Could it be that changes in barometric pressure might increase arthritic pain in some people causing (indirectly) less activity/exercise leading to change in INR?
From:
http://www.ascp.com/public/pubs/tcp/1999/nov/weather.shtml
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Several small studies have confirmed that for people with rheumatoid arthritis or osteoarthritis, pain increases when barometric pressure changes or when temperature or humidity are increasing.5,9,16 Further, people who have osteoarthritis tend to feel more pain on rainy days, and people with fibromyalgia are also sensitive to barometric changes. This tendency to feel pain may be more pronounced in women than in men.16 The key word here is change; changes in temperature or barometric pressure induce discomfort. Once the pressure or temperature has stabilized, discomfort does, too.9 In many older people, any weather front, change in barometric pressure, or blast of cold air can cause pain.10
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From that same site an article saying cold temps can produce increased fibrinogen (promoting coagulation).
From:
http://www.ascp.com/public/pubs/tcp/1999/nov/weather.shtml
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The reason for this trend is not clear. Scientists do know that the normal physiological response to cold includes a rapid change in many blood components. Those considered most important to circulation and coronary health are decreased total blood volume (leading to reduced oxygen supply10), increased fibrinogen (promoting coagulation), and increased blood viscosity and cholesterol levels (promoting arteriosclerosis17). It comes as no surprise that coronary events such as angina and myocardial infarction peak during autumn and winter,3,8,18 and circulatory ailments peak in January and February. 8,18
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