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1
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2
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3
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- Live sensibly, among a thousand people only one dies a natural death,
the rest succumb to irrational modes of living. Maimonides, 1135 - 1=
204
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4
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- Primary prevention
- Secondary prevention
- Tertiary prevention
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5
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- Keep disease from occurring at all by removing the risk factors
- Examples:
- immunization for communicable diseases
- stop smoking
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6
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- Detects disease early when it is still asymptomatic and when early
treatment can stop the disease from progressing
- Example: Pap smears for cervical cancer
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7
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- Clinical activities that prevent further deterioration or reduce
complications after a disease has become evident
- Example: use of beta-blocking drugs to decrease the risk of death in
patients who have recovered from myocardial infarction
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8
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9
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- Presumptive identification of an unrecognized disease by the applica=
tion
of tests, examinations, or other procedures which can be applied rap=
idly
- Sorts out persons who have a disease from those who probably do not<=
/li>
- Person with positive or suspicious findings must be referred for fur=
ther
diagnosis and treatment
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10
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- First decide which disease one wants to screen for
- Many time we order exams without knowing exactly what we are looking=
for
- Example: urinalysis ordered in a routine check up. What are we
interested in: diabetes or urinary tract infections or calculi, etc?=
- Which condition is it worth screening for?
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11
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- If early treatment is not effective it is not worth screening for the
disease
- Effective treatment
- it must work
- the patients accept it
- results of treatment are better early in the course of the disease
(asymptomatic) that later when disease is symptomatic and the patie=
nt
seeks medical care
- The best way to establish efficacy is by RCTs (it may take a long ti=
me)
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12
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- When evaluating effectiveness of a screening program it is important=
to
consider several sources of biases:
- Lead time bias
- Length/time bias
- Patient compliance
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13
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- Lead time is the period of time between the detection of a medical
condition by screening and when it ordinarily would have been diagno=
sed
because of symptoms
- People who are diagnosed by screening for deadly disease will live
longer from the time of diagnosis than people who are diagnosed beca=
use
of symptoms, even if early treatment is ineffective (disease time vs.
survival time)
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14
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15
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- A disease may be fast growing or slow growing
- Screening is more likely to find slow growing conditions (fast growi=
ng
will have already caused symptoms at the time of screening)
- Therefore, screening is more likely to detect diseases with better
prognosis (but the fact that screened persons have better prognosis =
is
not related to screening itself)
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16
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17
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- It depends on the propensity of patients to follow medical advice
- If we compare disease outcome between a group of people who voluntee=
red
for a screening program with outcomes in a group of people who did n=
ot
volunteer, better results in the volunteers might be due to other
differences and not to treatment after screening
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18
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- Is screening justified by the severity of the medical condition in t=
erms
of mortality, morbidity and suffering caused by the disease?
- Only conditions posing threats to life and health should be sought.<=
/li>
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19
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- Sensitivity and specificity (how the test performs)
- Simplicity and low cost
- Safety
- Acceptable to both patients and clinicians
- The labelling effect
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20
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- Describe how the test performs
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21
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- The test gives the correct answer when it is positive in the presenc=
e of
a disease or negative in the absence of the disease
- The test is misleading if it is positive when the disease is absent
(false positive) or negative when the disease is present (false
negative)
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22
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- The assessment of the test’s accuracy rests on its relationship to s=
ome
way of knowing whether the disease is truly present or not (the gold
standard)
- Why we want another test if we have already the gold standard?
- The gold standard is often elaborate, expensive, risky (biopsies) or=
is
available too late (autopsies)
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23
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- We want to know how good is a urine dipstick to detect asymptomatic
bacteriuria
- the gold standard is urine culture
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24
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- of 37 persons with bacteriuria 27 tested positive with the dipstick
(true positives)
- of 112 persons with no bacteriuria, 77 tested negative with the dips=
tick
(true negative)
- of 112 persons with no bacteriuria, 35 tested positive with the dips=
tick
(false positives)
- of 37 persons with bacteriuria 10 tested negative with the dipstick
(false negatives)
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25
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- Appearances to the mind are of four kinds.
- Things either are what they appear to be;
- or they neither are not appear to be;
- or they are and do not appear to be;
- or they are not, yet appear to be.
- Rightly to aim in all these cases
- is the wise man’s task
- Epictetus 2nd century AD
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26
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27
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- We are interested in knowing:
- The proportion of individuals with the disease who have a positive t=
est
for the disease (Sensitivity)
- The proportion of people without the disease who have a negative tes=
t (Specificity)
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28
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29
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- Sensitivity: individuals with positive result/individuals with
bacteriuria 27/37=3D73%
- Specificity: individuals with negative result/individuals with no
bacteriuria 77/112=3D69%
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30
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- A sensitive test will rarely miss people with the disease (chose a
sensitive test when there is an important penalty for missing the
disease - rule out the disease-, as in dangerous but treatable
conditions)
- A specific test test will rarely misclassify people without the dise=
ase
as diseased (use when you want to rule in a disease, e.g. when confirming a diagnosis befo=
re
starting a treatment)
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31
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- A test could be misleading (to some extent)
- Patients with the disease may test negative (false negative)
- Patients without the disease may test positive (false positive)
- Sensitivity: how good is the test in detecting as positive patients =
with
the disease
- Specificity: how good is the test in detecting as negative patients =
with
no disease
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32
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- Ideally the test should be:
- highly sensitive (not to mi=
ss the
usually few cases of disease that are present)
- highly specific (to reduce the number of people with false positive
results who require further investigations)
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33
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- The test should:
- take minimum time to perform
- require minimum preparation
- depend on no special appointment
- be inexpensive (think about further evaluation)
- Example: blood pressure determination
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34
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- Safety (a test considered safe in clinical practice when dealing with
symptomatic patients, could be seen as dangerous when used for scree=
ning
purposes in the general population)
- Acceptable to
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35
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- We do not know much about the psychological impact of test results=
li>
- The effect could be either
- positive (positive attitudes are reinforced)
- negative (the patient assumes the sick role)
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36
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- Disease can be prevented by primary or secondary prevention
- Screening makes sense if:
- early treatment is more effective than treatment at the usual time<=
/li>
- the disease causes a substantial burden of suffering
- a good screening test is available ( sensitive and specific enough)=
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