Table 3 Main study characteristics of the comparative risk of DVT

Total sample size

N = 320

N = 788

N = 988

N = 568

Mean age (SD)

66 (16) yrs

61 (17.2) yrs

59.1 yrs (17.3) cases, 58.1 yrs (16.8) in control group

54 yrs

Male (%)

188 (59%)

313 (40%)

454 (36%)

202 (36%)

Sources of cases

Consecutive patients hospitalised for DVT or PE

Consecutive individuals presenting to outpatients with confirmed DVT

Confirmed cases of DVT identified through general practices

Consecutive individuals with confirmed DVT presenting to an A&E Dept or primary care

Sources of control

No DVT or PE age-matched

Individuals where DVT were not confirmed Age & sex matched

No DVT (494) Age/sex matched

Individuals where DVT not confirmed

Case definition

Ultrasound echo-Doppler examination)

Compression ultrasonography, D-dimer assay and clinical follow ups and venography, ventilation perfusion lung scan or angiogram)

Venography, duplex ultrasonography, B mode untrasonography, and /or impedance plethysmography

Duplex ultrasonography

Exposure

Any travel > 4 hrs in last 4 weeks (self report)

Air travel > 3 hrs in last 4 weeks (Patient self report)

Any travel > 6 hrs in last 3 weeks (Patient self report – case notes)

Air travel > 3 hrs* in last 4 weeks (Patient self report)

Comments

Of those cases with DVT only 9 out of 39 patients travel by air.

The control group may be less likely to travel by air due to lower social class compared with cases.

If control individuals were ill in the last three weeks, they are less likely to have travelled compared with cases.

Many flew less than 5 hours.

People included in the study (cases or control) have several potential confounding factor they are not apparently healthy before the travel.

There are limited numbers of patients with DVT who travel by air in cases (4) and control, (13).

The number of continuous hours that was investigated was more than 3 hours, a period seems shorter than other studies.

Participants were asked about travel history before they knew the results of objective diagnosis to avoid referral bias.

Included patients in cases were individuals with surgery and plaster cast in previous 3 weeks flu or rhinopharygeal syndrome matched on sex and age +/- 10 years.

Long -distance travel and not specifically air travel that was addressed as risk factor for DVT. The length of travel was not specified.

If control individuals were ill in the last three weeks, they are less likely to have travelled compared with cases in the last 3 weeks.

The distribution of intrinsic and triggering risk factors are different between case and control patients.

>8 hrs of air travel was not reported as a significant factor.

The risk factor in relation to travel is only apparent if there was at least one risk factor