Evidence about Diagnosis

Question 1

Can you test for diagnostic accuracy with RCT?

Answer 1

NO- as diagnostic accuracy depends on whether Px has or doesn’t have disease

Question 2

What is diagnosing?

Answer 2

classifying individual as having ot not having disease

Question 3

what are things optometrists consider when making diagnosis?

Answer 3

symptoms
signs
characteristics
Hx
Examination Results

Question 4

How can we measure a diagnostic test?

Answer 4

Against a ‘gold standard’ or ‘reference test’

Question 5

Can a diagnostic test be more than one test?

Answer 5

YES- an examination as a whole can be considered to be a diagnostic test

Question 6

Best type of study to estimate diagnostic test accuracy?

Answer 6

• Consecutive sample
• Defined clinical presentation
• gold standard and test of interest performed in every individual
• SYSTEMATIC REVIEWS BEST

Question 7

What is better for testing diagnostic accuracy- consecutive or non-consecutive sample?

Answer 7

Consecutive sample

non-consecutive- sample bias

Question 8

Can we use case control studies to test diagnostic accuracy?

Answer 8

yes- but weak study

want to avoid

Question 9

Why are case control studies not great for diagnostic accuracy testing?

Answer 9

• subject to selection bias as non-consecutive sampling

- less likely to enrol Px’s with whole spectrum

Question 10

PICO for diagnostic accuracy

Answer 10

P- population
I- diagnostic test we’re interested in (rather than intervention)
C- gold standard
O- diagnosis in question

Question 11

For the following diagnostic accuracy question- what is the question:
You are working as an optometrist in a clinic
that performs refractive surgery on adults.
Your routine is to use both objective as well
as subjective refraction. A patient asks why
you do both. The question prompts you to
wonder if objective methods are as accurate
as the gold standard (subjective methods).

Answer 11

P: adults
I: objective refraction
C: subjective refraction
O: accuracy of measuring refractive error

Clinical question: In adults, how does objective refraction tests compare in accuracy to subjective refraction tests for Px’s considering Rx error surgery

Question 12

What Test used to Appraise Diagnostic Research Quality? What about RCT? What about prognosis? systematic reviews?

Answer 12

QUADAS-2

RCT- PEDro
(prognosis- CASP)
(sys reviews- AMSTAR or CASP)

Question 13

Is it appropriate to exclude difficult to diagnose Pxs from diagnostic accuracy study?

Answer 13

NO

Question 14

What types of studies report sensitivity and specificity?

Answer 14

Diagnostic accuracy studies

Question 15

What is Sensitivity?

Answer 15

Px’s with the disease who test +ve
Sensitivity = a/ (a+c)

D+	D- T+	a	b T-	c	d

Question 16

What is Specificity?

Answer 16

Px’s without the disease who test -ve
Specificity = d/ (d+b)

D+	D- T+	a	b T-	c	d

Question 17

What is a true positive and false positive

Answer 17

true positive- Px’s with the disease that get +ve result

False +Ve- Px without disease that gets +ve result

Question 18

What is a true negative and false negative

Answer 18

true neg- Px without disease that gets neg result

false neg- Px with disease that gets neg result

Question 19

Complete a table for a test on a sample of n=1000 Px’s with prevalence of 1% with disease

Answer 19

Test 1 D+ D-
T+ 6 99
T- 4 891
totals 10 990

Question 20

How do we diagnose?

Answer 20

Set a criterion

Question 21

What effect does decreasing criterion have on specificity and sensitivity?

Answer 21

increases sensitivity (more Px's tested as pos)
decreases specificity (more Px's with disease classified as +ve)

Question 22

High criterion- effect on sensitivity and specificity

Answer 22

low sensitivity

high specificity

Question 23

What is more useful that sensitivity and specificity?

Answer 23

Predictive values

Question 24

What are predictive values?

Answer 24

probability that the test result is accurate

Question 25

Does sensitivity and specificity change with the population?

Answer 25

NO- it is a property of the test itself

Question 26

calculate Positive Predictive Value (PPV):

Answer 26

PPV = probability of having disease when test is +ve
(go down test column)

PPV = a / (a+b)

D+	D- T+	a	b T-	c	d

Question 27

calculate Negative Predictive Value (NPV):

Answer 27

Probability of not having disease when test it -ve
(go down test column)

PPV = d/(d+c)

D+	D- T+	a	b T-	c	d

Question 28

Calculate PPV and NPV:
TEST 1	D+	D-
T+	         6	10
T-	        4	980
totals	10	990

Answer 28

PPV = 6/16 = 0.375

NPV = 980/984 = 0.995

Question 29

True or false- prevalence is not important for PPV and NPV

Answer 29

FALSE

Question 30

Effect of high prevalence on PPV and NPV?

Answer 30

increases PPV

decreases NPV

Question 31

Does prevalence effect sensitivity and specificity?

Answer 31

NO- property of test

Question 32

Likelihood ratio for positive test result

Answer 32

how much more likely a Px’s +Ve test would be for someone with disease compared to someone without disease

LR+ = sensitivity / (1-specificity)

Question 33

Likelihood ratio of negative test result

Answer 33

how much more likely a Px’s -ve test result would be for someone without disease compared to someone with disease

LR- = (1-sensitivity) / specificity

Question 34

What does a PPV of 0.03 mean on a RAT test

Answer 34

3% chance of having COVID if test result is +ve

not reliable test

Question 35

What does NPV of 0.998 mean on RAT test?

Answer 35

99.8% chance of not having COVID is test result is -ve