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Evidence Based Diagnosis
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Evidence based diagnosis

Aug 22, 2014

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Health & Medicine

Hesham Al-Inany

Evidence based medicine is now focusing on diagnostic tests: how accurate and useful could be ? sensitivity and specificity are no longer the important criteria for a test
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Page 1: Evidence based diagnosis

Evidence Based Diagnosis

Page 2: Evidence based diagnosis

When a Patient Has a Problem

The doctor reaches a diagnosis by:• Clinical data• Diagnostic tools

Page 3: Evidence based diagnosis

Increasing use of Diagnostic tests:

- Availability.- The urge to make use of new technology.

Page 4: Evidence based diagnosis

The evaluation of diagnostic techniques is less advanced than that

of treatments (NO phase I, II, III, IV).

New Diagnostic tests

Page 5: Evidence based diagnosis

Relevance• 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

Page 6: Evidence based diagnosis

Validity

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

Page 7: Evidence based diagnosis

Caution• reference standard used should be

acceptable (e.g HSG vs DL)• Both reference standard and test

should be applied to all patients

Page 8: Evidence based diagnosis

Independent• the decision to perform the reference

standard should ideally be independent of the results of the test being studied.

Page 9: Evidence based diagnosis

Ask yourself• the patient sample should include an

appropriate spectrum of patients to whom the diagnostic test will be applied in clinical practice

Page 10: Evidence based diagnosis

Rule of Thumb

• at least 100 participants to ensure an appropriate "spectrum" of disease

Page 11: Evidence based diagnosis

2 x 2 table comparing the results of a diagnostic test with a reference standard

reference standard

disease no disease

test abnormal true pos. [a] false pos. [b]

test normal false neg. [c] true neg. [d]

Page 12: Evidence based diagnosis

sensitivity• probability of a positive test among

patients with disease

• i.e Ability to diagnose

Page 13: Evidence based diagnosis

specificity• probability of a negative test among

patients without disease

• i.e Ability to exclude

Page 14: Evidence based diagnosis

2 X 2 Table

b(false positive)

a(true positive)

d(true negative)

c(false negative)

Page 15: Evidence based diagnosis
Page 16: Evidence based diagnosis

Keep in Mind• sensitivity and specificity by themselves

are only useful when either is very high (over typically, 95% or higher).

Page 17: Evidence based diagnosis

1000 individual10% disease prevalenceS

E

NS

IT

IV

IT

Y

SPECIFICITY

+VE PREDICTIVE VALUE

-VE PREDICTIVE VALUE

= a/a+c90/100 = 90%

= d/b+d720/900 =80%

= a/a+b90/720= 33%

= d/c+d720/730= 99.6%

+ve

-ve

disease No disease

90

10

180

720

100 900

270

730

a bc d

Page 18: Evidence based diagnosis

Who wants what?

main interest

Methodologistsensitivityspecificity

Doctor accuracy

Patient Probability

Page 19: Evidence based diagnosis

Likelihood 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

Page 20: Evidence based diagnosis

Likelihood 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

Page 21: Evidence based diagnosis

Interpretation LR

Large and often conclusive increase in the likelihood of disease >10

Moderate increase in the likelihood of disease 5 - 10

Small increase in the likelihood of disease 2 - 5

Minimal increase in the likelihood of disease 1 - 2

No change in the likelihood of disease 1

Minimal decrease in the likelihood of disease 0.5 - 1.0

Small decrease in the likelihood of disease 0.2 - 0.5

Moderate decrease in the likelihood of disease 0.1 - 0.2

Large and often conclusive decrease in the likelihood of disease <0.1

Page 22: Evidence based diagnosis
Page 23: Evidence based diagnosis

Why LR• The LR+ corresponds to the clinical concept of

"ruling-in disease" • The LR- corresponds to the clinical concept of

"ruling-out disease“

Page 24: Evidence based diagnosis

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."  

Page 25: Evidence based diagnosis

• 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?".

Page 26: Evidence based diagnosis

Is it Easy!!!• Assume that the overall risk of breast

cancer in any 45 year old woman, regardless of mammogram result, is

1%. Assume also that mammography is 90% sensitive and 95% specific. Then,

select your answer below:

1%    15%      60%      85%    95%

Page 27: Evidence based diagnosis

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

Page 28: Evidence based diagnosis

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

Page 29: Evidence based diagnosis

ROC curve is simply a graph of sensitivity vs (1-specificity)

Page 30: Evidence based diagnosis

ScoreSystematic Collaboration of Ovarian Reserve Evaluation

systematic reviews of Diagnostic tests

Page 31: Evidence based diagnosis

THANK YOU