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Life Support > Principle of cohort studies > Flashcards

Principle of cohort studies Flashcards

1
Q

What is a cohort study?

A
  • longitudinal, prospective study

following groups with different exposures forward over time, providing disease incidence

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2
Q

What is the design of a cohort study?

A

exposed subject vs unexposed subjects

do they develop disease or not?

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3
Q

What is the definition of a cohort study?

A

Hennekens et al, 1987

A prospective, cohort study is one in which a
group of people with different exposures is
followed over time to see if they acquire a
disease/outcome

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4
Q

What are the strengths of a cohort study?

A
  • strong research design (exposure measured before disease)
  • provide info on several parameters (RR, incidence and attributable risk)
  • several expos and several disease outcomes in one study

BUT not quick or cheap option

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5
Q

What are the classic cohort studies in CHD?

A

Global:

  • Framingham study (5000 people)
  • Seven countries study (12000 people)

UK based:

  • Whitehall (12000)
  • British Regional heart study (8000)
  • Scottish heart and health study (10000)
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6
Q

What are the key pitfalls in cohort study design?

A
  • hypothesis, confounding factors
  • size and statistical power
  • selection bias
  • follow up methods
  • approach to analysis
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7
Q

What is a confounding factor?

A

factor associated both with the exposure and with the outcome of interest

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8
Q

What are the important considerations for study size and statistical power?

A
  • strength of association (RR)
  • is expo common?
  • incidence rate in unexposed group
  • what p value will be statistically significant
  • what are the chances of detecting an association if present (study size)?
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9
Q

What is important to consider when selecting a cohort study population?

A
  • range of exposures low to high
  • general population would be helpful: representative

2 options:

  • geographically define
  • well-define groups *(e.g. occupation)
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10
Q

How can cohort studies inform on exposures that are uncommon in general population?

A

e.g. occupational hazards - asbestos

highly exposed occupational group
vs
low expo comparison group"
(+ve/-ve control)
(internal comparisons in same factory w/o expo and external group of similar exposure)
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11
Q

How can you define exposure as accurately as possible in a cohort study?

A
  • questionnaire, examination, blood, medical records
  • make OBJECTIVE measurement
  • repeat assessment if possible: more accurate associations
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12
Q

Where does the ‘outcome data’ for cohort studies come from?

A

N.B multiple outcomes may be measures

  • routine surveillance/registers
  • medical records
  • questionnaires
  • physical examination
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13
Q

How is risk presented from a cohort study?

A

usually expressed as number of outcomes per 1000 recruited in a defined period (expo vs. non-expo)

relative risk or attributable risk

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14
Q

What is relative risk (RR)?

A

RATIO

measured as risk in exposed/ risk in unexposed

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15
Q

What is attributable risk (AR)?

A

EXCESS risk (difference)

risk in exposed MINUS risk in unexposed

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16
Q

What are the ways that cohort data may be analysed?

A
  • logistic regression

- Cox proportional hazards modelling

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17
Q

What is logistic regression?

A

takes account of WHETHER an event has occurred during follow up ( 0 or 1)

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18
Q

What is Cox proportional hazards modelling?

A

takes account of WHETHER and WHEN and event has occurred

Can account for graded expos and for confounding

MORE PRECISE

19
Q

What criteria are used to assess whether an association is causal?

A

Bradford-Hill criteria

20
Q

What are the Bradford Hill criteria?

A
  • Strong
  • Independent
  • Dose response
  • Temporal sequence (exposure precedes outcome)
  • Consistent
  • Specific
  • Reversible
  • Biological plausibility (biological mechanisms)
  • Analogy (similar to other studies)
21
Q

What factor lead to high total and LDL-cholesterol levels in the population?

A
  • high dietary saturated fat intake
  • high dietary sat fat: polyunsaturated fat ratio
  • obesity
  • genetics
22
Q

What is familial hypercholesterolaemia?

A

genetic predisposition to elevated total and LDL cholesterol

autosomal dominant

1:500 hets

23
Q

What are the determinants of high HDL cholesterol levels?

A
  • high levels of exercise
  • higher EtOH intake
  • lower adiposity
  • non-smoker
    (potentially protective factors)
24
Q

What are the important causes of CHD identified in cohort studies?

A
  • high blood cholesterol
  • high BP
  • smoking
  • DM
  • adiposity
25
Q

What makes the identified causes of CHD ‘casual’?

A
  • strong evidence
  • RR associated with exposure are high
  • risks are widespread in population
26
Q

What happens when there are multiple causes of CHD co-existing?

A

Risks MULTIPLY together

27
Q

What are the other contributing factors that INCREASE risk of CHD?

A
  • DM (>2x elevated risk, F>M)

- obesity

28
Q

What are the other contributing factors that REDUCE risk of

A
  • physical activity
  • EtOH (light to moderate)
  • high HDL-chol.
29
Q

What are the other potential causes of CHD?

A
  • poor foetal growth and nutrition
  • factor related to inflammation
  • micronutrient status (low vit D or C or low folate)

BUT the evidence is inconsistent

30
Q

What is a disease cause?

A

a factor which, of itself, increases

the risk of a disease occurring

31
Q

What is a risk factor?

A

any characteristic which
IDENTIFIES a group at increased (decreased)
risk of disease now or in the future

32
Q

What are the non-modifiable causes of CHD?

A

Age
Sex
Ethnicity
FHx

33
Q

What are the modifiable causes of CHD?

A
  • BP
  • blood cholesterol
  • smoking
  • obesity
  • T2DM
34
Q

What are the main potential biases in cohort studies?

A
  • SELECTION BIAS: marked and selective loss to follow up
  • INFO BIAS
    misclassification of outcome status (disease or not)
    usually influences by knowing their expo status
35
Q

What can be done to ensure that bias is limited in cohort studies?

A
  • make follow-up as complete as possible
  • standardised reference for assessment of outcome
  • blinding to exposure status
36
Q

What is the validity of case-control vs cohort study?

A
  • cohort is a stronger study design: exposure (potential cause) is studies prior to disease onset)
  • bias can occur in either study type but is much more likely in case-control
37
Q

What can be added to cohort studies to strengthen the design?

A
  • v. large cohorts
  • pooling of data (statistical power and precision)
  • nested case-control within cohort study
  • retrospective cohort studies
38
Q

What is advantage of using ver large cohorts in cohort studies?

A

good for studying less strong associations

good for studying interplay of multiple factors

39
Q

What is the UK biobank prospective cohort?

A

500, 000 UK men and women
aged 40-69
extensive baseline questions, assessment and stored samples

40
Q

What is a nested case-control study design?

A

= prospective case-control

4 groups:
exposed vs non-exposed
disease vs no disease

41
Q

What is a historical cohort study?

A

exposures have already occurred and been assessed

cohort established whilst follow up is occurring or has occurred

42
Q

What are the main advantages of cohort studies?

A
  • exposure measured before disease onset (reduced bias)
  • temporal sequence of events
  • multiple outcomes/disease can be studied from one exposure
  • incidence can be measures (expos vs. non-expo)
43
Q

What are the main pitfalls of cohort studies?

A
  • slow, expensive and complex
  • need large number for long time
  • data collection may change participant behaviour
  • expo knowledge may impact outcome ascertainment
  • selection bias: losses in follow up

Life Support (13 decks)

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