Transcript Study Strategies for NAVLE: (When time gets short….)

Epidemiology made easy
-or“Lies, damn lies, and statistics”
Steven I McLaughlin DVM, MPH, ACVPM
[email protected]
www.zukureview.com
“Epidemiology is just common
sense,
…made complicated”
Dr. Jerry Gibson, State epidemiologist,
South Carolina Dept. of Health and Environmental Control
Lies, damn lies, and statistics
• You WILL see a couple questions on:
– Sens, Spec
– Predictive Val Pos, Predictive Val Neg
• The key is to draw a 2x2 table
• And remember your “ABCs”
The only table that matters: The 2 x 2
“Truth” or “Gold Std” test
Your test
Pos
Neg
Pos
Neg
a
b
c
d
Remember: You are comparing TWO TESTS when
you calculate Sens, Spec, PVP, PVN
You are comparing your test to a gold standard
For reference:
• Sensitivity- a/a+c
% true pos, correctly identified by my test
a/a+c
• Specificity- d/b+d
% true neg, correctly identified by my test
• Predictive Value Pos - a/a+b
Of those my test says are pos, how many are truly pos ?
a/a+b
• Predictive Value Neg - d/c+d
Of those my test says are neg, how many are truly neg?
You serologically test 140 wallabies for a disease.
35 wallabies test seropositive and 105 test seronegative.
However, postmortem data reveals 5/35 of the seropositive wallabies
are disease free and 4/105 of the seronegative wallabies are
diseased.
What is the predictive value positive (PVP) of this serologic test?
A. 39/101
B. 35/39
C. 105/140
D. 30/35
E. 101/105
PVP/PVN
PVP=a/(a+b)=30/35
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
PVP = 86%
30 a
4
a+c
5
35
b a+b
105
c 101 d c+d
b+d
140
a+b+c+d
You serologically test 100 Siberian box turtles for galloping halitosis.
27 turtles test seropositive and 73 test seronegative.
However, molecular testing reveals 3/27 of the seropositive turtles
are disease free and 10/73 of the seronegative turtles are diseased.
What is the sensitivity of your serologic test?
Sens=a/(a+c)=24/34
“Gold Std” test
Pos
Neg
Sens/Spec
A. 66/73
B. 63/66
C. 27/100
D. 10/63
E. 24/34
Pos
Your test
Neg
Sens = 70%
27
24 a
3
b a+b
10 c
63
d c+d
a+c
34
b+d
66
73
100
a+b+c+d
You serologically test 140 wallabies for a disease.
35 wallabies test seropositive and 105 test seronegative.
However, postmortem data reveals 5/35 of the seropositive wallabies
are disease free and 4/105 of the seronegative wallabies are
diseased.
What is the specificity of this serologic test?
Spec=d/(b+d)=101/106
“Gold Std” test
Pos
Neg
Sens/Spec
A. 101/106
B. 105/140
C. 30/34
D. 30/35
E. 106/140
Pos
Your test
Neg
Spec = 95%
30 a
4
a+c
5
35
b a+b
105
c 101 d c+d
b+d
140
a+b+c+d
You serologically test 100 Siberian box turtles for galloping halitosis.
27 turtles test seropositive and 73 test seronegative.
However, molecular testing reveals 3/27 of the seropositive turtles
are disease free and 10/73 of the seronegative turtles are diseased.
What is the predictive value negative (PVN) of your serologic test?
A. 91%
B. 89%
C. 86%
D. 73%
E. 70 %
PVP/PVN
PVN=d/(c+d)=63/73
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
PVN = 86%
27
24 a
3
b a+b
10 c
63
d c+d
a+c
34
b+d
66
73
100
a+b+c+d
You are using a SLAP® heartworm antigen test with a reported sensitivity 96%
and specificity of 98%.
Assuming the prevalence of heartworm in your area is 10%, what is the
predictive value positive (PVP) of your test ?
A. 886/900
B. 96/114
C. 882/886
D. 100/114
E. 882/1000
PVP/PVN
PVP=a/(a+b)=96/114
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
96 a
4 c
a+c
100
18
PVP = 84%
114
b a+b
886
882 d c+d
b+d
900
1000
a+b+c+d
Correct: PVP is 92%. The trick with this kind of question is to pick an
imaginary number of animals that you test, like 1000, and fill out your 2x2
table from there. Follow the links to see diagrams step by step.
If prev is 10% then there must be 100/1000 dogs w/ heartworm and 900
dogs that are disease-free.
A 96% sensitive test will correctly call 96/100 positive (box "a"), and INcorrectly call 4/100 negative, (box "c": these are the false negs).
