Summary of findings 8. Vitamin K antagonists versus aspirin.

Vitamin K antagonists compared with aspirin for primary thromboprophylaxis in ambulatory cancer patients receiving chemotherapy
Patient or population: ambulatory cancer patients receiving chemotherapy Settings: outpatient clinics Intervention: VKA Comparison: aspirin
Outcomes	Relative effect (95% CI)	Illustrative comparative risks* (95% CI)		Difference2 (95% CI)	No of Participants (studies)	Quality of the evidence (GRADE)	What it means
		Assumed risk1	Corresponding risk
		With aspirin	With VKA
Symptomatic VTE: Follow‐up: median 2.1 years	RR 1.50 (0.74 to 3.04)	Intermediate‐risk population3		27 per 1000 more events (14 fewer to 211 more)	440 (1)	⊕⊕⊕⊝ moderate4	VKA probably increases the incidence of symptomatic VTE when compared to aspirin in multiple myeloma.
		55 per 1000	82 per 1000 (40 to 166)
Major bleeding Follow‐up: median 2.1 years	RR 0.14 (0.01 to 2.75)	Intermediate‐risk population3		12 per 1000 fewer events (14 fewer to 24 more)	440 (1)	⊕⊕⊝⊝ low5	VKA may reduce the incidence of major bleeding when compared to aspirin in multiple myeloma.
		14 per 1000	2 per 1000 (0 to 38)
Symptomatic PE Follow‐up: median 2.1 years	RR 1.00 (0.25 to 3.95)	Intermediate‐risk population3		0 per 1000 fewer events (14 fewer to 54 more)	440 (1)	⊕⊕⊕⊝ moderate4	VKA is probably as effective as aspirin in the prevention of symptomatic PE in multiple myeloma.
		18 per 1000	18 per 1000 (5 to 72)
Symptomatic DVT Follow‐up: median 2.1 years	RR 1.75 (0.75 to 4.09)	Intermediate‐risk population3		27 per 1000 more events (9 fewer to 112 more)	440 (1)	⊕⊕⊕⊝ moderate4	VKA probably increases the incidence of symptomatic DVT when compared to aspirin in multiple myeloma.
		36 per 1000	64 per 1000 (27 to 149)
1‐year mortality Follow‐up: NA	NA6	Intermediate‐risk population3		NA	NA	NA	We do not know how VKA affects 1‐year mortality when compared to aspirin in multiple myeloma.
		NA	NA
Clinically relevant bleeding Follow‐up: NA	NA6	Intermediate‐risk population3		NA	NA	NA	We do not know how VKA affects clinically relevant bleeding when compared to aspirin in multiple myeloma.
		NA	NA
*The basis for the assumed risk (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; DVT: deep vein thrombosis; NA: not applicable; PE: pulmonary embolism; RR: risk ratio; VKA: vitamin K antagonists; VTE: venous thromboembolism
GRADE Working Group grades of evidence High quality: Further research is very unlikely to change our confidence in the estimate of effect. Moderate quality: Further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate. Low quality: Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate. Very low quality: We are very uncertain about the estimate.

¹The assumed risk is calculated from the observed control group risk in Palumbo 2011.
 ²Difference calculated as the absolute risk difference between the assumed risk and corresponding risk, expressed per 1000.
 ³Intermediate‐risk population refers to the median observed risk to experience symptomatic VTE in the trials contributing to the analyses (55 per 1000). Rates between 2% and 7% are considered intermediate risk (Khorana 2008).
 ⁴Downgraded (1 level) because of imprecision. Although attrition bias may have occurred, it is unlikely to have changed the results in a clinically relevant manner.
 ⁵Downgraded (2 levels) because of imprecision. Although attrition bias may have occurred, it is unlikely to have changed the results in a clinically relevant manner.
 ⁶No trials contributed to this outcome.