Diagnosis of venous thromboembolism in pregnancy - technical
The clinical diagnosis of deep venous thromboses and pulmonary thromboembolism is unreliable and objective testing is required if there is substantial clinical suspicion. Anticoagulant treatment should be employed in women with clinical features consistent with venous thromboembolism until an objective diagnosis is made.
Ultrasound venography is the first line diagnostic test for deep venous thromboses in pregnancy. If ultrasonography is negative but there is a high level of clinical suspicion, then the patient should be anticoagulated and ultrasonography repeated in one week, or an alternative test such as radiological venography should be considered. If repeat testing is negative, anticoagulant treatment should be discontinued.
If pulmonary thromboembolism is suspected, ultrasound venography of the leg veins can also be performed: if positive anticoagulation can be given; if negative a chest radiograph and ventilation–perfusion scan or CT pulmonary angiogram should be performed. None of these investigations is considered to pose a significant radiation risk to the fetus in the context of the diagnosis of pulmonary thromboembolism, and they should not be withheld in pregnancy because of fetal considerations when pulmonary thromboembolism is suspected.
The average fetal radiation dose with CT pulmonary angiogram is less than that with ventilation–perfusion lung scanning during all trimesters of pregnancy. However, this is offset by the relatively high radiation dose (≥0.02 Gy) to the mother’s thorax and in particular breast tissue, which may be especially sensitive to radiation exposure during pregnancy. The delivery of 0.01 Gy to a woman’s breast has been calculated to increase her lifetime risk of developing breast cancer by up to 14%. This emphasizes the continued role for ventilation–perfusion scans in pregnancy.
In an appropriate clinical context outside pregnancy an increased level of D-dimer suggests that thrombosis may be present and an objective diagnostic test for deep venous thromboses and/or pulmonary thromboembolism should be performed. In pregnancy, D-dimer can be elevated due to the physiological changes in the coagulation system, and levels become ‘abnormal’ at term and in the post-natal period in most healthy pregnant women. Furthermore, D-dimer levels are increased if there is a concomitant problem such as pre-eclampsia.
Thus a ‘positive’ D-dimer test in pregnancy does not indicate the presence of venous thromboembolism and objective testing is required, but a low level of D-dimer in pregnancy is likely—as in the nonpregnant—to suggest that there is no venous thromboembolism. However, it is important to note that in the nonpregnant, even with a high pretest probability and a highly sensitive D-dimer assay, around 4% of DVTs will not be identified by a D-dimer test. Hence in patients with a moderate or high pretest probability—which accounts for most pregnant patients—it would be inappropriate to rely on D-dimer to exclude venous thromboembolism in pregnancy.