If 100/1000 animals are infected, then 900/1000 are Dz-free. Your 98%
specific test will correctly call 882/900 Dz-free
(box "d": 0.98 X 900=882) and IN-correctly call 18/900 positive, (box "b":
these are the false pos).
Now your a,b,c,d boxes are all filled, it is easy to calculate PVP
=a/(a+b)=96/(96+18)=84%
Ref: Hennekens and Buring, Epidemiology in Medicine 5th ed., pp. 332-39.
You are using a SLAP® heartworm antigen test with a reported sensitivity 96%
and specificity of 98%.
Assuming the prevalence of heartworm in your area is 10%, what is the
predictive value negative (PVN) of your test ?
A. 80%
B. 85%
C. 88%
D. 92%
E. 99%
PVN=d/(c+d)=882/886
PVP/PVN
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
PVN = 99%
96 a
18 b 114
a+b
4 c
886
882 d c+d
a+c
100
b+d
900
1000
a+b+c+d
Correct: PVN is 99%. The trick with this kind of question is to pick an
imaginary number of animals that you test, like 1000, and fill out your 2x2
table from there. Follow the links to see diagrams step by step.
If prev is 10% then there must be 100/1000 dogs w/ heartworm and 900
dogs that are disease-free.
A 96% sensitive test will correctly call 96/100 positive (box "a"), and INcorrectly call 4/100 negative, (box "c": these are the false negs).
If 100/1000 animals are infected, then 900/1000 are Dz-free. Your 98%
specific test will correctly call 882/900 Dz-free
(box "d": 0.98 X 900=882) and IN-correctly call 18/900 positive, (box "b":
these are the false pos).
Now your a,b,c,d boxes are all filled, it is easy to calculate PVN
=d/(c+d)=882/(882+4)=99%
Ref: Hennekens and Buring, Epidemiology in Medicine 5th ed., pp. 332-39.
Congratulations!
You can now handle the most common epi questions you are
likely to see
All you need is a 2x2 table
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
And these 4 equations
Sensitivity - a/a+c
Specificity- d/b+d
Predictive Val Pos (PVP) - a/a+b
Predictive Val Neg (PVN) - d/c+d
a+c
a
b a+b
c
d c+d
b+d
a+b+c+d
Three extra concepts
(In case your brain hasn’t exploded yet.)
If your head is about to pop,
just try to remember the next slide.
Then go home and pet your dog
You suspect hyperadrenocorticism in a 9-year old female spayed dog with a twomonth history of increased appetite, thirst and urinary accidents.
Which diagnostic test do you trust the most if it has a positive result?
Urine Cortisol Creatinine Ratio (UCCR). Sensitivity=90% , Specificity =25%
ACTH Stimulation. Sensitivity=80% , Specificity =85%
Low Dose Dexamethasone Suppression. Sensitivity=95% , Specificity =50%
A - Cannot say without knowing the predictive value negative
B - Low Dose Dexamethasone Suppression (LDDS)
C - Urine Cortisol Creatinine Ratio
D - ACTH Stimulation
E - Cannot say without knowing the predictive value positive
Remember your Ps and Ns.
ACTH Stim is most sPecific (fewer false Pos, so trust a POS test more).
LDDS is more seNsitive (fewer false Negs, so trust NEG test more).
Why high SPECIFICITY means you can trust a POSITIVE test
result: Because false pos are LOW
“Truth” or “Gold Std” test
Pos
Pos
True
Pos
Your test
Neg
0.10 prev X 1000=
100 infected
False
Pos (1)
a
False
Negs
b
True
Negs (899)
c
a+c
Where did that 900
come from ?
Neg
d
a+b
c+d
b+d
900
SENS
Sens = a
a+c
Spec = d
b+d
1000-100= 900 Dz-free
Total
a+b+c+d
If Spec =99.9%; Prev = 10%
and test 1000 animals
Spec =d/(b+d)=899/(900)
Only 1 False Pos
A positive result by your test
is likely correct
Why high SENSITIVITY means you can trust a
NEGATIVE test result : Because false negs are LOW
“Truth” or “Gold Std” test
Pos
Pos
Your test
Neg
True
Pos (99)
c
a+c
0.10 prev X 1000=
100 infected
False
Pos
a
False
Negs (1)
Where did that 100
come from ?
