economy class syndrome” is the formation of a blood clot in a vein deep within the body, usually in the legs. Not only can DVT be painful, but it can also lead to a potentially fatal condition called pulmonary embolism if the clot breaks free and travels through the circulatory system to the lungs. In addition to environmental factors, there are genetic variations that can raise a person’s chances of having DVT. But providing genetic testing to everyone is not feasible, leading researchers from the Leiden University Medical Center in the Netherlands to question whether a family history of DVT could also be of value as a risk indicator. The researchers looked at records and family histories of 3,764 people — 1,605 who had had DVT and 2,159 who had not. Their results, published online this week in the Archives of Internal Medicine, show that in people with no environmental or genetic risk factors, having a first-degree relative (parentor sibling) with DVT increased the odds of having a first episode of DVT by 2.5 times. This was about the same as the increase in odds (2.3 times) caused by genetic risk factors in people with no family history or environmental risk factors. Only 29.7% of people with family history of DVT carried a known genetic risk factor, but a family history was more common in DVT patients than controls with similar known genetic and environmental risk factors. This suggests that there are unknown, probably genetic, factors at work. The researchers say that unknown genetic factors probably have small effects that contribute to DVT only when other, larger risk factors are present, such as the factor V Leiden genetic variation or an environmental risk factor such as surgery, immobilization or oral contraceptive use. Based on the fact that a positive family history increases the risk of DVT to about the same degree as known genetic risk factors, and the fact that most people with a family history of DVT don’t actually carry a known genetic risk factor, the researchers conclude that genetic testing for variations associated with DVT is of little additional value compared to collecting family history alone. However, while a cost-effectiveness argument might be made, the authors’ own data shows that known genetic risk factors substantially increase the odds of DVT whether or not a person has a family history of the condition. A family history of DVT or a known genetic risk factor alone increases the odds of DVT by about two and a half times, but the presence of both increases the odds by 6.3 times. The presence of an environmental risk factor raises the odds of DVT about nine and half times. Add in a positive family history and the odds go up to more than 16 times those of someone who lacks known risk factors (both genetic and environmental) as well as a family history of DVT. But in those unlucky enough to have a family history of DVT, an environmental risk factor and a known genetic risk factor, the odds of DVT are increased a whopping 64 times.