Evidence based medicine

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Evidence based medicine

• Diagnostic tests

Ross Lawrenson
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Diagnostic tests

• When looking at a paper about a diagnostic test
  we ask ourselves three questions.

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Diagnostic tests

• Is this test useful?

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Diagnostic tests

• Is this test useful?
• Is it reliable?

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Diagnostic tests

• Is this test useful?
• Is it reliable?
• Is it valid?

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Is this test useful?

• The test should have been researched in a study
  population relevant to the individual or
  population in whom it is to be used.

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Reliability

• Reliability refers to the repeatability or
  reproducibility of a test.
• It can be assessed by repeating the test using
  the same or different observers.

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Validity

• Relates to whether the test measures what it
  purports to measure. Is the result true?

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Validity

• For example if you measure blood pressure in an
  obese patient and use a cuff that is too small
  you are likely to get a falsely high reading. The
  reading maybe reliable (you get the same blood
  pressure if you do it again) but it lacks
  validity.

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Sensitivity and specificity
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Sensitivity and specificity
Disease
Healthy
Total
Test
a
b
ab
Test -
c
d
cd
Total
ac
bd
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Sensitivity and specificity
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Sensitivity

• The probability that the test will be positive if
  the disease is present
• a/ac

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Sensitivity

• The probability that the test will be positive if
  the disease is present
• a/ac
• A sensitive test is likely to also record a
  number of false positive tests

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Sensitivity
If the cut off point of this test is set low
then it will be sensitive (all patients with
disease will test positive) but there will also
be a number of false positives
Diseased
Healthy
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Specificity

• Theprobability that the test will be negative if
  the disease is truly absent.
• d/bd

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Specificity

• Theprobability that the test will be negative if
  the disease is truly absent.
• d/bd
• In this situation there is a high likelihood of
  false negatives.

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Accuracy of the test
(ad)/(abcd)
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Example
5000 women underwent a test for
blood glucose at 24 weeks following
a glucose load. 243 women were
found to have a blood glucose
greater than 6.8 mmol/L and were
referred for an OGTT. 186 were
found to have gestational diabetes.
Four women who initially had tested
negative were diagnosed as having
diabetes later in their pregnancy.
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Example
Prevalence
Sensitivity
Specificity
Positive predictive value
Negative predictive value
Likelihood ratio test
Likelihood ratio - test
Accuracy
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Diabetes
No diabetes
Total
Positive
186
57
243
Negative
4
4753
4757
Total
190
4810
5000
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Example
Prevalence
190/5000
Sensitivity
186/190
Specificity
4753/4810
Positive predictive value
186/243
Negative predictive value
4753/4757
Likelihood ratio test
(186/190)/(57/4810)
Likelihood ratio - test
(4/190)/(4753/4810)
Accuracy
1864753/5000
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Gold standard
.
.
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Gold standard

• The gold standard is the test or battery of tests
  that will most accurately diagnose a particular
  disease or condition.
• Thus traditionally the OGTT has been seen as the
  gold standard when testing for diabetes. Other
  diagnostic tests may have a gold standard that is
  too expensive or invasive for routine use e.g.
  fluoroscein angiography for diabetic retinopathy.
  
• Sometimes the gold standard is a battery of tests
  or symptoms e.g. the Jones criteria for rheumatic
  fever

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Percent agreement
Abnormal
Suspect
Normal
Abnormal
A
B
C
Suspect
D
E
F
Normal
G
H
I
Percent agreement (AEI) / Total
X100
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Percent agreement
Melanoma
Indeterminate
Benign
Melanoma
10
1
10
Indeterminate
0
0
0
Benign
0
0
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Percent agreement (10016)/37 X100 70
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KAPPA
Second
Exam
Normal
Retinopathy
Total
First
Normal
46
10
56
Exam
Retinopathy
12
32
44
Total
58
42
100
Observed agreement 46 32/100
78
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KAPPA
Second
Exam
Normal
Retinopathy
Total
First
Normal
58x56
42x56
56
Exam
Retinopathy
58x44
42x44
44
Total
58
42
100
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KAPPA
Second
Exam
Normal
Retinopathy
Total
First
Normal
32.5
23.5
Exam
Retinopathy
25.5
18.5
Total
Agreement expected by chance
32.518.5/100
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