Diagnosis of venous thromboembolism
VTE can occur at any time in pregnancy; however, it is noteworthy that over 50% of events occur prior to 20 weeks’ gestation (24, 25). The clinical features were noted earlier, but are not in themselves sufficiently reliable to make a firm diagnosis.
As clinical diagnosis is unreliable then given the importance of the condition, objective assessment is required when there is clinical suspicion of an event. Less than 10% of clinically suspected cases of VTE are confirmed on objective testing (10). At present, pretest probability assessment is not validated in pregnancy in contrast to the non-pregnant where the Wells score is often used (26). Chan et al. (27) have demonstrated that the pretest probability of gestational DVT using three variables with their LEFt rule (left leg (L), (E for (o)edema), calf circumference difference of at least 2 cm, and first trimester presentation (Ft)) may be effective in excluding DVT. Supporting this, a post hoc analysis of 157 women with suspected gestational DVT (28) reported that the LEFt rule could accurately identify those in whom the incidence of confirmed DVT was very low. The modified Wells score (MWS) has been assessed for risk stratification in the diagnosis of pulmonary embolism (PE) in pregnancy (29): a score of 6 or higher had 100% sensitivity, 90% specificity, and a positive predictive value of 36% for objectively confirmed PE, while a MWS less than 6 had a negative predictive value of 100%. Although encouraging, these data are not yet sufficient to support the introduction of such assessment into routine practice, and prospective evaluation of pretest probability assessment tools is still required,Compression duplex ultrasonography is the first-line investigation for suspected gestational DVT (30, 31). Where there is a negative ultrasound examination and a high level of clinical suspicion, the ultrasound examination should be repeated within 3-7 days (32).
While awaiting the repeat test anticoagulation is usually discontinued. Serial compression duplex ultrasonography has been reported, in a prospective cohort study of over 200 pregnancies, to have a negative predictive value of 99.5% (95% CI 96.9-100%) (32). Occasionally, when iliac vein thrombosis is suspected and where DVT is not confirmed on ultrasonography, alternative techniques such as magnetic resonance imaging or X-ray venography may be used (30).Where PE is suspected and where there are clinical features of possible DVT, ultrasound examination of the leg may be useful. Where a DVT is confirmed, then anticoagulation can be started, as treatment is the same for both conditions, so avoiding the need for specific radiation-based thoracic imaging, unless life-threatening PE is present. Clearly a negative ultrasound result cannot exclude PE and where specific investigations for PE are required in pregnancy, ventilation-perfusion (V/Q) lung scans may be preferred to CTPA (26). This is because of their high negative predictive value, low maternal radiation dose, and the relatively low prevalence of comorbid pulmonary problems in pregnancy, which outside pregnancy often lead to non-diagnostic intermediate probability results (26). With a normal chest X-ray, the ventilation component may be omitted so minimizing the radiation dose. Computed tomography pulmonary angiography (CTPA) is often avoided in pregnancy despite its high sensitivity and specificity and ability to identify other pathologies such as aortic dissection, because of the maternal radiation dose (up to 20 mGy), which may be associated with an increased risk of breast cancer (26). However, by employing bismuth breast shields the maternal radiation exposure can be reduced considerably (33). CTPA, V/Q, and low-dose perfusion (Q) lung scans are reported to have similar negative predictive values for PE diagnosis in pregnancy of 99% or more (34, 35). CTPA can be of value where the V/Q scan result shows intermediate probability for PE. There has sometimes been a reluctance to use radiation-based investigations for PE in pregnancy because of concerns relating to exposure of the fetus to potentially harmful radiation (27). However, the fetal radiation dose from such investigations is minimal. For example, a chest X-ray at any gestation exposes the fetus to a negligible dose of radiation (of therapeutic LMWH and LMWH should not be given for at least 4 hours after the use of spinal anaesthesia or after the epidural catheter has been removed. The epidural catheter should not be removed within 12 hours of the last LMWH injection (26). There is an increased incidence of wound complications in women receiving peripartum anticoagulation (44), so it is prudent to anticipate this and with caesarean delivery the use of drains and interrupted skin sutures may be of value in preventing or draining wound haematomas (26).