1.       Cystic Fibrosis.

 

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MCQ Paper 2

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a.

is X-linked.

False

b.

about 1:25 of Caucasians is a carrier.

True

c.

the condition is less common in Asians and Afro-Caribbeans.

True

d.

meconium ileus occurs in 90% of affected neonates.

False

e.

most affected males are infertile.

True

f.

affected individuals are at increased risk of diabetes mellitus.

True

g.

affected individuals are at risk of fat malabsorption.

True

h.

carrier status can be detected in all individuals.

False

i.

the condition is due to a unique mutation.

False

j.

affected pregnancies can be detected by chorionic villus biopsy.

True

k.

affected pregnancies can be detected by amniocentesis.

True

l. women with cystic fibrosis should be discouraged from becoming pregnant. False
m. women with cystic fibrosis who inadvertently become pregnant should be encouraged to have termination of pregnancy. False
n. if a woman with cystic fibrosis becomes pregnant, at best the baby will be a carrier of cystic fibrosis. True
o. if a women with cystic fibrosis becomes pregnant and her partner screens negative for carrier status, there is no risk of the baby being affected. False
p. pulmonary hypertension is a bad prognostic feature for the woman with cystic fibrosis who becomes pregnant. True
q. neonatal screening became routine in the UK in 2007. True

List of contents.

  1. abbreviations

  2. key facts for the DRCOG

  3. expanded information for the MRCOG

    1. suggested reading and sources of information

    2. clinical features 

    3. the pancreas: malabsorption and diabetes

    4. treatment

    5. meconium ileus

    6. infertility

    7. pathophysiology 

    8. genetics

    9. screening for the mutations

    10. can cffDNA be used for screening or diagnosis?

    11. can PIGD be used for the high-risk couple?

    12. risk of transmission of CF

    13. woman whose sibling has CF

    14. EMQ1

    15. EMQ2

    16. EMQ3

    17. EMQ4

    18. EMQ variants

    19. amniocentesis, CVS and cffDNA

    20. cffDNA

    21. CF and pregnancy

    22. neonatal screening

    23. will early diagnosis improve outcomes?

    24. reason for "l" and "m" being false

       

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Abbreviations.

CF:            cystic fibrosis.

cffDNA:      cell-free fetal DNA.

CF-EU2:    Elucigene CF-EU2.

cffDNA:     cell-free fetal DNA. (See MCQ12, question 19.)

CFTR:       cystic fibrosis trans-membrane conductance regulator

CVS:         chorionic villus sampling

EMQ:         extended matching question.

PIGD:        Pre-implantation Genetic Diagnosis

SAQ:         short answer question.

TOG:         The Obstetrician and Gynaecologist.

OGRM:      Obstetrics, Gynaecology and Reproductive Medicine.

 

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Key facts for the DRCOG.

Causes progressive lung disease and early death.

Causes malabsorption.

Causes pancreatic disease and diabetes.

Affected males are infertile due to absence of the vas deferens.

Affected females have reduced fertility.

May cause meconium ileus in the neonate.

Autosomal recessive condition with > 1,000 mutations.

One in four pregnancies will be affected when both parents are carriers.

There is variation in the prevalence of carrier status, and so of the disease, between ethnic groups.

Caucasians have the highest prevalence, with ~ 1 in 25 being carriers.

This makes it the commonest hereditary disease in Caucasians.

Universal neonatal screening has been established in the UK since the spring of 2007.

This should give more accurate figures for the prevalence in non-Caucasian populations.

Life expectancy is about 31 years and improving.

It may be extended by heart & lung transplant.

Life expectancy for the child born today is thought to be about 50 years.

 

Clinical features.

There is a very good resume here.             

So, I haven't gone into great detail here.

Neonate:         

Child:

Adult:

Meconium ileus.

Bronchiectasis. 

Bronchiectasis.

Respiratory infection.

Cor pulmonale.

Chronic pancreatitis.

Failure to thrive. Pancreatitis. Pulmonary hypertension.

Malabsorption.

Diabetes.

Cor pulmonale

Steatorrhoea.

Biliary cirrhosis.

Diabetes.

Rectal prolapse

Gall stones.

Malabsorption

 

Portal hypertension.

Gall stones.

 

Hypersplenism. 

Bile duct strictures & cirrhosis.

 

Heat exhaustion.