Neg
b
a+b
d
c+d
True
Negs
b+d
100
SENS
Sens = a
a+c
Spec = d
b+d
1000-100= 900 Dz-free
Total
a+b+c+d
If Sens =99%; Prev = 10%
and test 1000 animals
Sens =a/(a+c)=99/(100)
Only 1 False Neg
A negative result by your test
is likely correct
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
270 a
30 c
As Prevalence decreases, PVP decreases too
If Prev=30%,
Sens and Spec=90%, 1000 turtles
70 b 340
a+b
PVP=a/(a+b)=270/(270+70)
660
630 d c+d
PVP = 79%
a+c
300
b+d
700
1000
a+b+c+d
“Gold Std” test
Pos
Neg
Pos
If Prev DECREASES to 1%,
Sens and Spec=90%, 1000 turtles
PVP=a/(a+b)=9/(9+99)
PVP decreases to ~1%
Your test
Neg
~9 a
~99 b 108
a+b
~1 c
892
~891d c+d
a+c
10
b+d
990
1000
a+b+c+d
As prevalence of a disease goes DOWN, PVP of your serologic test also
goes DOWN. That is, as your disease becomes more and more rare, the
predictive value of your same old test gets WORSE. That’s all you need to
know. Read more only if you want to see the math, (but you don’t need it.)
>>>>>>>>
Lets say prevalence of pedunculated giblet disease is 30% in 1000 turtles
(that’s 300 infected, 700 dz-free, then).
A test w/ 90% sensitivity would correctly Dx 270 (cell "a") with the dz (true
pos) and INcorrectly say 30 were negative (cell "c"); a test w/ 90%
specificity would correctly Dx 630 (cell "d") as dz-free (true negs) and
INcorrectly say 70 were positive (cell "b"): PVP=a/(a+b)=270/(270+70)=
79%.
Now, do the same math on 1000 turtles, but assume prevalence is now
only 1% (0.01) after your vaccination program. You will see PVP goes
down to ~1%.
Ref: Hennekens and Buring, Epidemiology in Medicine 5th ed., pp. 332-39.
You’re on your
way!
“The art of
epidemiological
thinking is to draw
conclusions from
imperfect data”
George W.
Comstock
Extra practice questions,
if you want them
You serologically test 140 wallabies for a disease.
35 wallabies test seropositive and 105 test seronegative.
However, postmortem data reveals 5/35 of the seropositive wallabies
are disease free and 4/105 of the seronegative wallabies are
diseased.
What is the sensitivity of this serologic test?
Sens=a/(a+c)=30/34
“Gold Std” test
Pos
Neg
Sens/Spec
A. 95%
B. 96%
C. 88%
D. 86%
E. 77 %
Pos
Your test
Neg
Sens = 88%
30 a
4
a+c
5
35
b a+b
105
c 101 d c+d
b+d
140
a+b+c+d
You serologically test 100 Siberian box turtles for galloping halitosis.
27 turtles test seropositive and 73 test seronegative.
However, molecular testing reveals 3/27 of the seropositive turtles
are disease free and 10/73 of the seronegative turtles are diseased.
What is the specificity of your serologic test?
Spec=d/(b+d)=63/66
“Gold Std” test
Pos
Neg
Sens/Spec
A. 95%
B. 91%
C. 86%
D. 73%
E. 70 %
Pos
Your test
Neg
Spec = 95%
27
24 a
3
b a+b
10 c
63
d c+d
a+c
34
b+d
66
73
100
a+b+c+d
You serologically test 100 Siberian box turtles for galloping halitosis.
27 turtles test seropositive and 73 test seronegative.
However, molecular testing reveals 3/27 of the seropositive turtles
are disease free and 10/73 of the seronegative turtles are diseased.
What is the predictive value positive (PVP) of your serologic test?
PVP=a/(a+b)=24/27
“Gold Std” test
Pos
Neg
Sens/Spec
A. 91%
B. 89%
C. 86%
D. 73%
E. 70 %
Pos
Your test
Neg
PVP = 89%
27
24 a
3
b a+b
10 c
63
d c+d
a+c
34
b+d
66
73
100
a+b+c+d
You serologically test 140 wallabies for a disease.
35 wallabies test seropositive and 105 test seronegative.
However, postmortem data reveals 5/35 of the seropositive wallabies
are disease free and 4/105 of the seronegative wallabies are
diseased.
What is the predictive value negative (PVN) of this serologic test?
A. 95%
B. 96%
C. 88%
D. 86%
E. 77 %
PVP/PVN
PVN=d/(c+d)=101/105
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
PVN = 96%
30 a
4
a+c
5
35
b a+b
105
c 101 d c+d
b+d
140
a+b+c+d
You are using an FeLV test with a sensitivity of 90% and a specificity of
95%.
Assuming the prevalence of feline leukemia in your area is 5%, what is
the predictive value positive (PVP) of your test ?
A. 45%
B. 48%
C. 55%
D. 88%
E. 90%
PVP/PVN
PVP=a/(a+b)=45/93
“Gold Std” test
Pos
Neg
Pos
Your test
Neg
PVP = 48%
45 a
48 b 93
a+b
5 c
907
902 d c+d
a+c
50
b+d
950
1000
a+b+c+d