Epidemiology Flashcards by Élise Girouard

List the basic types of Epidemiologic studies

Descriptive epidemiology (distribution, description)

2. Analytical epidemiology (hypothesized causal relationship, evaluating interventions, risk factors)

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Recognize primary health outcomes (“the five D’s”)

Death
Disease
Discomfort
Disability
Dissatisfaction

** can add destitution (financial cost of illness)

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prevalence

proportion of person in population who have the disease at a specific point
= number of existing cases of a disease / total population (at a given time)

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describe sensitivity

Sensitivity (ability of a test to correctly identify those with the disease, aka true positive rate)
= True positives/(True positives + false negatives)

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What are the major biases that complicate the interpretation of studies of screening results.

VOLUNTEERISM: people who attend screening differ from those who don’t. The benefits of screening will exaggerate the benefits.
LEAD TIME BIAIS: in evaluating the benefit of screening on reducing mortality, a bias in the comparison of survival tomes may result if lead time is not taken into account. In a RCT, we have to compare mortality, nor survival.
LENGHT -TIME BIAIS: Screening will more likely detect those persons having a longer preclinical disease phase resulting in over representation of slowly progressing disease

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however something is measured, we want the mesurement to be…

Validity: does a measurement reflect the truth? (FREE OF BIAS)
Reliability: precision, reproducibility
Responsiveness: does it change as the phenomenon changes?
Interpretable: Do people reading it know what it means?

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Incidence

number of NEW events in a population of individuals AT RISK during interval

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what are the 2 specific types of incidence mesures ?

cumulative incidence

2. incidence rate (incidence density)

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cumulative incidence

the proportion of individuals INITIALLY FREE of the disease of interest who develop the outcome during a defined time period
CI = # of new cases during a given period/total population at risk

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Incidence rate

number of events divided by the amount of person-time observed, measures how quickly people are developing the disease
DI = # of new cases of a disease in a given period of time/total person-time of observation

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what’s the link between prevalence, incidence and duration ?

prevalence = incidence x duration

lower prevalence = high incidence, short duration (people who have the disease die fast)
higher prevalence = low incidence/long duration (people live with the disease for a long time)

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name 2 special types of incidence mesures and why they’re used

mortality rate

- attack rate (a type of cumulative incidence , for accute epidemis)

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why is prevalence mesure useful ?

most useful for health care providers
to assess the public health impact of a specific disease within a community
to project medical care needs for affected individuals

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why is incidence mesure useful ?

assessing exposure disease relationship

- risk factors for disease

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what are adjusted rates ?

used to be able to compare rates between two populations that are different in terms of some important characteristics (age structure, sex, etc.)

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what would be the best Dx test ?

valid
reliable
inexpensive
easy to administer
of minimal discomfort to the patient

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what are the most useful test in clinical practice ?

those who produce the biggest changes from pretest to post-test probabilities

specificity

ability of the test to correctly identify those without the disease, aka true negative rate
= true negative / (true negative + false positive)

predictive value of a positive test (PPV)

probability that subjects with a positive screening test truly have the disease
= true positive / (true positive + false positive)

predictive value of a negative test (NPV)

probability that subjects with a negative screening test truly don’t have the disease
= true negative / (true negatives + false negatives)

do sensitivity and specificity change with prevalence ?

No, these remain fixed characteristics independant of what population the test is performed/disease prevalence

on what depend PPV and NPV ?

sensitivity
specificity
prevalence of the disease

as prevalence increase, what PPV and NPV increase or decrease ?

PPV increase and NPV decrease

what are likelihood ratios ?

a way to incorporate the sensitivity and specificity of a test in a single mesure
LIKELIHOOD OF A GIVEN TEST RESULT IN A PERSON WITH THE DISEASE COMPARE TO THE LIKELIHOOD OF THIS SAME RESULT IN A PERSON WITHOUT THE DISEASE

LR+

Likelihood ratio of a positive test (LR+) if LR+ is 10, the people who have the disease are 10x more likely to test positive = sensitivity / 1-specificity

LR-

Likelihood ratio of a negative test (LR-) if LR- = 0.1, people who have the disease are about 0,1 fois more likely to test negative than those who do. (so want it as close to 0 as possible) = 1-sensitivity / specificity

what does high LR+ help to do

ruling in the disease = conclusive changes from pre to post-test probabilities

what does low LR- value help to do ?

RULING OUT the disease

a test with a high sensitivity but a low specificity produce

- no false negative | - many false positive

a test with a low sensitivity but a high specificity produce

- many false negative | - no false positive

how is, for any disease determined the optimal cut-off point ?

it has to be selected depending on the consequences of missing a few positive if the cut-off is set higher or of falsely classifying more negatives as positives if the cut-off point is set lower.

what's the receiver operating curve ?

a plot of what happens to sensitivity and specificity as different cutoffs are used (for a particular test)

what's the preclinical phase ?

the phase between disease onset and symptoms (possible screening)

what are the 2 steps of screening ?

1. screening test | 2. follow-up for definite diagnosis

the feasibility of screening is based on those requirements

1. burden of the disease 2. availability of effective treatment 3. performance of test

what are the requirement for screening

1. accuracy (high sensitivity and specificity) 2. reliability (reproducible) 3. safe and acceptable 4. simple and inexpensive

best trial to compare a screening program ?

RCT - randomized control trials

what are the negative consequences of screening ?

- physical harm - psychological harm - false reassurance - financial harm

what are the disease's requirements for screening ?

1. severe, common and perceived to be an important population health problem 2. must be able to significantly alter outcomes such as death or severe disability 3. significant lead time

what are the different types of data ?

- nominal (in categories) - ordinal (some sense of order, but interval not specified) - interval (inherent order with specified intervals)

what are the 3 ways of defining abnormality ?

- abnormal = UNUSUAL - abnormal = ASSOCIATED WITH BAD OUTCOME - abnormal = TREATABLE (target levels)

what's the regression to the mean ?

when you retest someone with an abnormal result, particulary if extreme, the second test will usually come back closer to the mean, for that population