Duodenal ulcer.

Intestinal obstruction.

 

 

Azospermia.

There is progressive lung damage from repeated infection.

Pulmonary hypertension and cor pulmonale result.

Malabsorption leads to feeding problems and malnutrition is common.

Most men with cystic fibrosis have azoospermia.

Women have reduced fertility.

Treatment.

You will get all the information you need here.

Treatment consists of:

    physiotherapy to get rid of thickened lung secretions,

    mucolytic drugs,

    antibiotics,

    digestive enzymes.

    dietary supervision and supplementation: high energy and protein with added vitamins A, D & E.

Screening does not exclude carrier status. 

It just reduces the risk.

If a pregnancy is to be tested, it will usually be by chorionic villus biopsy. (MCQ9, question 10)

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Pregnancy and CF.

There have been lots of pregnancies in women with cystic fibrosis.

And both OGRM and TOG have had articles on the subject.

So MRCOG candidates need to prepare an essay.

The TOG article give easily enough detail for a gold medal standard essay!

It is estimated that 30 - 40 women with CF will embark on pregnancy each year in the UK.

Presumably there will be more who choose TOP.

One would hope that the pregnancies would always be planned and after careful consideration.

A highly significant fact is that average life expectancy for women with cystic fibrosis is only about 31 years.

This means that they are not likely to see their children into adulthood.

Outcomes are determined by the severity of the disease.

If pulmonary hypertension exists, pregnancy is likely to be lethal.

Pulmonary hypertension should be a knee-jerk for the exam.

It tends to be lethal in pregnancy, full stop.

Similarly, marked reduction in pulmonary function is a poor sign and may worsen during pregnancy.

Whether this is permanent is debateable.

Lung colonisation with Burkholderia cepacia is a poor prognostic phenomenon.

Similarly, serious pancreatic involvement with diabetes or malabsorption are unhelpful.

The malabsorption that goes with CF means that most have a continuous battle to maintain a normal weight.

There was an extremely moving TV programme a few years ago about a brilliant young musician at Eton.

He wanted to conduct the school orchestra in a major work and the programme followed his progress.

He was seen on a number of occasions with his CF consultant.

On one, she stressed the need for him to maintain a high calorie intake.

As this was by way of eating confectionary of one kind or another, it is the sort of advice that would be welcome by most children.

But he was finding it a real struggle and got no pleasure from it.

The point of this long tale is that the woman who is underweight will have more problems with pregnancy.

And some women may not be able to increase their calorie intake sufficiently for breast feeding.

 

 

 

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Expanded information for the MRCOG and to help facts stick.

The above is what you need to know. 

The following is a more detailed explanation to help the facts stick.

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Expanded explanation for the MRCOG and to help facts stick.

Suggested reading.

There is good, brief information here.

There is a lot of good information on the Cystic Fibrosis Trust's website.

TOG had an article in 2009 on CF and pregnancy.

OGRM published an article in January 2010 on respiratory disease and pregnancy, with a section on CF.

There is a good article in Emedicine.

Wikipedia has a detailed article with a smattering of the history.

There is a model essay on this website.

The "Handbook of Obstetric Medicine" by Nelson-Piercy has a useful section.

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Clinical features.

Clinical manifestations are (with lung and pancreas being especially important).

There is a very good resume here.             

Neonate:          

Child:

Adult:

Meconium ileus.

Bronchiectasis.

Bronchiectasis.

Respiratory infection.

Cor pulmonale.

Chronic pancreatitis.

Failure to thrive.

Pancreatitis.

Diabetes.

Malabsorption.

Malabsorption.

Malabsorption.

Steatorrhoea.

Diabetes.

Gall stones.

Rectal prolapse

Biliary cirrhosis.

Bile duct strictures.

 

Gall stones,

Intestinal obstruction.

 

Portal hypertension.

Duodenal ulcer.

 

Hypersplenism.

Azoospermia.

 

Heat exhaustion.

Female sub-fertility

There is progressive lung damage from repeated infection.

Pulmonary hypertension and cor pulmonale result.

Malabsorption leads to feeding problems and malnutrition is common.

Most men with cystic fibrosis have azoospermia.

Women have reduced fertility.

     

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Pathophysiology.

The CTFR gene encodes for a protein involved in water and chloride transport across cell membranes.

