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