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Paediatric Biochemistry > Principles of Screening and Ante-natal Screening > Flashcards

Principles of Screening and Ante-natal Screening Flashcards

1
Q

What are features of screening?

A
  • Applied to a large population with a low incidence of disease
  • Aims to identify a subset of individuals at higher risk of disease
  • Does not diagnose
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2
Q

What are types of Screening?

A

Population screening - large scale screening of whole population group

  • Newborn blood spot screening
  • Newborn hearing screening
  • Antenatal screening

Selective screening - Pre-selected high risk groups in the population

  • Diabetic retinopathy screening
  • AAA screening-men only!

Surveillance - Long term observation of individuals or populations

  • Screening for HCC over time in individuals with cirrhosis.
  • Earlier breast screening for patients with BRCA2 mutation.

Case Finding or “opportunistic” screening - The patient initiates the contact to seek medical help and the opportunity is used to screen for other conditions

  • HIV
  • Hepatitis
  • Health checks- “diabetes screens”, “CHD screens
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3
Q

Wht is the WHO criteria for a good screening test?

A

The Condition

  • The condition screened for should be an important one
  • There should be an acceptable treatment for patients with the disease
  • Should be a recognised latent or early symptomatic stage

The Test

  • There should be a suitable test or examination which has few false positives –specifity - and few false negatives – sensitivity
  • The test or examination should be acceptable to the population
  • The facilities for diagnosis and treatment should be available

The Cost

  • The cost, including diagnosis and subsequent treatment, should be economically balanced in relation to expenditure on medical care as a whole

Other Considerations

  • Prevalence must be known
  • Screening test must be simple, safe and scaleable
  • Diagnostic procedure following a positive screening results should be ethically acceptable to patients and public
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4
Q

What are ethical issues of screening?

A
  • Offered to the “well population”
  • Initiated by the health professional NOT the patient
  • Screening creates patients out of people
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5
Q

What must informed consent provide to patients before screening?

A
  • The disorder being screened for
  • Its clinical impact
  • The performance of the screening test.
  • Implications of a positive result
  • Subsequent diagnostic procedure
  • Future treatment options
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6
Q

What is Sensitivity, specificity and prevelance?

A
  • Prevalence: How common the disease is in the overall population
  • Sensitivity: How good the test is at detecting the disease
  • Specificity: How good the test is at not detecting normal individuals as having the disease
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7
Q

What is positive and negative predictive value?

A
  • Positive Predictive Value: The proportion of positive tests that are true positives
  • Negative Predictive Value: The proportion of negative results that are true negatives
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8
Q

What are considerations made before picking a “cut-off” value?

A

Consequences of Missing a Case

  • How serious is the illness or condition?
  • Would it be thought beneficial even if only a small percentage were detected?
  • What are the consequences economically / socially?
  • What would be publicly acceptable?
  • Cost

Consequences of Classifying an unaffected person as Positive

  • Anxiety
  • Unnecessary procedures
  • What are the dangers of these procedures and are they justified?
  • What would be publicly acceptable?
  • Cost
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9
Q

What is Antenatal Screening?

A
  • During pregnancy women are offered a series of screening tests for their fetal-maternal health
  • Women should be counselled and consent to the individual screening tests
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10
Q

What are types of Antenatal Screening?

A
  • Infectious Disease in Pregnancy (IDPS)
  • Sickle cell and alpha thalassaemia screening (SCT)
  • Foetal anomaly screening programme (FASP)
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11
Q

What is involved in Infectious Disease screening?

A

Measure with consent:

  • HIV
  • Hepatitis B
  • Syphillis

Failure to identify these puts the mother and baby at risk.

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12
Q

What happens if an infectious disease is confirmed by screening and an alternative method?

A

Referral to specialist care for the woman to be monitored throughout pregnancy

  • Hep B - Post-natal vaccination
  • Syphillis - Antenatal vaccination
  • HIV - Partner testing, Advice from specialist midwife, Drugs, Birth plan, Feeding advice and Partner and baby testing
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13
Q

How is Sickle cell and alpha thalassaemia screening (SCT) conducted?

A

Antenatal and Neonatal programme

  • Women should be screened as early as possible in pregnancy
  • Both are disorders of haemoglobin morphology, and can be life-threatening
  • Screened by HPLC analysis for haemoglobinopathy
  • If variant identified, partner screening may be offered
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14
Q

What is done if Sickle cell and alpha thalassaemia screening (SCT) is positive?

A
  • Diagnostic testing, amniocentesis or fetal blood sampling.
  • Most women referred to specialist care
  • If condition severe some women may terminate the pregnancy
  • Advice about how to live with the condition, and available treatments
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15
Q

Which conditions are screened in foetal anomaly screenings?

