Deep vein thrombosis (DVT), sometimes called “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.

(The researchers considered several genetic risk factors, including the factor V Leiden and prothrombin variations reported in the 23andMe Venous Thromboembolism Clinical Report. A table summarizing the results of the current study is included at the end of this post.)

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.


Notes on the results table:

• Odds are given relative to someone with a negative family history and no known risk factors for DVT.
• A family history of DVT is the presence of the condition in any parent, brother or sister.
• Environmental risk factors considered were surgery, injury, immobilization and pregnancy or delivery within three months of the study, use of oral contraceptive or hormone therapy at the time of the study, and cancer diagnosis within five years before or within six months after the start of the study.
• Genetic risk factors considered were the factor V Leiden variation, the prothrombin 20210A variation, and low levels of antithrombin, protein C or protein S.

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I’m a bad patient.

Every time I go to a doctor she asks, “Do you have a family history of any diseases?”

I’m ashamed to say that my answer is always “I don’t think so.”

Your family health history is a powerful tool for your doctor; it can help him or her make vital screening and treatment decisions. Yet a recent survey (Porter Novelli Healthstyles 2004) found that although 96% of Americans recognize the importance of their family history, only 33% have ever tried to gather and organize this data for themselves.

23andMe helps people see what their DNA might have to say about a host of common and not-so common diseases and conditions; but we don’t want anyone to think that purchasing our Personal Genome Service™ is a substitute for a detailed family health history.

For example, the effect of the SNPs 23andMe reports on for breast cancer pales in comparison to that of mutations in the well-known BRCA1 and BRCA2 genes, which run in families and can increase a woman’s breast cancer risk three- to seven-fold (23andMe does not report data on these mutations). So even if your 23andMe data says that you have average or even below-average risk for breast cancer, a strong family history of the disease indicates the strong possibility of a genetic mutation that trumps the information we have gleaned from your SNPs.

In other cases, your family health history might dovetail with your 23andMe data, giving a fuller picture of, and perhaps increasing, your predicted risk for a certain disease. For example, a report published just last week in the Journal of the National Cancer Institute showed that a SNP recently associated with lung cancer (customers can see their data in Health and Traits) has a significantly larger effect in people where three or more first-degree relatives have been diagnosed than in people who develop a sporadic case of the disease.

The bottom line: It is very helpful to be aware of your family health history and share it with your medical provider. Here are a few resources to help you get you started:

• The U.S. Surgeon General’s Family History Initiative
  For the past four years the Surgeon General has designated Thanksgiving, a time when families naturally gather together, as National Family History Day. You can use the online tool “My Health Portrait” to help guide you in collecting your own family’s health history. (Also available in Chinese, Polish, Spanish, French and Portuguese from the Brigham and Women’s Hospital National Family Health Initiative Website.)

• The Genetic Alliance
  Links to sites that can help with collecting a family health history, including resources for family reunion organizers who want to incorporate this into their next gathering.

Mom, get ready for some questions.

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