14 Population Screening Flashcards
Who do we want to give screening tests to?
Those who are more likely to be helped than harmed on further treatments. Those that we might get early prevention or treatment for.
Those what are higher risk and so help the sensitivity and specificity of the test.
What are the benefits of screening?
Early diagnosis for more effective treatment.
Prevent disease in at risk individuals.
Inform family planning.
Raise awareness of health status so they can take charge of their own health.
Be a cost effective solution at controlling disease at a population level.
What are the harms of screening?
Not 100% accurate. False positive - treatment they don’t need. False negative - Won’t take treatment.
People that could always be asymptomatic get stress and treatment they don’t need.
Disclosure of information in terms of affected relatives.
Implications for health insurance, only HD in UK.
What criteria are there for new screening programmes?
Test should be ethically and socially acceptable.
Patient should be informed and give consent.
Test should have acceptable accuracy.
A positive result must lead to some benefit.
It must show reasonable cost benefit analysis.
What is the difference in role between screening tests and diagnostic tests?
Screening detects disease markers.
Diagnosis detects disease.
What’s the difference in test population between screening tests and diagnostic tests
Screening - large numbers of at risk individuals.
Diagnostic - Symptomatic individuals, or those with positive screen
What’s the difference in test method between screening tests and diagnostic tests
Screening - cheap, simple, non-invasive.
Diagnostic - More expensive, complex, invasive, but justified.
What’s the difference in accuracy between screening tests and diagnostic tests
Screening - High sensitivity to avoid missing affected individuals.
Diagnostic - High specificity.
What the impact of a positive result of a screening vs diagnostic test?
Screening - suggestive of disease.
Diagnostic - Definite diagnosis.
For genetic tests, why is the distinction between a screening and diagnostic test sometimes unclear?
Diagnostic tests can be fairly low invasiveness, and positive screening results can be a definitive result
Screening programmes can be systematic such as breast cancer screening. Explain this.
Breast cancer screening is offered to all women 50-64 every 3 years. It’s to everyone as an offer, often a letter. This is common when stigma is low and the disease prevalence is high.
Screening programmes can be opportunistic like HIV in pregnant women. Explain this.
There is a stigma around it, so it is offered when individuals are present at clinical services for other reasons. Included in a series of tests
When are these screening tests?
Down’s syndrome, neural tube defects
Prenatally
What are some neonatal screening tests?
PKU, hypothyroidism, CF
When is preconception screening done?
For potential carriers that want to be parents. Such as Tay Sach’s disease
What are some presymptomatic/adult-onset screens?
Bowl/breast cancer, haemochromatosis
What will prenatal screens lead to?
People killing their babies
All pregnant women can have screens for downs syndrome but….
They need to opt in
Risk of trisomies increased with age, how much?
> 37 years old has a 1 in 200 risk.
Why is the age of 37 used as a screening age for downs syndrome specifically?
Because then the risk of downs syndrome = the risk of amniocenetsis inducing miscarriage
How is down syndrome prenatal testing done?
Aminocentesis and then chromosome analysis
Prenatal downs screening detects _____
35% of all downs syndrome cases (because many women are having children younger)
There’s a quadruple test that looks for increased HCG or inhibin A, or decreased alpha-fetoprotein or unconjugated oesteriols. What does this test do to help screening?
Those positive on a quadruple test AND >37 combined are only 5% of pregnancies, but it’s able to increase the number of detections of downs syndrome. It decreases the number of invasive tests, and the test is more cost effective.
What screening tests do use neonate blood spot cards for?
PKU, hypothyroidism, sickle cell anaemia, thalassaemia, CF, MCADD, and inherited metabolic diseases
How many are affected by PKU?
1 in 10,000 births. 250 cases/year
What is PKU?
Lack of enzyme PAH so amino acid phenylalanine cant be metabolised to tyrosine.
What test was initially used to detect PKU and what is used now?
Initially the Guthrie growth assay, now tandem mass spectrometry
If you don’t treat PKU the high levels of Phe can affect a child’s IQ. How can you tackle it?
Change diet to improve outcomes
When are babies screened for PKU and how?
5-8 days for Phe levels. Those with High Phe levels are tested again a few days later as well as for Tyr.
