evidence-based diagnosis
TRANSCRIPT
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Evidence Based Diagnosis
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When a Patient Has a ProblemWhen a Patient Has a Problem
The doctor reaches a diagnosis by:The doctor reaches a diagnosis by:
• Clinical dataClinical data
• Diagnostic toolsDiagnostic tools
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Increasing use of Diagnostic Increasing use of Diagnostic tests:tests:
- Availability.- The urge to make use of new technology.
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The evaluation of diagnostic techniques is less advanced than that
of treatments (NO phase I, II, III, IV).
New Diagnostic testsNew Diagnostic tests
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RelevanceRelevance
• First, the test should be one that is feasible for you in your community
• Example: brain biopsy is an accurate test for diagnosing dementia, it’s not practical for my (living) patients!
• Can I apply the test to my patients? (Availability, Cost) e.g MRI
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ValidityValidity
The degree to which the results of a study are likely to be true and free from bias.
• It should be compared to a gold reference standard
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CautionCaution
• reference standard used should be acceptable (e.g HSG vs DL)
• Both reference standard and test should be applied to all patients
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IndependentIndependent
• the decision to perform the reference standard should ideally be independent of the results of the test being studied.
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Ask yourselfAsk yourself
• the patient sample should include an appropriate spectrum of patients to whom the diagnostic test will be applied in clinical practice
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Rule of ThumbRule of Thumb
• at least 100 participants to ensure an appropriate "spectrum" of disease
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22 x 2 tablex 2 table comparing the results of a diagnostic test with a reference standard
reference standard
diseaseno disease
test abnormaltrue pos. [a]false pos. [b]
test normalfalse neg. [c]true neg. [d]
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sensitivitysensitivity
• probability of a positive test among patients with disease
• i.e Ability to diagnose
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specificityspecificity
• probability of a negative test among patients without disease
• i.e Ability to exclude
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22 X 2 TableX 2 Table
a(true positive)
b(false positive)
c(false negative)
d(true negative)
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Keep in MindKeep in Mind
• sensitivity and specificity by themselves are only useful when either is very high (over typically, 95% or higher).
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Who wants whatWho wants what? ?
main interest
Methodologistsensitivity
specificity
Doctoraccuracy
PatientProbability
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Likelihood RatioLikelihood Ratio
The "positive likelihood ratio" (LR+) tells us how much to increase the probability of disease if the test is positive
The "negative likelihood ratio" (LR-) tells us how much to decrease it if the test is negative
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Likelihood RatioLikelihood RatioLR=+
probability of a +ve test in those who have the disease___probability of a +ve test in those who do not have the disease
= sensitivity 1-specificity
LR=- probability of a -ve test in those who have the disease___
probability of a -ve test in those who do not have the disease
= 1-sensitivity specificity
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LRInterpretationInterpretation >10
Large and often conclusive increase in the likelihood of disease
5 - 10Moderate increase in the likelihood of disease
2 - 5Small increase in the likelihood of disease
1 - 2Minimal increase in the likelihood of disease
1No change in the likelihood of disease
0.5 - 1.0
Minimal decrease in the likelihood of disease
0.2 - 0.5
Small decrease in the likelihood of disease
0.1 - 0.2
Moderate decrease in the likelihood of disease
<0.1Large and often conclusive decrease in the likelihood of
disease
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Why LR
• The LR+ corresponds to the clinical concept of "ruling-in disease"
• The LR- corresponds to the clinical concept of "ruling-out disease“
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Patient oriented!!!!!!!
• Your 45 year old patient has a mammogram. The study is interpreted as "suspicious for malignancy" by your radiologist.
• Your patient asks you:"Does this mean I have cancer?", and you (correctly) answer "No, we have todo further testing."
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• Your patient then asks, "OK, I understand that the mammogram isn't the final answer, but given what we know now, what are the chances that I have breast cancer?".
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Is it Easy!!!
• Assume that the overall risk of breast cancer in any 45 year old woman,
regardless of mammogram result, is1%. Assume also that mammography is 90% sensitive and 95% specific. Then,
select your answer below:
1% 15% 60% 85% 95%
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If you know that the risk of breast cancer in any 45 year old woman is 1% and that mammography is 90% sensitive and 95% specific.
What do you think your patient’s probability of having breast cancer is?
LR+=Sens/100-Spec
=90/5=18
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Diseaseruled IN
Disease ruled OUT
Disease not
ruled in or out
Above this point,treat
Below this point,no further testing
Determined by:Complications of untreated disease
Risks of therapyComplications of tests
Cost
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ROC curve is simply a graph of sensitivity vs (1-specificity)
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ScoreSystematic Collaboration of Ovarian Reserve Evaluation
systematic reviews of Diagnostic tests
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THANK YOU