who largely have been excluded from diagnostic studies.

Objectives: 

To determine the diagnostic accuracy of computed tomography pulmonary angiography (CTPA), lung scintigraphy and magnetic resonance angiography (MRA) for the diagnosis of pulmonary embolism during pregnancy.

Search strategy: 

We searched MEDLINE and Embase until July 2015. We used included studies as seeds in citations searches and in 'find similar' functions and searched reference lists. We approached experts in the field to help us identify non-indexed studies.

Selection criteria: 

We included consecutive series of pregnant patients suspected of pulmonary embolism who had undergone one of the index tests (computed tomography (CT) pulmonary angiography, lung scintigraphy or MRA) and clinical follow-up or pulmonary angiography as a reference test.

Data collection and analysis: 

Two review authors performed data extraction and quality assessment. We contacted investigators of potentially eligible studies to obtain missing information. In the primary analysis, we regarded inconclusive index test results as a negative reference test, and treatment for pulmonary embolism after an inconclusive index test as a positive reference test.

Main results: 

We included 11 studies (four CTPA, five lung scintigraphy, two both) with a total of 695 CTPA and 665 lung scintigraphy results. Lung scintigraphy was applied by different techniques. No MRA studies matched our inclusion criteria.

Overall, risk of bias and concerns regarding applicability were high in all studies as judged in light of the review research question, as was heterogeneity in study methods. We did not undertake meta-analysis. All studies used clinical follow-up as a reference standard, none in a manner that enabled reliable identification of false positives. Sensitivity and negative predictive value were therefore the only valid test accuracy measures.

The median negative predictive value for CTPA was 100% (range 96% to 100%). Median sensitivity was 83% (range 0% to 100%).

The median negative predictive value for lung scintigraphy was 100% (range 99% to 100%). Median sensitivity was 100% (range 0% to 100%).

The median frequency of inconclusive results was 5.9% (range 0.9% to 36%) for CTPA and 4.0% (range 0% to 23%) for lung scintigraphy. The overall median prevalence of pulmonary embolism was 3.3% (range 0.0% to 8.7%).

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