Colin Dale

Question 1

Subtypes of screening

Answer 1

1. OPPORTUNISTIC
   - (screening patients with unrelated problem)
   - `ad hoc’ screening
2. SYSTEMATIC
   - (purposive screening, screening programme)

Question 2

What are the subtypes of systematic screening?

Answer 2

1. MASS SCREENING
   (screening aimed at unselected population)
2. SELECTIVE SCREENING
   (screening aimed particularly at subjects at high risk of
   developing a disease)

Question 3

What are the four categories of assessment of screening programme by the NSC>

Answer 3

1. The condition
2. The test
3. The treatment
4. The screening programme

Question 4

What are the NSC conditions for the screening?

Answer 4

1. Important public health problem

2 Epidemiology and natural history of the condition
should be adequately understood

3. There should be a detectable risk factor or disease
   marker (forms basis for screening test)

4.  The condition should have a latent period or early
symptomatic stage (with good prognosis)

5. All cost-effective primary preventive interventions
   should have been implemented as far as possible

Question 5

What are NSC criteria for the test?

Answer 5

1. There should be a simple, safe, precise and validated
   screening test
2. The test should be acceptable to the population
3. The distribution of test values in the target population
   should be known and a suitable cut-off level defined and
   agreed
4. There should be an agreed policy on the further diagnostic
   investigation of individuals with a positive test result and on
   the choices available to those individuals

Question 6

What are the screning tests for CRC?

Answer 6

Combination of direct imaging and indirect testing for
faecal blood

1. Faecal occult blood (FOB) (`guaiac’ test, FOB based on
   immunochemical test)
2. Flexible sigmoidoscopy
3. Colonoscopy

Question 7

Describe the FOB test and its sensitivity and specificity

Answer 7

Traditional guaiac test detects haem

• Sensitivity for detection of colorectal cancer between 25 and
  50% (Sensitivity – ability of test to identify true disease correctly)
• Specificity around 75%, not high (does not discriminate human
  and animal haemoglobin, meat in diet can give positive test, also
  other bowel lesions [e.g. piles]) (Specificity – ability of test to identify real non-cases correctly)

Question 8

PPV of FOB

Answer 8

 Positive predictive value
(Given a positive test, what is the chance that
disease is really present?)

-1 in 2 (50%) positive screening tests have
some form of neoplasia
-1 in 8.5 (12%) have colorectal cancer

Question 9

Describe the use of flexible sigmoidoscopy

Answer 9

• Flexible sigmoidoscope (60 cm) can reach more than
  50% of colorectal cancers (these mostly occur on LEFT
  side of colon + rectum)
• Sensitivity for identification of large adenomas and
  colorectal cancer on L side very high (~80% +)
• Can be diagnostic test (+ biopsy) in presence of cancer
• Also, presence of left sided adenomas a good marker
  for need for colonoscopy

Question 10

What is the gold standard for Dx of CRC

Answer 10

Colonoscopy

Question 11

Diagnostic test req. after FOBT

Answer 11

Colonscopy/Bx

Question 12

Diagnostic test req. after flexible sigmoidoscopy

Answer 12

Bx

Question 13

Diagnostic test needed after colonoscopy

Answer 13

Bx

Question 14

NSC criteria for treatment

Answer 14

1. There should be an effective treatment for patients identified
   through early detection
2. There should be evidence that earlier treatment leads to better
   outcomes than late treatment
3. There should be agreed evidence-based policies defining
   which individuals should be offered treatment and the
   appropriate treatment to be offered
4. Clinical management of the condition and patient outcomes
   should be optimised by all health care providers prior to
   participation in a screening programme

Question 15

NSC criteria for the screening programme

Answer 15

1. There should be sufficient evidence from high quality randomised
   controlled trials that the screening programme is effective in reducing
   mortality and morbidity
2. There should be evidence that the complete screening programme
   (test, diagnosis and treatment) is clinically, socially and ethically
   acceptable to health professionals and the public and that the benefits
   outweigh the harms
3. The opportunity cost of the screening programme should be
   economically balanced in relation to medical care expenditure as a
   whole
4. There should be a plan for managing and monitoring the screening
   programme and an agreed set of quality assurance standards
5. Adequate staffing and facilities prior to commencement
6. All other options for managing the condition should have been
   considered (e.g. improving treatment)

Question 16

Difference between screening and diagnosis

Answer 16

Detection of disease in asymptomatic people
= screening

• Detection of disease in symptomatic people
= diagnosis

Question 17

Practical explanation of specificity

Answer 17

Specificity = 94%

94% of people without disease test negative

Question 18

Practical explanation of sensitivity (DETECTION RATE)

Answer 18

Detection rate (sensitivity) = 80%
80% (or 8 out of 10) people with disease test positive

Question 19

Pratical eexplanation of flase positive rate

Answer 19

False positive rate = 6%

6% of people without disease test positive

Question 20

Practical explanation of PPV

Answer 20

PPV = 71%
among people with test positive results 71% have the
disease

Question 21

What can a likelihood ratio tell us

Answer 21

How much more likely are you to get a given
test result (positive or negative) in someone
who does have the disease than in
someone who does not.

Question 22

What is the likelihood ratio for a positive/negative result

Answer 22

probability of a positive/negative result if the disease is present divided by the probability of a positive/negative result if the disease is absent

Question 23

Interpretation of likelihood ratio

Answer 23

Positive test result
LR = 80%÷ 6% = 13
People with disease are
13x as likely to test
positive than people
without disease

Negative test result
LR = 20%÷ 94% = 0.2
People with disease are
0.2 x as likely to test
negative as people
without disease

Question 24

Significance of likelihood ratios

Answer 24

• Greater than 1 make your hypothesis more likely
• Less than 1 make your hypothesis less likely
• To significantly change odds which are not already
large needs a likelihood ratio of >10 or <0.1
• Are a fixed characteristic of a test
• Are thus independent of disease prevalence
• Provide a clinically meaningful idea of the value of a
test
• Are insufficiently quoted and used
• Are key to accurate diagnostic reasoning
– Allows individualization of risk prediction
– More on this next time……

Question 25

Problems with making a diagnosis

Answer 25

• For a dichotomous test sensitivity and / or specificity
may be low
• For a quantitative test there may be considerable
overlap between normal and abnormal distributions
• The likelihood ratios are low i.e. close to 1
• The test does not move your hypothesis far along
the probability line
• You therefore need larger numbers of tests

Question 26

THERES A WORKSHEET THAT I SHOULD PROBABLY DOOOO

Answer 26

THERES A WORKSHEET THAT I SHOULD PROBABLY DOOOO