secondary prevention

Question 1

Define screening.

Answer 1

identifying apparently healthy people (i.e. not aware they have a condition) that may be at increased risk of a disease or condition.
once identified, they can be offered information, further tests and treatment to reduce their risk and/or complications of disease or condition.

Question 2

Name 4 methods of screening and an example of each.

Answer 2

• questionnaire e.g. Geriatric Depression Scale
• Examination e.g. BP measurement
• Lab test e.g. Pap smear
• Imaging e.g. mammography

Question 3

Define sensitivity (of a test).

Answer 3

number of true positive results / (number of true positive results + number of false negative results)

= how good a test is a picking up those with the disease

Question 4

Is a highly sensitive test more useful when the (positive) result is positive or negative?

Answer 4

negative - don’t want to miss anybody who is at risk of a life-threatening condition

Question 5

Define specificity (of a test).

Answer 5

number of true negative results / (number of true negative results + number of false positive results)

= how good a test is at correctly excluding those who don’t have the condition

Question 6

Is a highly specific test more useful when the (negative) result is positive or negative?

Answer 6

positive - don’t want to incorrectly identify people as having the condition when they don’t

Question 7

Define the positive predicted value (PPV) of a test.

Answer 7

number of true positive results / (number of true positive results + number of false positive results)

= how likely you actually have the condition if you test positive

Question 8

Define the negative predicted value (NPV) of a test.

Answer 8

number of true negative results / (number of true negative results + number of false negative results)

=how likely it is that you don’t actually have the condition if you test negative

Question 9

Define prevalence (in relation to a test).

Answer 9

(number of true positives + false negatives) / number of test results (i.e. all four groups)

= number of people with the disease/total number of people tested

Question 10

what is the relationship between the prevalence of disease and positive predicted value (PPV)?

Answer 10

a rarer disease will have a lower PPV meaning more people who test positive for the disease are unlikely to have it.

Question 11

what is the relationship between the prevalence of disease and negative predicted value (NPV)?

Answer 11

a rarer disease will have a higher NPV meaning more people who test negative for the disease are likely to not have it.

Question 12

what is the difference between screening tests and screening programmes?

Answer 12

screening programmes are screening tests PLUS options for diagnosis and treatment, and take into account population at risk and screening intervals.

Question 13

What are the potential harms of screening?

Answer 13

• unnecessary costs and hazards of screening for those who test true negative
• further tests required for those testing false positive so further costs and hazards
• also false positive people may become anxious and fear future screening
• those who test false negative end up with false reassurance and disregard their symptoms > harmful
• also false positive people receive required treatment/intervention a lot later than they should because of incorrect testing

Question 14

who assess screening programmes in the UK before they are used?

Answer 14

National Screening Committee

Question 15

when is it appropriate to screen for a particular disease?

Answer 15

• if disease has a pre-clinical detectable period
• if detecting disease is going to make a difference to outcome of patient - is there treatment?
• if the test is practical

Question 16

Describe the criteria used by the NSC to assess a screening test.

Answer 16

• knowledge of the disease and pathogenesis
• the treatment - is it effective? who can receive it? are the facilities adequate?
• the test - is it simple/safe/precise/valid/acceptable? are the cut-off values clear? is there an agreed policy for positive tests?
• the programme - is there RCT evidence for its effectiveness? is the information in the programme easy to understand by participants? is it socially and ethically acceptable? DO THE BENEFITS OUTWEIGH THE HARMS? is it cost-effective?

Question 17

what is volunteer bias?

Answer 17

people who volunteer to participate in screening tests have some different characteristics, privileges, and lifestyles from those who do not volunteer

-may send most severe patients in > not getting a complete overview of the whole population

Question 18

what is lead time bias?

Answer 18

supposedly if diagnosed by screening then have longer time to live/ advanced screening increases survival

when actually could have the same time to live but just because disease detected earlier, it seems like the person has longer to live
e.g. person detected with disease by screening 3 years after onset and therefore has another 7 years to live compared to a person who is diagnosed 5 years after onset, not by screening, and has five years to live - both have same time to live.

Question 19

what is length time bias?

Answer 19

those detected by screening more likely to have a better prognosis (less aggressive disease)

however this is not due to earlier treatment, this is because those with more aggressive disease will have quicker disease progression and die more quickly. if two people have disease onset at the same time but one is more aggressive than the other, the more aggressive patient may die before the other patient has even been screened for the disease and therefore screening results show that screening leads to better outcome/prognosis.

Question 20

describe some factors influencing screening policy.

Answer 20

• public values and opinions e.g. US mammography screening policy which was initially condemned for insufficient evidence to warrant usefulness but then passed due to pressure from population who demanded regular screening, saying it was cheating women to not let them have screening
• commercial interests e.g. European HPV testing which was stated in the media as something of much benefit when in fact it is no better then cytology and can in fact cause more harm due to over-detection and stigma. eventually found out that manufacturing company for HPV tests were the ones pushing for the screening policy in disguise.