If this is not working correctly in mucous membranes, mucus becomes abnormally thick.

This reduces clearance of mucus and bacteria leading to infection and tissue damage.

Average life expectancy is about 31 years and improving.

Death is mainly from lung damage and heart-lung transplant may prolong life.

Studies are being done to see if it is possible to introduce normal genes to improve the prognosis.

But these are in their infancy.

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Genetics.

CF is an autosomal recessive condition (see MCQ8, question 2).

And the commonest hereditary disease in the Caucasian population.

It is due to abnormalities of the CFTR gene on chromosome 7.

When I checked in October 2010 the Cystic Fibrosis Mutation Database had information on 1819 mutations.

Only four occur at a frequency of 1% or above in the UK.

The most common is F508del (previously known as DF508) – del or “D” stands for “deletion”.

It accounts for about 75% of cystic fibrosis in the UK.

F508del is a 3 base-pair deletion which results in the deletion of a phenylalanine amino acid residue at position 508.

But nobody is going to ask you.

80 - 90 % of carriers will be identified by seeking these four.

There is variation in the prevalence of carrier status, and so of the disease, between ethnic groups.

Caucasians have the highest incidence, with ~ 1 in 25 being carriers.

From this you can deduce that the risk of both partners being carriers is about 1 in 625 (1 in 25 x 1 in 25).

We used to think that cystic fibrosis was rare in the Asian and Afro-Caribbean populations.

Genetics Home Reference says that CF affects about 1 in 17,000 African Americans and 1 in 31,000 Asian Americans.

Martin Schwarz (see below) reckons the prevalence of carrier status in both these populations is at least 1 in 50.

Exact figures are unknown.

I imagine that the policy of screening all neonates for cystic fibrosis will yield more accurate data.

For these populations, the risk of both partners being carriers is ~ 1 in 2,500 (1 in 50 x 1 in 50).

If both parents are carriers, each pregnancy has a 1 in 4 risk of being affected.

This results from the possible combinations of genes:

Mother:   has one normal gene (N) + one CF gene (CF).

Father:    has one normal gene (N) + one CF gene (CF).

The mother’s normal gene can combine with either of the father’s genes giving:

1. Maternal N + paternal N -        babe has no CF genes (‘normal’).

2. Maternal N + paternal CF -      babe is a carrier.

The mother’s CF gene can combine with either of the father’s genes giving:

3. Maternal CF + paternal N -      babe is a carrier.

4. Maternal CF + paternal CF -    babe has cystic fibrosis.

This means that:

one baby gets two normal genes,

one baby gets two CF genes and cystic fibrosis

and two babies get one normal and one CF gene and become carriers. 

 

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Prevalence.

We are interested in the prevalence of the gene and cystic fibrosis itself.

The prevalence is only known accurately for the Caucasian population.

1 in 25 of the Caucasian population has the gene.

This gives a risk of a couple both being carriers of 1 in 25 x 1 in 25.

This works out at 1 in 625.

The couple who are both carriers have a risk that 1 child in 4 with have CF.

So, the overall prevalence of CF in the Caucasian population works out at about 1 in 625 x 1 in 4.

This is about 1 in 2,500.

But it only applies to neonates.

Mortality rates are much higher for those with CF than the general population.

So the 1 in 2.500 figure falls off rapidly with age.

The prevalence of carrier status in Afro-Caribbeans and Asians is less.

Unfortunately, exact figures are not yet known.

The risk used to be thought very small, but is now reckoned to be higher, partly due to better identification of cystic fibrosis in these populations.

Martin Schwarz (see below) reckons that the prevalence of carrier status in the Indian sub-continent could be at least 1 in 50, with a similar figure for Afro-Caribbeans.

A 1 in 50 chance of being a carrier gives a figure for the prevalence of cystic fibrosis of 1 in 10,000 (1 in 50 x 1 in 50 x 1 in 4).

As cystic fibrosis is uncommon in these populations and under-diagnosed, it is no surprise that when you talk to doctors from Africa, the Caribbean and Asia, most have not encountered a case before coming to Europe.

Routine screening of all neonates for CF status (see below) is routine in the UK.

This will give us accurate figures for the prevalence of carrier status in non-Caucasian populations in the UK.

 

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Screening for the mutations.

The problem in screening in cystic fibrosis is the multiplicity of mutations (see the genetics section).

