Broccoli and spinach are not likely to affect INR

If you are undergoing treatment with anticoagulant drugs such as Marevan, you should regularly be tested with a blood test called INR.
This blood test is designed to estimate if the dose you receive is correct, but it should also prevent overdosing in which the blood would get "too thin". This condition is dangerous and can result in internal bleeding.
(Vitaviva's comment: Vitamin K has the opposite effect of anticoagulants and is therefore suspected of being able to abolish the effect of these drugs; for this reason, people taking anticoagulants have since long been advised against taking vitamin supplements containing vitamin K and against eating vegetables that are rich in vitamin K, such as broccoli, spinach, and kale.)
12 healthy volunteers were included in a study in which they were given a correct dosage of anticoagulants for 13 weeks and adjusted to a maintenance dose with a constant and stable INR value that would prevent them from forming blood clots.
Then, they were given increasingly large daily doses of vitamin K from 50 mcg. to 500 mcg. during the course of one week. Not until the dose reached 150 mcg. of vitamin K a day taken as a dietary supplement, was any effect on INR observed. Even at this dose, INR was only affected in 3 out of the 12 trial subjects.
When the trial subjects were given food that is particularly rich in vitamin K, i.e. broccoli and spinach, there was no clinically relevant effect on INR because the effect was so transient, and the authors suggest that the reason might be a poor bioavailability of the vegetables. This may be surprising, as kale, spinach, and broccoli can contain up to 400 mcg. of vitamin K per 100 g.
Doses of 100 mcg. vitamin K as an easily absorbable dietary supplement had no effect on INR.
If this study on healthy, young trial subjects can be repeated with the same result on patients with a predisposition to forming blood clots, it would make life significantly easier on a great number of people who every day stare in despair at the long list of foods containing vitamin K that they are not allowed to eat while taking Marevan.
Reference:
Schurgers LJ, Shearer MJ, et al: Effect of Vitamin K Intake on the Stability of Oral Anticoagulant Treatment. Dose-Response Relationships in Healthy Subjects. Blood 2004;104(9):2682-2689.