What’s the inheritance pattern for PKU?
Autosomal recessive
How many are affected by congenital hypothyroidism?
1 in 4000. 300 babies a year. 2:1 girl to boy ratio.W
Hat is the cause of Congenital hypothyroidism?
Lack of thyroxine hormone.
What are the symptoms of congenital hypothyroidism?
Growth problems, mental and physical disability.
What do blood spot tests look at for congenital hypothyroidism? And how do you treat the baby.
Increased TSH levels. If high levels, treat the baby with thyroxine tablets to allow normal development
How many births affected by CF?
1 in 2,500
CF is what inheritance pattern?
Autosomal recessive
What is defective in CF?
The Chloride ion transporter
What does a defective chloride ion transporter in CF lead to? AKA symptoms
Sticky mucus secretions, recurrent chest infections, digestive problems, pancreatic insufficiency
What do babies with CF need?
Antibiotics and physiotherapy and high energy diet
A CF screening test consists of what?
The blood spot is analysed for IRT levels. High levels identify a high risk group. These then have a diagnostic sweat test.
A genetic test can be used to confirm a borderline sweat test for CF. What are on the panels?
Firstly a panel of the 4 most common CF mutants are tested in an ARMS assay. Then if only one variant is detected, a second panel with 53 variants is performed.
There are >2000 CF variants, but not many are common
What downsides are there to CF screening?
The IRT test interpretation can be inconclusive leading to stress during retesting. Also a cause of worry for carrier parents.
What are some metabolic disorders that are screened for presymptomatically?
IVA, Glu1, HCU, Maple syrup urine disease, LCHAD
Why does screening for metabolic disorders need to be carefully considered?
Because they are very rare, but very severe. What’s the cost-benefit analysis like? Are they treatable
What’s the incidence and inheritance of Haemochromatosis?
1 in 4000 in the UK. It’s autosomal recessive.
What’s the onset of haemochromatosis?
30-50 years old
A mutation in the HFE gene in haemochromatosis leads to what? (Symptoms)
Build up of iron causes liver cirrhosis, cardimyopathy, diabetes mellitus, arthritis.
How can you treat haemochromatosis?
Venesection
What are the pros for screening for haemochromatosis?
The variants are common.
The test is available.
Treatment is available
What are the cons for screening haemochromatosis?
Low penetrance of the disease, only 1 in 10 homozygotes get symptoms.
What is cascade screening?
When family members have a greater chance to be affected when there is already an affected individual in the family, the screening if offered therefore to the family.
What’s great about cascade screening?
People are more likely to take it up, and the results are better. Difference between a 1 in 400 chance of having mutations for haemochromatosis, compared to 2/3 chance of carrying it if a sibling has it.
What’s the pros and cons of preconception carrier testing?
Provide information for reproductive decisions. Warns you about a baby’s potential disease.
Stigmatises certain ethnic groups or children. Inappropriate use by health insurance.
Why is PIGD strictly regulated?
Embryos can be damaged by removing a cell to perform it.
And the test is not fully reliable.
What choices do people have when they’ve had preconception carrier testing?
Have a pregnancy, find a different partner, assisted reproduction, adoption, PIGD in IVF, tests during pregnancy.
What is an amniocentesis or CVF?
Aminiotic fluid sample,
chorionic villus sample from placenta
When are amnio samples taken? What’s the chance of miscarriage?
> 15 weeks.
0.5-1%
When are CVF samples taken? What’s the chance of miscarriage
11-14 weeks.
1-2% chance.
What are some high carrier frequency diseases in the Ashkenazi jewish population?
Gaucher disease (1:13)
Bloom syndrome (1:100)
Tay Sach’s (1:30) AR (affects 1 in 4000)
What are the symptoms of Tay Sach’s disease?
A neurodegenerative disorder causing loss of hearing and vision and swallowing problems. Fatal at 1-3 years with no cure.
What is the cause of Tay Sachs?
A deficiency of hexosaminidase A for fatty acid metabolism
What is the Tay Sach’s screening programme?
Measuring HEXA enzyme activity in jewish populations. Screening happens in schools.
What does Dor Yeshorim do?
Arranges marriages for Ashkenazi jewish populations with genetic info to reduce carriers marrying other carriers
