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Epidemiology made easy-or-Lies, damn lies, and

statistics

- Steven I McLaughlin DVM, MPH, ACVPMsteve_at_zukurev

iew.com 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

Remember You are comparing TWO TESTS when you

calculate Sens, Spec, PVP, PVNYou are comparing

your test to a gold standard

For reference

- Sensitivity- a/ac
- true pos, correctly identified by my test
- Specificity- d/bd
- true neg, correctly identified by my test

- Predictive Value Pos - a/ab
- Of those my test says are pos, how many are

truly pos ? - Predictive Value Neg - d/cd
- Of those my test says are neg, how many are

truly neg?

a/ac

a/ab

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

PVPa/(ab)30/35PVP 86

35

5

30

105

4

101

140

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? A.

66/73 B. 63/66 C. 27/100 D. 10/63 E. 24/34

Sensa/(ac)24/34Sens 70

27

3

24

73

10

63

100

66

34

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? A. 101/106 B. 105/140

C. 30/34 D. 30/35 E. 106/140

Specd/(bd)101/106Spec 95

35

5

30

105

4

101

140

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

PVNd/(cd)63/73PVN 86

27

3

24

73

10

63

100

66

34

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

PVPa/(ab)96/114PVP 84

114

96

18

886

4

882

1000

100

900

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 IN-correctly 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 900882) 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/(ab)96/(9618)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

PVNd/(cd)882/886PVN 99

114

18

96

886

4

882

1000

900

100

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 IN-correctly 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 900882) 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/(cd)882/(8824)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

And these 4 equations Sensitivity - a/ac

Specificity- d/bd Predictive Val Pos (PVP) -

a/ab Predictive Val Neg (PVN) - d/cd

Three extra concepts

- (In case your brain hasnt 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 two-month 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). Sensitivity90

, Specificity 25ACTH Stimulation.

Sensitivity80 , Specificity 85Low Dose

Dexamethasone Suppression. Sensitivity95 ,

Specificity 50A - Cannot say without knowing

the predictive value negativeB - Low Dose

Dexamethasone Suppression (LDDS)C - Urine

Cortisol Creatinine RatioD - 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

Where did that 900 come from ?0.10 prev X 1000

100 infected 1000-100 900 Dz-free

False Pos (1)

True Pos

False Negs

True Negs (899)

900

If Spec 99.9 Prev 10 and test 1000

animals Spec d/(bd)899/(900)Only 1 False

PosA positive result by your test is likely

correct

Why high SENSITIVITY means you can trust a

NEGATIVE test result Because false negs are LOW

Where did that 100 come from ?0.10 prev X 1000

100 infected 1000-100 900 Dz-free

False Pos

True Pos (99)

False Negs (1)

True Negs

100

If Sens 99 Prev 10 and test 1000

animals Sens a/(ac)99/(100)Only 1 False

NegA negative result by your test is likely

correct

As Prevalence decreases, PVP decreases too

If Prev30, Sens and Spec90, 1000

turtles PVPa/(ab)270/(27070)PVP 79

340

70

270

660

30

630

1000

700

300

108

99

9

If Prev DECREASES to 1, Sens and Spec90, 1000

turtles PVPa/(ab)9/(999)PVP decreases to 1

892

1

891

1000

10

990

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. Thats all you need to know. Read more

only if you want to see the math, (but you dont

need it.) gtgtgtgtgtgtgtgt Lets say prevalence of

pedunculated giblet disease is 30 in 1000

turtles (thats 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")

PVPa/(ab)270/(27070) 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.

Youre on your way!

The art of epidemiological thinking is to draw

conclusions from imperfect dataGeorge 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? A. 95 B. 96 C.

88 D. 86 E. 77

Sensa/(ac)30/34Sens 88

35

5

30

105

4

101

140

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? A.

95 B. 91 C. 86 D. 73 E. 70

Specd/(bd)63/66Spec 95

27

3

24

73

10

63

100

66

34

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? A. 91 B. 89 C. 86 D.

73 E. 70

PVPa/(ab)24/27PVP 89

27

3

24

73

10

63

100

66

34

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

PVNd/(cd)101/105PVN 96

35

5

30

105

4

101

140

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

PVPa/(ab)45/93PVP 48

93

48

45

907

5

902

1000

950

50

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