Clinical features

• recurrent venous thromboembolism ^a
• a known clotting disorder ^b
• a history of clotting disorders in first-degree relatives ^c

• Thrombophilia is quite common, especially in patients who develop a DVT aged less than 25. ^c
• Factor V_Leiden is the commonest disorder. ^b

A family history and early or recurrent venous thromboembolism makes thrombophilia more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
suspected thrombophilia c (pre-test probability: 8%)	thrombophilia (thrombophilia blood tests)	family history (first degree relative)	2.2 (1.3 to 3.5)	16%	0.67 (0.45 to 1.0)	6%
		family history and first episode aged < 41	3.2 (1.4 to 7.0)	22%	0.81 (0.63 to 1.0)	7%
		family history, first episode aged < 41 and previous PE, DVT	4.7 (1.3 to 17)	30%	0.89 (0.76 to 1.1)	7%

Factor V _Leiden is the commonest thrombophilia

Thrombophilia	% (95% CI)
factor V Leiden b c	21% (17% to 26%)
factor V Leiden aged < 45	23%
factor V Leiden aged < 25	42%
protein C deficiency	3% (1% to 5%)
protein S deficiency	2% (0% to 4%)
plasminogen deficiency	1% (0% to 2%)
antithrombin III deficiency	1% (0% to 2%)

Thrombophilia increases the risk of venous thromboembolism

Outcome	Risk Factor	PEER	OR (95% CI)	NNH (95% CI)
venous thromboembolism b	factor V Leiden independent	0.021%	9.3 (3.6 to 24.1)	570 (200 to 1800)
venous thromboembolism b	homocystinaemia not independent	0.021%	2.95 (2.08 to 4.17)	2400 (1500 to 4400)