Epidemiology of VTE Flashcards
What are the types of observational studies?
- case-control
- cohort
observing = no exposure implementation
What are the outcome of comparative studies?
- estimate of RELATIVE RISK (ratio)
- cohort and RCTs: info on incidence in expo vs non-expo (gives estimate for attributable risk)
What is Virchow’s triad?
[INCREASED VTE RISK]
- reduced blood flow
- increased coagulation
- damage to venous endothelium
What factors place airline passengers at elevated VTE risk?
- immobility (low blood flow)
- increased blood coagulability (dehydration, hypoxia)
What is the time frame for developing VTE after a flight?
occurs days rather than hours after flight
TREND: usually 3 days-2 weeks
What are the strengths of case-control studies?
- quick to carry out
- cheap
- good approach for rare disease
- can look at several approaches (but only one disease)
What are the challenges of a case-control study?
- confounding is likely
- bias (systematic error)
both of these can occur in cohort studies, but they are more likely in CCS
What are the 2 general types of error that can occur?
- random error
- systematic error (bias)
What is random error?
random imprecision, but without systematic inaccuracy.
Still on target – more observations provide better estimate of target position
What is systematic error?
= bias
can occur
- in the selection of cases and controls
- in the collection of information from cases and controls on exposure to the potential cause
What are main limitations of a case-control study?
- hypothesis and confounding factors
- size and statistical power
- selection of cases and controls
- analysis approach
What factors inform how big a study should be?
How strong an association? (relative risk)
How common is the exposure?
What p value will be statistically significant?
What chance do you want to have of detecting an association if it is really present? (often 80-90%)
What guiding principles can be used for selection of cases in CCS?
Standard definition of cases
Newly diagnosed (incident) cases or established (prevalent) cases?
Advantages of incident cases
- Closer to causes of disease
- Less chance of exposure changes
- Not assessing determinants of survival
How should controls be selected for a CCS?
SELECTION
avoid selection which will systematically affect exposure of interest (in this case flying)
PARTICIPATION
Will control population be reasonably willing to take part?
INFORMATION
will controls provide information in a similar way to cases?
What are the key principles in assessing exposure in a CCS?
Ensure that opportunity to recall exposure is similar in cases and controls
maintain consistency in:
- Setting (hospital/home)
- Instrument/method used
- Informant
- Incentives
How do we ensure that assessment of exposure is not biased?
OBJECTIVITY IS KEY
- Participants should be blind to hypothesis
- Use an objective assessment method if possible
- Self-admin questionnaire preferred to interviewer; if observer used, blind to hypothesis and case-control status if possible
- Standard protocol, observer training etc
How is a CCS analysed?
basic outcome: RELATIVE RISK (ratio)
usually provided by an odds ratio
What complex analyses can deal with confounding in CCS?
LOGISTIC REGRESSION
(0 or 1 score)
analysis examines relationship between exposure and outcome at each stratum of confounder
Can then pool the analysis across all strata to obtain adjusted odds ratio
What does a CCS not provide?
- disease incidence (since patients are not followed up over time and they already have the disease)
What are the 2 types of bias present in a CCS?
- info bias
- selection bias
What is selection bias?
In which the selection of either the cases or the
controls (or both) is systematically related to the
exposure under scrutiny
What is information bias?
In which the information on exposure obtained from
the cases and controls is not comparable
How is selection bias minimised?
Cases and controls drawn from a similar population
insofar as possible, and in a way which will not affect
their risk of exposure
How is info bias minimised?
Information on exposure is obtained from cases and
controls in exactly the same wa
What factors may impact a true association?
- chance
- bias
- confounding
- reverse causation
What is reverse causation?
presence of disease has affected exposure, or the estimation of exposure
What factors might whether the association is a true null (no difference between groups)?
- small sample size (limited statistical power)
- inaccuracy of exposure measure
- absence of association = bias or confounding
Where are the different risk measures useful?
RELATIVE RISK
important for defining cause, but not helpful for guiding indivudal in Rx
ATRRIBUTABLE (excess) RISK
more helpful in helping decision making in the individual pt
How is attributable risk calculated?
Absolute risk in those exposed
MINUS
Absolute Risk in those not exposed
(baseline risk)
Attributable risk provides an indication of the extra risk (due to air travel) in the individual
BUT attributable risk is highly dependent on baseline risk
What measures could be taken to prevent flight-related VTE?
FOR HIGH RISK pt
- Avoidance of flights (especially long-haul flights)
- Use of elastic compression stockings
- Consider taking aspirin in advance
- Subcutaneous low molecular weight heparin
What are the general tips given to people on long haul flights?
FOR ALL RISK pts
- Lower limb movement during flight
- Keep well hydrated
What are the main barriers to in-flight VTE prevention?
- Seats discourage movement
- Difficulty leaving seats
- Passages narrow; trolleys
- Film screens
