Imaging for Elevated D-dimer at 16 Weeks Gestation
Proceed directly to bilateral compression ultrasonography of the lower extremities with Doppler imaging of the iliac veins as your first-line imaging study, regardless of the D-dimer result. 1, 2
Why Ultrasound First, Not D-dimer-Driven Decisions
D-dimer cannot be used to exclude VTE in pregnancy because levels rise physiologically by approximately 39% each trimester, reaching mean values of 0.58 mg/L (first trimester), 0.83 mg/L (second trimester), and 1.16 mg/L (third trimester). 1
At 16 weeks gestation (early second trimester), an elevated D-dimer has minimal diagnostic value—you must image regardless. The American Society of Hematology and American College of Chest Physicians both recommend that imaging should not be delayed while awaiting or interpreting D-dimer results when clinical suspicion exists. 3, 1
A negative D-dimer in early pregnancy may lower the likelihood of VTE but must never be used as the sole rule-out tool; proceed immediately to compression ultrasonography. 1
The Correct Imaging Protocol
Perform bilateral proximal compression ultrasonography examining the common femoral and popliteal veins, combined with Doppler assessment of the iliac veins, as a single examination. 3, 2, 4
If the initial ultrasound is positive for DVT, start therapeutic-dose low-molecular-weight heparin (LMWH) immediately without any confirmatory testing. 1, 2
If the initial ultrasound is negative but clinical suspicion remains or D-dimer is markedly elevated, repeat proximal compression ultrasound on day 3 and day 7. 3, 1, 5 Serial testing detects up to 24% of DVTs that are missed on initial examination. 1
Special Consideration: Iliac Vein Thrombosis
Pregnancy-related DVT has an 85% left-sided predominance and frequently involves the iliac veins due to compression by the gravid uterus and right iliac artery. 6
If the patient has swelling of the entire leg or complains of flank, buttock, or back pain, and standard proximal ultrasound is negative, obtain Doppler ultrasound of the iliac vein or magnetic resonance venography (without gadolinium). 3, 2 Standard compression ultrasound cannot adequately visualize the iliac veins. 2
Approximately 65% of pregnancy-related DVTs are isolated to the iliofemoral veins, and 12% are isolated iliac vein thromboses. 5
If Pulmonary Embolism Is Suspected
If the lower-extremity ultrasound is negative but the patient has respiratory symptoms or the clinical picture suggests pulmonary embolism, proceed to chest imaging. 3, 1
Computed tomography pulmonary angiography (CTPA) is preferred over ventilation-perfusion (V/Q) scanning because it delivers lower fetal radiation doses (approximately 13–300 µGy) compared with V/Q scans, and all doses remain far below the 50,000 µGy threshold considered hazardous. 1
The American Thoracic Society/Society of Thoracic Radiology previously recommended V/Q scanning over CTPA primarily for maternal breast radiation concerns, but more recent evidence favors CTPA for superior diagnostic accuracy and lower fetal exposure. 3, 1
Critical Pitfalls to Avoid
Never rely on D-dimer alone to exclude VTE in pregnancy—the false-negative rate is unacceptably high due to physiologically elevated baseline levels, and specificity drops to approximately 15% by the third trimester. 1, 7
Do not skip serial ultrasound testing after an initial negative scan if clinical suspicion persists; up to 24% of DVTs are detected during follow-up examinations rather than at presentation. 1, 2
Avoid whole-leg ultrasound as routine initial testing; proximal compression ultrasound with iliac Doppler is sufficient and reduces false-positives from incidental calf vein findings. 3, 2
Do not delay imaging while awaiting D-dimer results when clinical suspicion is moderate to high—compression ultrasonography should be performed promptly. 1