A
  • Down’s Syndrome (T21)
  • Edwards’ Syndrome (T18)
  • Patau’s Syndrome (T13)
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16
Q

What are features of Down’s Syndrome?

A
  • Extra copy of chromosome 21
  • Typical facial features: Slanted eyes, flat nasal bridge, protruberant tongue
  • Small stature
  • 1 in 700-1000 live births
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17
Q

What are clinical features of Down’s Syndrome?

A
  • Hypotonia, delayed development, severe learning difficulties
  • Congenital heart defects
  • Duodenal atresia
  • Recurrent Respiratory Infections
  • Hearing and visual impairment
  • Hypothyroidism
  • Early onset Alzheimers
  • Penetrance is highly variable, and some Down’s affected individuals lead long fulfilling lives.
18
Q

What is Edwards Syndrome?

A
  • Extra copy of Chromosome 18 (3 in 10,000 births)
  • Most babies die before or at birth (70%)
  • Following birth one-year survival is 13.5%*
  • 5-year survival is 12.5%* of the 13.5%
  • Medical interventions can prolong life
19
Q

What is the clinical features of Edwards Syndrome?

A
  • Small head with prominent back of head
  • Low set ears
  • Small jaw often cleft palate
  • Fists clenched with overlapping fingers
  • Club foot or rocker-bottom feet
  • Severe learning disabilities
  • Developmental delay
  • 90% will have congenital heart defect
  • Epilepsy is also common
20
Q

What is Patau’s syndrome?

A
  • Extra copy of Chromosome 13
  • Affects 2 in 10,000 births
  • Approx 50% babies die before or at birth
  • Following birth one-year survival is 11.5%*
  • 5-year survival is 9.5%* of the 11.5%
  • Medical interventions can prolong life
21
Q

What are clinical features of Patau Syndrome?

A
  • Small head and eyes
  • Significant facial dysmorphisms i.e. no eyes, single eye
  • Cleft palate
  • Extra digits
  • Significant developmental delay and learning disabilities
  • 80% with have heart condition
  • Kidney defect
  • Exomphalos
22
Q

What is Mosaicism is chromosomal defects?

A
  • Most trisomies are complete i.e. A full extra copy of the chromosome is present in every cell
  • This is because they are derived from the division of the maternal (almost always) gamete cells (egg cells)
  • The extra chromosome is carried through into all the cells of the developing child
  • However in some individuals a trisomy can occur in the developing embryo in some of the cell lines
  • This means that some cells are normal and some have a trisomy- this is mosaicism
  • Often the individual will display traits of the condition but to a lesser extent (not always)
  • Estimates 2 in 100 T21 cases are mosaic
23
Q

What are some choices given to women of the screening programme?

A
  • Decline screening
  • Have screening for Down’s syndrome only
  • Have screening for Edwards’/Patau’s syndromes only
  • Have screening for all 3 conditions
24
Q

How is featal anomaly screening conducted in the first trmester?

A

The woman is given a chance for Down’s and a combined chance for Edwards’ and Patau’s available 10-14+1 weeks of pregnancy

  • Measure free –BHCG & PAPP-A (pregnancy associated plasma protein A). Done on AutoDelfia/Delfia Xpress
  • Measure NT nuchal translucency measurement on scan (only able to do so between 11+2-14+1weeks).
25
Q

How is featl anomaly screening conducted in the 2nd trimester?

A

In the second trimester only Down’s syndrome can be screened for by a blood test. The test for Edwards’ and Patau’s is the 18-20 week foetal anomaly scan. Quadruples test

  • Measures AFP, free BHCG, uE3, Inhibin A
  • Taken 14+2-20 weeks of pregnancy. 15-16 weeks is optimal
  • Available for late bookers
  • No Edwards’ and Patau’s chance available
26
Q

What is Pregnancy associated plasma protein (PAPP-A)?

A
  • Trimester: First
  • Description: Produced by the placenta. Involved in the regulation of feto-placental growth
  • Changes in normal pregnancy: Blood concentration increases with gestational age
27
Q

What is Free-beta human chorionic gonadotrophin (BhCG)?

A
  • Trimester: First and second
  • Description: Produced by the placenta. Important for the development of the early fetal-placental unit
  • Changes in Normal Pregnancy: Blood concentration increases in the early weeks of pregnancy, then begin to fall after 10 weeks gestation
28
Q

What are characteristics of Alpha-feto Protein (AFP) as a marker fetal anomaly screening?

A
  • Trimester: Second
  • Description: Produced by the fetal yolk sac and liver, but does not appear to be required for normal fetal development
  • Changes in normal pregnancy: Blood concentration increases with gestational age
29
Q

What are characteristics of Inhibin-A as a marker in fetal anomaly screening?

