https://vimeo.com/152035436 How do you decide if you are going to treat a patient with strep throat, or send a test to rule in or rule out the disease or just send him home on NSAIDs? What if instead of strep throat you are suspecting a pulmonary embolism, what do yo do? One of the key things before thinking in ordering a specific test, is assigning that particular disease PRETEST probability, and then knowing what your therapeutic and diagnostic threshold are for that specific disease. It is the only way of knowing if the test you ordered is good enough to rule in or rule out that disease!

1. Dr. Nicolás Pineda
@nfpineda
Associate Director MUE – USS
@MUE_14
Diagnostic Tests
& LR

2. The bottom
line
There is no 100%, there
is no 0%... Get used to
it!!!
We need to establish a
PRETEST probability of
disease
LR’s: consider
sensitivity, specificity &
prevalence of disease
Utility of diagnostic test
is not only determined
by LR number

4. Patients do not have disease, only
probability of disease
Diagnostic tests are merely revision of
probabilities
Test interpretation should precede test
ordering
If you are not going to change
managemente with the result of a test, you
shouldn’t order it

6. 0%
100%
20%
40%
60%
80%
Diagnostic threshold
Therapeutic threshold

7. 0%
100%
20%
40%
60%
80%
FORGET FORGET
T
E
S
T
T
E
S
T
T
R
E
A
T
T
R
E
A
T

8. There is no 0%
There is no 100%

9. 0%
100%
20%
40%
60%
80%
0%
100%
20%
40%
60%
80%

10. Depends on disease
Institutional variability
Personal variability

11. 0%
100%
20%
40%
60%
80%
FORGET FORGET
T
E
S
T
T
E
S
T
T
R
E
A
T
T
R
E
A
T

12. Need to think… system 2
Depends on Gestalt
Depends on patient

13. Personal
variability
Institutional
variability

14. 0%
100%
20%
40%
60%
80%
OLVIDO OLVIDO
B
U
S
C
O
B
U
S
C
O
T
R
E
A
T
T
R
E
A
T

15. Como
seguimos?
Test

16. 0%
100%
20%
40%
60%
80%
FORGET FORGET
T
E
S
T
T
E
S
T
T
R
A
T
O
T
R
A
T
O

17. History
Physical
Labs
Radiology

18. Characteristics
Sensitivity
Capability of detecting the disease among
patients with the disease
TP
TP+ FN

19. Characteristics
Specificity
Capability of detecting the absence of
disease
TN
TN+ FP

20. 2318 407
459 4952
The truth
Test
Result
+
-
Sick Healthy
Lean WL et al. Rapid diagnostic tests for group A streptococcal pharyngitis:
a meta-analysis Pediatrics, 2014 Oct;134(4):771-81
S:
2318/(459+23
18)
83%
Sp:
4952/(407+49
52)
92%

21. Characteristics
Predictive values (+ & -)
Probability that a positive test is due to presence
of disease (TP)
Probability that a negative test is due to the
absence of disease (TN)

22. The truth
2318 407
459 4952
Test
Result
+
-
Sick Healthy
Lean WL et al. Rapid diagnostic tests for group A streptococcal pharyngitis:
a meta-analysis Pediatrics, 2014 Oct;134(4):771-81
S:
2318/(459+23
18)
83%
Sp:
4952/(407+49
52)
92%
PPV
2318/(2318+4
07)
NPV
4952/(459+49
52)
85%
92%

23. Prevalence
Low prevalence
- test (+): TP or FP?

24. How do we take in
consideration sensitivity,
specificity & prevalence
of disease?

25. L.R.

26. L.R.
Utility of a diagnostic test
How likely is that the patient has
the disease
Ratio of probabilities

27. L.R. +
Probability that patient test (+) because he has
the disease DIVIDED by the probability that a
patient without the disease tests (+)
LR+ = sensitivity/ 1 - specificity
Prob True(+) / prob False(+)

28. L.R. +
The bigger the number,
better LR+
LR+ = sensitivity/ 1 - specificity
Prob True(+) / prob False(+)

29. L.R. -
Probability that patient with the disease tests
(-) DIVIDED by the probability that a patient
without the disease tests (-)
LR- = 1 - sensitivity/ specificity
Prob False (-) / Prob True(-)

30. L.R. -
The closer to 0,
better LR-
LR- = 1 - sensitivity/ specificity
Prob False (-) / Prob True(-)

32. The truth
2318 407
459 4952
Test
Result
+
-
Sick Healthy
Lean WL et al. Rapid diagnostic tests for group A streptococcal pharyngitis:
a meta-analysis Pediatrics, 2014 Oct;134(4):771-81
S:
2318/(459+23
18)
83%
Sp:
4952/(407+49
52)
92%
LR+: 10.3
LR-: 0.2

34. How do
we use a
LR?
LR+: 10.3
LR-: 0.2

35. 0%
100%
20%
40%
60%
80%
0%
100%
20%
40%
60%
80%

36. Pre-Test
Probability

37. The bottom
line
There is no 100%, there
is no 0%... Get used to
it!!!
We need to establish a
PRETEST probability of
disease
LR’s: consider
sensitivity, specificity &
prevalence of disease
Utility of diagnostic test
is not only determined
by LR number

38. www.facebook.com/mue.cl
www.muebe.org
@nfpineda
@MUE_14