Epidemiological screening

Question 1

What is epidemiological screening and how is performed?

Answer 1

• Investigate apparently healthy individuals to detect unrecognized disease so can prevent/delay development or improve prognosis
• If earlier detection does not offer any hope of improved outcome then screening is generally not indicated
• Divide into 2 groups (more/less likely to have disease) and investigate more likely more

Question 2

What are indications for screening?

Answer 2

• Early diagnosis for prevention
  detection of disease at an early stage leads to improved prognosis
• Identify high risk individuals
  where interventions will reduce that risk
• Identify those posing risk to others and reduce risk
  infectious disease where treatment or other control measures will improve the outcome for the individual and reduce transmission

Question 3

What is a the point of a screening test and what should a screening test be?

Answer 3

• Identify most/least likely to have condition
• Not diagnostic necessarily
• Applied when people feel healthy
• Can have true/false positives or false/true negatives

simple, safe, acceptable, inexpensive, repeatable and valid

Question 4

What are the chalanges with screening?

Answer 4

• Screen for low prevalence population
  
  • So need high specificity
  • So lower sensitivity

Question 5

What are current screening programs in the UK?

Answer 5

Antenatal

• Fetal anomaly
• Infections (HIV, syphilis)
• Sickle cell disease
• Physical abnormalities

Newborn
- Physical exam 
- Hearing test
- Blood spot
Adult cancer
- Breast, cervical, bowel

Other

• Abnormal aortic aneurysm
• Diabetic retinopathy

Additional programs

• NHS vascular health checks
• Prostate cancer
• Chlamydia screening

Cervical cancer screening

• 35-64 y/o
• Regular screening leads to reduction in cancer and saving lives

HIV screening in pregnancy

NHS breast screening program

Question 6

What is the validity of a screening test?

Answer 6

ability to distinguish between subjects with the condition and those without

must know definitive disease status of individuals before

Question 7

What is sensitivity?

Answer 7

ability of the test to correctly identify people with the disease

sensitivity = a ÷ (a+c)

Question 8

What is specificity?

Answer 8

ability of the test to correctly identify people without the disease

specificity = d ÷ (b+d)

Question 9

What is positive predictive value?

Answer 9

likelihood that a patient with a positive test result will actually have the disease

positive predictive value = a ÷ (a+b)

Question 10

What is negative predictive value?

Answer 10

likelihood that a patient with a negative test result will not have the disease

negative predictive value = d ÷ (c+d)

Question 11

What does predictive value depend on?

Answer 11

sensitivity and specificity AND the prevalence of the condition in the population

Question 12

What are Receiver Operator Characteristics (ROC) curves

Answer 12

used to determine a cut-off value for a diagnostic or screening test

graphical display of the how the proportions of true positives and false positives change for each of the possible pre-determined values.

proportion of true-positives and false-positives are calculated for possible values.

choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity

Question 13

What are approaches to screening?

Answer 13

mass

targeted

may be a systematic programme where people are called for screening

opportunistic programme when a person presents to the doctor for some other reason and they are offered a test

Question 14

What is the disease criteria for screening?

Answer 14

important global health problem
detectable at early age
well understood natural diseas ehistory
long period between signs and disease

Question 15

What is the test criteria for screening?

Answer 15

cheap
reliable 
valid 
safe
simple

Question 16

What is the diagnosis and treatment criteria for screening?

Answer 16

adquate fascilities
effective, acceptable and safe
cost effective
sustainable

Question 17

What does feasability depend on?

Answer 17

how easy it is to organise the population to attend for screening, whether the screening test is acceptable, whether facilities and resources exist to carry out the necessary diagnostic tests following screening.

Question 18

What are possible biases in screening efficetivity

Answer 18

selection

lead time: screening identifies disease that would otherwise be identified at a later stage. This may result in an apparent improvement in the length of survival due to screening which is really due to the earlier date of diagnosis.

length: some conditions may be slower in developing to a health threatening stage, that is, they have a longer preclinical stage. This means they are more likely to be detected at that stage but they may also have a more favourable prognosis leading to the false conclusion that screening is beneficial in lengthening the lives of those found positive.

Question 19

What are ethical considerations of screening?

Answer 19

For the individual the screening test can do harm as well as giving benefit
There may be a risk attached to the screening test or subsequent diagnostic test
A false positive result can cause unnecessary anxiety
There may be other unplanned effects of a positive test
A false negative result will give false reassurance