## Clinical Context This patient presents with classic signs of deep vein thrombosis (DVT)—a known complication of combined oral contraceptive use due to increased thrombotic risk from estrogen-induced hypercoagulability. ## Investigation of Choice for DVT **Key Point:** Doppler ultrasonography (compression ultrasound) is the gold standard, first-line investigation for suspected DVT in the lower limb. ### Why Doppler Ultrasonography? - **Non-invasive** and **no radiation exposure** - **High sensitivity (95–100%)** and **high specificity (98–99%)** for proximal DVT - **Real-time visualization** of venous flow and compressibility - **Readily available** in most centers and can be repeated serially - **Operator-dependent** but standardized technique allows reliable diagnosis - **Cost-effective** compared to CT or MRI **High-Yield:** Compression ultrasound (inability to compress the vein under probe pressure) is the diagnostic hallmark of DVT on Doppler imaging. ## Role of Other Investigations | Investigation | Role | Limitation | |---|---|---| | D-dimer assay | Screening tool; high sensitivity but low specificity | Cannot confirm DVT; used to rule out in low-risk patients | | CT pulmonary angiography (CTPA) | Investigation of choice for suspected pulmonary embolism (PE) | Not first-line for DVT diagnosis; reserved for PE workup | | Impedance plethysmography | Older non-invasive test; less sensitive for calf DVT | Largely replaced by Doppler ultrasound; poor sensitivity for distal DVT | **Clinical Pearl:** In a patient on COCPs with acute leg swelling, DVT must be ruled out urgently because of the risk of pulmonary embolism. Doppler ultrasound provides rapid, definitive diagnosis without delay. **Warning:** D-dimer is NOT diagnostic—it is elevated in many conditions (infection, malignancy, surgery) and cannot confirm or exclude DVT on its own.
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