Factor V Leiden

Factor V Leiden is a heritable tendency toward venous thrombosis, not a diagnosis that automatically requires anticoagulation. The same result can lead to very different care: an asymptomatic heterozygote usually needs risk counselling rather than medication, whereas a patient with a pulmonary embolism, homozygous disease, pregnancy-related risk, or a prior VTE may need a substantially different plan. The clinical questions are therefore sequential. First, does the patient have an acute DVT or PE, and is there haemodynamic instability? Second, would confirming Factor V Leiden change anticoagulation duration, pregnancy prophylaxis, or hormone counselling? Third, does the patient have additional risks—surgery, immobility, estrogen exposure, cancer, obesity, or another thrombophilia—that outweigh the low baseline risk carried by many heterozygotes? Canadian practice applies these principles through current ASH, CHEST, and SOGC guidance together with provincial protocols and local laboratory requirements.