Behind the Headlines

Tuesday May 13 2008

Man at desk

Counter prolonged periods at the desk with walking

“Desk-bound workers at double DVT risk,” reads the headline in The Daily Telegraph today. Workers who “sit at a desk for eight hours a day and spend more than three hours without stretching double their risk of developing deep vein thrombosis (DVT)”, the newspaper says.

The newspaper story is based on a study which looked at people admitted to hospital for blood clots or heart problems and compared how long they had sat at a desk for each day in total and how long the longest period was that they stayed seated for. However, the results should be interpreted cautiously because of the study’s limitations; the estimate of a two-fold increase in risk was not statistically significant and therefore cannot be seen as a reliable estimate. The risk of developing a blood clot for most office workers will be very low, however, it seems sensible to suggest that remaining immobile for long periods in any setting should be avoided.

Where did the story come from?

Dr Jill West and colleagues from the Capital & Coast District Health Board, Medical Research Institute of New Zealand, the University of Otago Wellington and the University of Southampton in the UK carried out this research. The study reported no sources of funding. It was published in the Journal of the Royal Society of Medicine, a peer-reviewed medical journal.

What kind of scientific study was this?

This was a case control study. The researchers enrolled 97 adults (cases) aged under 65 who consecutively attended an outpatient venous thromboembolism (VTE) clinic between October 2005 and December 2006. These people had been discharged from hospital in the previous 12 months after treatment for deep vein thrombosis (DVT, 53 people), pulmonary embolism (PE, 29 people) or both (15 people). The diagnoses were confirmed by standard methods. The researchers also enrolled 106 similarly aged adults admitted to the coronary care unit (controls).

Cases and controls were interviewed and asked questions about their VTE or heart problem and about risk factors for VTE, including age, family history of VTE, medical risk factors (such as pregnancy), surgery or trauma, prolonged travel, prolonged seated immobility at work in the four weeks before they suffered from VTE and the total prolonged seated immobility (work, travel to and from work, and at home seated at the computer) in the four weeks before they suffered from VTE. Prolonged seated immobility was defined as:

  • being seated at least eight hours a day and at least three hours at a time without getting up; 
  • being seated at least 10 hours a day and at least two hours at a time without getting up; or 
  • being seated at least 12 hours a day and at least one hour at a time without getting up.

The researchers then compared periods of immobility for cases and controls. They adjusted for the other risk factors for VTE in their analyses (not including surgery/trauma, as only one person had this risk factor).

What were the results of the study?

The average age of the case group was 44.9 years and the control group was 52.4 years. Most (about two thirds) of the controls had been admitted to hospital for heart disease. Although the odds of developing VTE was increased about two-fold in people who had prolonged periods of immobility at work or in total, this increase was no longer statistically significant when the analysis was adjusted for other risk factors for VTE.

"Prolonged seated immobility at work represents a risk factor for VTE."

Jill West, lead author

The researchers found that an increase of an extra hour in the maximum total time seated at work increased odds of VTE by 10%, and an extra hour in the maximum total time seated at work without getting up increased odd by 20%. Both of these increases only just reached statistical significance, both before and after adjustment for other risk factors.

What interpretations did the researchers draw from these results?

The researchers concluded that their study provided “preliminary evidence that prolonged seated immobility at work may represent a risk factor for VTE requiring hospital admission”.

What does the NHS Knowledge Service make of this study?

This study has a number of limitations, and there are some points that it is important to keep in mind when interpreting this study:

  • The increase in the odds of VTE with prolonged periods of immobility at work or at work and home were not statistically significant. This means that it is not possible to say confidently that periods of prolonged immobility at work and home (as defined by this study) increase the risk of VTE. Although the increase in the odds of VTE per additional hour seated was significant, it was only just significant, and this leads to a reduction in confidence in this estimate. 
  • The cases and controls were not matched for any key features, such as age (which was significant different between the groups), or the other risk factors for VTE. Although the researchers did try to adjust for these factors in their analyses, matching the participants would have reduced these differences, potentially leading to a more reliable result.
  • The controls chosen all had heart problems; these people will not be representative of the general population. 
  • The questionnaires asking people about periods of immobility were administered retrospectively, and people’s recollection of the time they spent seated may not have been accurate. People who had experienced a VTE may have been likely to overestimate how long they spent seated if they felt it might be related to their VTE.

The risk of developing a VTE for most office workers will be very low. Although the estimate of increased risk from this study may not be reliable, prolonged periods of immobility, for example during travel, have been linked to an increase risk of blood clots in other studies, and it seems sensible to avoid long periods of immobility in any setting if possible. The researchers call for larger studies and for evaluation of chair design to further an understanding of the mechanisms involved.

Sir Muir Gray adds...

Every hour get active; I am planning for computers to go down every hour for two minutes only to be reactivated by action.

Analysis by Bazian

Edited by NHS Choices

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