A
  • Trimester: Second
  • Description: Produced by the ovary and placenta. Acts at pituitary to prevent ovulation during pregnancy.
  • Changes in Normal Pregnancy: Blood concentrations remain stable throughout the second trimester.
30
Q

What are characteristics of Unconjugated oestriol as a marker in fetal anomaly screening?

A
  • Trimester: Second
  • Description: Steroid hormone, synthesised by fetal adrenals, liver and placenta. The major estrogen of pregnancy
  • Changes in normal pregancy: Blood concentrations tend to increase with gestational age
31
Q

How is the Down’s Chance calculated?

A
  • Women are classed as higher chance if their risk is 1 in 150 or higher.
  • Women at higher chance are offered an invasive diagnostic procedure (amniocentesis or chorionic villus sampling) and karyotyping which will give a DIAGNOSTIC result
  • A low chance result does not mean that a pregnancy will be unaffected.
32
Q

What is the Multiples of Median?

A
  • Marker levels change throughout gestation.
  • Need to assess what is “normal” for a particular day of gestation.
  • Work out the median for each day of gestation
  • Calculate MoM = multiple of the median
  • Must know the gestational age of the pregnancy on the day of the blood sample. MoM is a indicator of how far away from the median a particular value is and once calculated is independent of gestation
33
Q

What is the equation for the MoM?

A

MoM = Marker conc in woman / Median marker conc for that day of gestation

34
Q

How is the chance calculation made?

A
  • Likelihood ratios are calculated for every marker and combined with the age risk.
  • Each woman has a prior age chance as chance of trisomy is higher in older women
35
Q

How are Higher Chance calculations resported?

A
  • Higher chance results 1:150 or higher are telephoned and emailed to Midwife
  • Electronic reporting
  • Midwife will contact woman and counsel regarding options
  • Woman will be offered invasive diagnostic procedure
36
Q

What is an amniocentesis?

A
  • Most common invasive procedure
  • Only viable from 15 weeks of pregnancy
  • Involves collection of amniotic fluid from amniotic sac under US guidance
  • Amniotic fluid contains fetal cells which can be karyotyped at Cytogenetics
  • Procedure-related loss around 1%
37
Q

What is Chorionic Villus Sampling?

A
  • Is used from 10 weeks gestation
  • Is offered to women high risk from first trimester screening.
  • Is more uncomfortable for the woman but can provide an early diagnostic result.
  • Procedure related loss is 1%
  • Overall pregnancy loss is 2-3% due to background risk at this gestation.
38
Q

What are factors that affect risk calculations ?

A
  1. Gestation​
    • Accurate gestation is essential and has large bearing on risk
    • Scan measurements must be provided CRL/HC + NT if appropriate
  2. Weight​​​
    • Larger women have bigger blood volume which dilutes blood markers and vice versa
    • Laboratory correct for this so accurate weight at time of sampling is essential
  3. Ethnicity
    • ​PAPP-A in 60% higher in Afro–Caribbean’s, lab corrects for this
    • Different ethnic groups have different levels of markers
  4. Smoking
    • Smoking affects the levels of some markers (reason not known)
    • Inhibin is 50% higher in smokers and PAPPA 20% lower in smokers. Lab corrects for this.
  5. IVF
    • Age of egg important, not age of woman, so need to know if egg/embryo frozen, or donated, need to know age of donor
    • HCG is slightly higher in IVF pregnancies, correct for this
  6. IDDM
    • AFP is slightly lower in Type 1 (reason not known) and PAPPA lower in Type 2 not on insulin
    • Lab corrects for this.
39
Q

What are sample requirements for risk calculations?

A
  • Minimum 5 ml blood sample in gel tube
  • Sample should be spun and separated ideally within 24 hours.
  • K-EDTA tubes must not be sent, K-EDTA chelates the Europium from the Delfia assays!
40
Q

What is dectection rate and screen postivie rate in fetal anomaly screening?

A

Detection Rate (sensitivity)

  • The proportion of screened Down’s/Edwards/Patau’s pregnancies that were identified as “higher chance” by the screening programme

Screen Positive rate

  • The proportion of all screened pregnancies that are given a higher chacne result
41
Q

How is Quaity control conducted in Fetal Anomaly screening?

A
  • Internal QC, 3 levels beginning, end of plate and spread through the plate
  • External QA- NEQAS every month
  • Weekly monitoring of screen positive rate and median MoMs
  • Monthly monitoring of gestation plots
42
Q

What is DQASS?

A
  • Run by fetal anomaly screening programme
  • Labs send 6 monthly anonymised data to DQASS and feedback performance on how well median equations are being controlled by labs
  • Also feedback to sonographers regarding NT performance

Paediatric Biochemistry (31 decks)

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