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

1. Evidence Based Diagnosis

2. When a Patient Has a Problem
The doctor reaches a diagnosis by:
• Clinical data
• Diagnostic tools

3. Increasing use of Diagnostic
tests:
- Availability.
- The urge to make use of new
technology.

4. The evaluation of diagnostic
techniques is less advanced than that
of treatments (NO phase I, II, III, IV).
New Diagnostic tests

5. 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

6. 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

7. Caution
• reference standard used should be
acceptable (e.g HSG vs DL)
• Both reference standard and test
should be applied to all patients

8. Independent
• the decision to perform the reference
standard should ideally be independent of
the results of the test being studied.

9. Ask yourself
• the patient sample should include an
appropriate spectrum of patients to whom
the diagnostic test will be applied in clinical
practice

10. Rule of Thumb
• at least 100 participants to ensure an
appropriate "spectrum" of disease

11. 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]

12. sensitivity
• probability of a positive test among
patients with disease
• i.e Ability to diagnose

13. specificity
• probability of a negative test among
patients without disease
• i.e Ability to exclude

14. 2 X 2 Table
b
(false positive)
a
(true positive)
d
(true negative)
c
(false negative)

16. Keep in Mind
• sensitivity and specificity by themselves
are only useful when either is very high
(over typically, 95% or higher).

17. 1000 individual
10% disease prevalenceS
E
N
S
I
T
I
V
I
T
Y
S
P
E
C
I
F
I
C
I
T
Y
+VE PREDICTIVE VALUE
-VE PREDICTIVE VALUE
= a/a+c
90/100 =
90%
= d/b+d
720/900
=80%
= a/a+b
90/720=
33%
= d/c+d
720/730
= 99.6%
+ve
-ve
disease No disease
90
10
180
720
100 900
270
730
a b
c d

18. Who wants what ?
main interest
Methodologist
sensitivity
specificity
Doctor accuracy
Patient Probability

19. 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

20. Likelihood Ratio
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
sensitivity=
1-specificity
LR-=
ve test in those who have the disease___-probability of a
probability of a -ve test in those who do not have the disease
sensitivity-1=
specificity

21. InterpretationLR
Large and often conclusive increase in the likelihood of
disease
> 10
Moderate increase in the likelihood of disease5 - 10
Small increase in the likelihood of disease2 - 5
Minimal increase in the likelihood of disease1 - 2
No change in the likelihood of disease1
Minimal decrease in the likelihood of disease0.5 - 1.0
Small decrease in the likelihood of disease0.2 - 0.5
Moderate decrease in the likelihood of disease0.1 - 0.2
Large and often conclusive decrease in the likelihood of
disease
< 0.1

23. Why LR
• The LR+ corresponds to the clinical concept of
"ruling-in disease"
• The LR- corresponds to the clinical concept of
"ruling-out disease“

24. 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 to
do further testing."

25. • 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?".

26. 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%

27. 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

28. Disease
ruled 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 therapy
Complications of tests
Cost

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

30. Score
Systematic Collaboration of
Ovarian Reserve Evaluation
systematic reviews of Diagnostic
tests

31. THANK YOU