PHRM3031 - screening and risk assessment

Question 1

population-based screening

defintion

Answer 1

a test is offered systematically to all individuals in the defined target group within a framework of agreed policy, protocols, quality management, monitoring and evaluation

Question 2

opportunistic case-finding

Answer 2

a test is offered to an individual without symptoms of the disease when they present to a health care practitioner for reasons unrelated to that disease

Question 3

screening (WHO)

Answer 3

• presumptive identification of unrecognised disease or defects by means of tests, examinations or other procedures that can be applied rapidly
• intended for all people, in an identified target population, who do not have symptoms of the disease or condition being screened for
• process can identify: a pre-disease abnormality early disease; or disease risk markers
• aim of screening for a disease or a risk marker or a disease –> reduce the burden of the disease in the community including incidence of disease, morbidity from the disease or mortality

Question 4

WHO principles of early disease detection

Condition

Answer 4

• the condition should be an important health problem
• there should be a recognisable latent or early symptomatic stage
• the natural history of the condition, including development from latent to declared disease should be adequately understood

Question 5

WHO principles of early disease detection

Test

Answer 5

• there should be a suitable test or examination

- the test should be acceptable to the population

Question 6

WHO principles of early disease detection

Treatment

Answer 6

-there should be an accepted treatment for patients with recognised disease

Question 7

WHO principles of early disease detection

Screening program

Answer 7

• agreed policy on whom to treat as patients
• facilities for diagnosis and treatment should be available
• the cost of case-findings should be economically balanced in relation to possible expenditure on medical care a whole
• case-findings should be a continuing process and not a ‘once and for all’ project

Question 8

Screening Tests

Answer 8

• can reduce the risk of developing or dying from a disease, but it does not guarantee that disease will not occur or if it occurs, that it can be cured
• benefits, harms and costs –> ethical obligations to maximise benefits and minimise harms
• overall benefits >harms
• need community consensus that the benefits justify the expense when using community resources to fund screening
• often laboratory tests that detect particular markers of a specific disease
• many medical evaluations and tests may be thought of as screening procedures
• screening test not equally to diagnostic tests
• diagnostic tests are done if you already have symptoms of a disease –> prove that disease is present
• definitive diagnosis generally requires more extensive, sometimes invasive and more reliable evaluations

Question 9

The test must be…

Answer 9

• validated
• safe
• highly sensitive –> identify those with disease
• highly specific –> exclude those without disease
• relatively high positive predictive value
• relatively high negative predictive value
• established criteria for what constitutes + or - test results (suitable cut-off) + continuous to dichotomous value
• any harm caused, or that may be caused by the screening test should be acknowledged, communicated to those undergoing screening and accurately measured

Question 10

Screening Programs in Australia

Answer 10

• BreastScreen
• National Bowel Cancer Screening Program
• National Cervical Screening Program

Question 11

Sensitivity

Answer 11

• the proportion of people with the disease (or condition) who have a positive test result
• a sensitive test will rarely miss people with the disease

Question 12

Specificity

Answer 12

• the proportion of people with the disease (or condition) who have a negative test result
• specific test will rarely misclassify people as having the disease when they do not

Question 13

when to use a sensitive test

Answer 13

• there is an important penalty for missing the disease i.e the disease is dangerous but treatable
• it is early in diagnostic work up when several diagnoses are considered –> reduce the number of possibilities
• **highly sensitive test is most helpful when the result is negative

Question 14

when to use a specific test

Answer 14

• proportion of people without the disease who have a negative test
• you want to confirm or ‘rule in’ a diagnosis as it is rarely positive in the absence of disease
• false positives can harm the patient physically, emotionally or financially
• **highly specific test is most helpful when the result is positive

Question 15

trade offs: sensitivity & specificity

Answer 15

ideally increase sensitivity and increase specificity

• for most clinical tests there is a threshold at which it is agreed the disease is present
• sensitivity and specificity are affected by changes to the agreed threshold
• trade-off between Se and Sp-if attempt to increase Se will lead to reduction of Sp

Question 16

predictive value of tests
defintion
PPv
NPV

Answer 16

-does the person have the disease given the test results?
-clinicians are often more concerned about predictive value than about sensitivity and specificity (test)
Positive Predictive Value (PPV): the probability of having having the disease when the test results is positive
Negative Predictive Value (NPV): the probability of not having the disease when the test result is negative

Question 17

prevalence and screening

Answer 17

• predictive value is related to Se,Sp and prevalence of disease
• prevalence-proportion of persons in a defined population at a given point in time having the disease (burden to society of a disease or condition)
• sometimes called ‘prior (or pretest) probability (probability of the disease before the test results is known)
• assigning prior probability (eg. deciding if diagnostic test is required and interpreting those results)

Question 18

how to measure efficacy of screening?

Answer 18

effectiveness –> does it reduce morbidity and mortality of the disease? compare mortality: those diagnosed by screening vs symptoms

1. correlational studies correlate trends in disease-specific mortality over time with frequency of screening in a population
2. case-control and cohort studies
3. randomised clinical trials

Question 19

bias in screening studies

self-selection

Answer 19

participants tend to be healthier, have healthier lifestyles and they tend to adhere to therapy better –> better outcomes. also the “worried well” - people who a asymptomatic, but at higher risk

Question 20

bias in screening studies

lead time

Answer 20

screening detects disease earlier but if compare survival time from the point of diagnosis, the subject whose disease was identified through screening appears to survive longer, but only because their disease was identified

Question 21

bias in screening studies

length time

Answer 21

the length of the detectable pre-clinical phase can vary substantially from person to person (eg.prostate cancer is very slow growing tumour in many men, but very rapidly progressing and lethal in others)–> can exaggerate the apparent benefit of screening as there is greater chance that screening will detect subjects with long phases and therefore more benign disease (leads to over diagnosis)

Question 22

screening - harms

Answer 22

• many people with condition may remain asymptomatic and do require intervention
• if become symptomatic, can have treatment and the delay generally causes no problem (eg gallstones)
• even if condition is detected by screening, earlier treatment does not seem to substantially prolong survival (eg lung cancer)
• if disease prevalence is low –> screening is very inefficient
• screening may not change disease course and cause unnecessary harm by distress and worry and further invasive (and unnecessary) test

Question 23

screening - harms

overdiagnosis

Answer 23

-over-diagnosis: an inordinate emphasis on early diagnosis of disease and that the increasingly aggressive pursuit of abnormalities among people without symptoms is leading to actual harm and great cost without reaping any benefits

Question 24

screening -harms

false positives

Answer 24

people who test positive even though they really don’t have disease

Question 25

screening - harms

false negatives

Answer 25

people of test negative even though they really have the disease