Diagnosis (part 1)

Question 1

What types of studies assess diagnostic accuracy?

Answer 1

Cross-sectional studies
RCTs
Screening
Systematic reviews

Question 2

Describe cross-sectional studies

Answer 2

• Accuracy of the test at a point in time.
• Compare with gold standard
• Gold standard could be hindsight (correct diagnosis apparent with time)

Question 3

What are the two types of cross-sectional studies?

Answer 3

Cohort - sample population suspected of having diagnosis (not known until after enrolment)

Case-control - one group clearly has diagnosis and other doesn’t (known at enrolment)
— prone to bias

Question 4

When are RCTs used in diagnostics?

Answer 4

Usually to see if testing affects outcomes (e.g. Routine XR improve pt Mx with LBP?)

Question 5

How is screening used?

Answer 5

Applies diagnostic tests to subjects who are NOT presenting with a disorder/complaint.

Effectiveness of screening by RCT, e.g. early detection helps…

Question 6

What does SENSITIVity refer to?

Answer 6

Correctly identifying people who HAVE the disease.

Question 7

How is SENSITIVity calculated?

Answer 7

From people who actually have the disease:

Correctly diagnosed WITH disease/total with disease

Question 8

Is a high sensitivity always good?

Answer 8

No. Could just always be a positive test.

Question 9

What does SPECIFity refer to?

Answer 9

Identify “well” people (no disease)

Question 10

How is SPECIFity calculated?

Answer 10

From people who actually DON’T have the disease:

Correctly diagnosed WITHOUT disease/total without disease

Question 11

What is the PPV?

Answer 11

Positive predictive value.

- chance that a positive test result will be correct.

Question 12

How is the PPV calculated?

Answer 12

From only the POSITIVE test results:

Correctly diagnosed with disease/total DIAGNOSED

Question 13

What is the NPV?

Answer 13

Negative predictive value.

- the chance that a negative test result will be correct.

Question 14

How is the NPV calculated?

Answer 14

From only the NEGATIVE test results:

Correctly diagnosed WITHOUT the disease/total DIAGNOSED WITHOUT disease

Question 15

How does the sensitivity, specificity, PPV and NPV change with a disease prevalence?

Answer 15

Sensitivity and specificity stay the same.

PPV falls with lower prevalence but NPV will rise.

Question 16

If the specificity of a test is 88%, what percentage of patients will have the same symptoms but no disease?

Answer 16

12%

1 - specificity

Question 17

What is the likelihood ratio and how is it calculated?

Answer 17

How much more likely the test result would be seen in someone with the condition, than someone without.

Sensitivity/(1 - specificity)

Question 18

How are likelihood ratios interpreted?

Answer 18

> 1 = test is more often positive
3 = test is useful
10 = test is very useful

Question 19

What can sensitivity/specificity be used to describe, and what can’t they predict?

Answer 19

Describe:
- how abnormality/normality predicts particular test results

Can’t predict:
- risk of abnormality

Question 20

Assessment of diagnosis (3 steps)

Answer 20

1. Is the evidence about diagnosis valid?
2. Is it important?
3. Can you apply it clinically?

Question 21

1. Is the evidence valid? (3)

Answer 21

Herbert et al 2005
A. Comparison with adequate reference standard
B. Comparison bind?
C. Study sample consisted of subjects with diagnostic uncertainty?

Question 22

Describe 1A (4)

Answer 22

Ref standard

• ref standard will often have face validity, e.g. open surgical
• reliability of standard should be assessed
• ref standard may be hindsight
• imperfect ref standard = under estimate of the accuracy

Question 23

Describe 1B (2)

Answer 23

Blinding

• assessor should be blind to result of the ref test
• – knowledge would bias the index test

Question 24

Describe 1C (2)

Answer 24

Diagnostic uncertainty

• spectrum of patients enrolled similar as possible to clinical practice? (COHORT study)
• aim is to solve diagnostic dilemmas (not confirm obvious Dx/negative Dx) (CASE-CONTROL study)

Question 25

Is this evidence about a Dx test valid? (other list) (4)

Answer 25

Straus et al 2005

1. Independent, blind comparison with reference (gold) standard of Dx?
2. Appropriate spectrum of pts?
3. Ref standard applied REGARDLESS of Dx test result?
4. Test/cluster of tests validated in a second, independent group of pts?