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Dear Dr. Roach: Many members of my family and I have protein S deficiency and are prescribed warfarin. We are aware of the effects that food, exercise and medicines have in reaction to warfarin. Several of us have INR meters and do our testing at home. I have been on warfarin since 1991, with only a couple of clots, usually when dehydrated.

My grandson was on Eliquis, and had a DVT despite this, so he is back on warfarin. It is my understanding that none of the other “blood thinners” are testable to determine INR. Why do doctors prescribe them for protein S deficiency? Aren’t they approved just for atrial fibrillation? Why would a doctor prescribe Lovenox and warfarin together for over a year for my grandson?

Hematologists, not internists, should be the doctors to treat protein S deficiency.

— A.C.G.

Dear A.C.G.: Most people have probably not heard of protein S or protein C, but these are important regulators of the body’s capacity to form a clot. Unlike the other clotting factors, these act AGAINST the blood clotting cascade and so help prevent inappropriate clotting in the body.

When people have low levels of either of these proteins, they are at risk for developing a blood clot, such as a deep venous thrombosis or pulmonary embolism.

People with protein S deficiency are treated with anticoagulants if they have a clot, but are generally not treated with anticoagulants if they have not had a clot, even if they have known deficiency.

However, some people with protein S deficiency but no history of abnormal clotting may be given anticoagulants before surgery, for example, especially if there is a strong family history of abnormal clots.

The choice of anticoagulants includes both warfarin and direct oral anticoagulants such as apixaban (Eliquis). One major advantage to apixaban and similar drugs is that routine blood testing is not required. They do not work the same way warfarin does, so checking the INR (international normalized ratio) is not appropriate. Head-to-head testing of warfarin and the newer agents like apixaban has shown that apixaban is at least as good as warfarin.

Both apixaban and warfarin are Food and Drug Administration-indicated for treatment of deep venous thrombosis, but neither is specifically indicated in people with protein S deficiency.

Anticoagulants sometimes fail. If a person does develop a clot on one treatment, his or her physician may choose a different treatment, such as apixaban for a person who developed a clot on warfarin, or vice versa. Low molecular weight heparin, such as Lovenox, is an injection medication seldom used for prolonged periods of time, such as the year your grandson took it. But there have been cases.

I agree with you that difficult or complicated cases are ideally managed by a hematologist when available.

Readers may email questions to ToYourGoodHealth@med.cornell.edu.

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