2c screening and diagnosis Flashcards
What is the aim of screening?
To reduce the harm caused by a disease or its complications
define screening
Screening is a process of identifying apparently healthy individuals who may be an increased risk of a condition. They can then be offered information, further tests or appropriate treatment to reduce their risk of any complications arising from the disease or condition
who is the organization responsible for approving screening programmes in the UK?
The UK national screening commitee
What are the UK national screening committee responsible for
Responsible for introducing, modifying or ending screening programmes in the UK based on the best available evidence
Advise the UK chief medical officers on screening policy
Considers the evidence for screening benefits, harms and costs and evaluates these against its own criteria based on Wilson and Jungners WHO criteria for assessing the viability, effectiveness and appropriateness of a screening programme
Give examples of some UK ‘screening’ programmes which are not part of UK NSC national, systematic screening
- PSA (prostate screening)
-NHS health checks (CVS Screening) - national child measuring programme (obesity screening)
UK screening programmes: foetal anomaly screening
- All pregnant women
- about 12 weeks: downs, edwards, pataus (USS and blood)
- about 20 weeks: foetal anomaly scan (USS)
UK Screening: Sickle cell and thalassaemia
- All pregnant women
- fathers if the mother is a genetic carrier
- not always done through blood test, questionnaire may be used to determine likelihood of being affected
UK screening: Infectious diseases in pregnancy screening programme
- all pregnant women
- HIV, HepB, syphilis
UK screening: Newborn and infant physical examination
- All newborns
- with 72 hours of birth and 6-8 weeks
-looks for:
1. congenital cardiac disease
2. cryptorchidism
3. DDH
4. congenital cataract
UK screening: newborn hearing screeing
- all newborns in first 4-5 weeks
- uses automated otoacoustic emission tests
- if no clear response may need:
1. retest
2. use of automated auditory brainstem response
3. audiology referral
UK screening: cervical screening
- england women aged 25- 64 years
- 3 yearly until 49 years
- 5 yearly from 50 years -64 years
- tests for HPV, if present cells are examined for abnormalities
- may require:
- monitoring
- colposcopy
UK screening: diabetic eye disease
- all >12 year olds with diabetes
-2 yearly if no retinopathy on past 2 screens - may need:
1. monitoring
2. opthalmology referral
UK screening: AAA screening
- one off men aged 65 years
- USS
- may need:
1. monitoring
2. vascular referal
UK screening: breast
- women aged50 -71 years
- mammography 3 yearly
- may need:
1. USS, repeat mammogram, fine needle aspiration
UK screening: Bowel
- Faecal immunochemical test (FIT)
- men and women aged 60-74 years
- every 2 years
- may need: colonoscopy
sensitivity
The proportion of people who have the disease who test positive
Test specific, not affected by population
a/ a+c
Specificity
The proportion of people who do not have the disease who test negative
Test specific, not affected by population
b/b+d
Positive predictive value
Proportion of people who test positive who have the disease
a/ a+b
Population specific, affected by population prevalence
Increasing prevalence will result in higher PPV
Negative predictive value
The proportion of people who test negative who do not have the disease
d/c+d
Population specific, affected by population prevalence
Increasing population prevalence will result in lower NPV
Constructing a screening table
What goes where?
Disease ON TOP
test results ON SIDE
What is threshold setting and what needs to be considered
- Need to decide a cut off point at which a test is ‘abnormal’
- setting a threshold will be a compromise between sensitivity and specificity. a test sensitive enough to detect all cases will also likely label a few without the disease as abnormal
-Receiver operating characteristic curves can help in making this decision
what are receiver operating characteristic curves and what are they used for?
- curve that plots sensitivity (true positive rate) on Y axis against 1- specificity on the X axis (false positive rate)
- for each possible threshold the sensitivity and specificity are calculated and plotted on this curve
- a straight diagonal line depicts a useless test (ie a true positive is as likely as a false positive- you might as well flip a coin)
- curves which are up nearer the top left hand corner are better performing
- ROC curves can be used for:
1. threshold setting
2. assessing the accuracy and validity of a test
3. comparing accuracy and validity of multiple diagnostic tests
What is AUROC and what is it used for
-Area under receiver operating characteristic curve
- perfect test (100% sensitive and 100% specifi) would have an AUROC of 1
- an AUROC of 0.5 indicates a useless test- you are as likely to get a true positive as a false positive
What is parallel testing in screening, give an example and what is its impact?
When multiple tests are done to look for the condition on the same individual at the same time.
ie quadruple test for downs syndrome antenatally
leads to higher sensitivity but lower specificity
What is serial testing in screening, give an example and what is its impact?
When multiple tests are done on the same individual sequentially to screen for a disease
ie cervical screening, samples are tested for HPV first and then under go cytology if positive
Serial testing increases the specificity
In what situations might you want to set a threshold for a higher sensitivity?
- Serious disease with effective treatment
- risk of infectivity to others
- subsequent diagnostic tests are cheap and acceptable
In what situations might you want to set a threshold for a higher sensitivity?
Subsequent diagnostic tests/ treatment are unpalatable/ expensive
what are screening, diagnosis and case finding and how are they similar/ different
SCREENING
Assessing risk of disease in asymptomatic individuals. Used as a guide as to whether or not to offer a diagnostic test
DIAGNOSIS
Offered to people with symptoms, signs or screened positive. Used to determine whether or not the individual has the disease
CASE FINDING
A systematic process of identifying people/ groups at higher risk of a disease or adverse event. Involves systematically searching for people rather than waiting for them to present with active disease