And the fact that some are extremely rare.

 Clearly it would not be feasible to screen every potential carrier for every known mutation.

And, even if you could, it is likely that others remain to be identified.

Usually you will be screening because a family member has the disease.

If it is due to one of the rare mutations, you will not be able to identify a carrier, or exclude carrier status unless you know what the mutation is.

Even knowing the mutation, you could not guarantee that the individual did not have another.

For this reason potential carriers are not given a zero risk of being a carrier after screening, merely a much reduced one.

Martin Schwarz tells me (October 2010) that the routine test used in the North West is the Elucigene CF-EU2.

This is a commercial kit which screens for the 50 most common mutations found in the European population.

The European population is far from uniform.

Even in a small island like Britain there are significant regional variations in patterns of mutation frequency.

We are a bunch of mongrels with a rich assortment of ancestors.

CF-EU2 has a detection rate of 93-94% detection rate in the white population of the North West of England.

It is widely used throughout England and gives 90% coverage overall in the white population.

Let's imagine we test a white person from the NW with a background carrier prevalence of 1:25.

We know the test has an accuracy of about 93% in this population.

A negative result would reduce the risk to 7% of 1:25 or approximately 1: 350.

The test does not work anything like so well for other populations.

Martin tells me that the situation for other ethnic groups is no so good.

For example, CF-EU2 only has a sensitivity of only 25-30% in UK Pakistanis and work is being done to remedy this.

cffDNA  has been used to make the diagnosis.

It may one day have a role in screening and diagnosis for a range of anomalies, including CF.

But it has not yet been developed sufficiently to meet these challenges.

 I hope this makes some sense!

 

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Cell-free fetal DNA.

It is hoped that cffDNA will become a viable, non-invasive screening and diagnostic test.

At the moment is is a research tool and not a practical test for CF.

 

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Pre-implantation Genetic Diagnosis.

Couples at high risk of having a baby with CF may opt for PIGD.

This is one of the most common reasons for PIGD in the UK.

It cannot completely exclude CF, the problem of the number of mutations applying here too.

It sounds very attractive, but it means the woman going through the risks and stresses of IVF.

And several attempts may be necessary before getting a successful pregnancy.

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Pregnancy in women with CF.

There have been lots of pregnancies in women with cystic fibrosis.

One would hope that these would always be planned and after careful consideration.

A highly significant fact is that average life expectancy for women with cystic fibrosis is only about 31 years.

This means that they are not likely to see their children into adulthood.

There was an article in TOG in 2009.

This makes it a topic MRCOG candidates need to prepare for a possible SAQ.

Outcomes are determined by the severity of the disease.

If pulmonary hypertension exists, pregnancy is likely to be lethal .

Pulmonary hypertension should be a knee-jerk for the exam.

It tends to be lethal in pregnancy, full stop.

Similarly, marked reduction in pulmonary function is a poor sign and function may worsen during pregnancy.

Whether this is permanent is debateable.

Lung colonisation with Burkholderia cepacia is a poor prognostic phenomenon.

And serious pancreatic involvement with diabetes or malabsorption are unhelpful.

“l” and "m" are false because it is not your business to encourage or discourage the woman from becoming or staying pregnant.

It is your job to ensure that she fully understands the risks.

These will suffice to discourage most women.

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Risk of transmission of CF or carrier status to the fetus.

This has been covered in the genetics section, but is worth repeating.

If both parents are carriers, each pregnancy has a 1 in 4 risk of producing a baby with CF.

This results from the possible combinations of genes.

If you are not already comfortable with this, get a piece of paper and jot down all the possible gene combinations.

The Mother has one normal gene (N) + one CF gene (CF).

The Father has one normal gene (N) + one CF gene (CF).

We will start by considering the mother's "normal" gene and how it can combine with the paternal genes.

Then we will look at the mother's CF gene and how it can combine with the paternal genes.

There are only 4 possible combinations, so it is not that difficult!

 

The mother’s ‘normal’ gene can combine with either of the father’s genes giving:

1. Maternal N + paternal N -        babe has no CF genes (‘normal’).

2. Maternal N + paternal CF -      babe is a carrier.

 

The mother’s CF gene can combine with either of the father’s genes giving:

3. Maternal CF + paternal N -      babe is a carrier.

4. Maternal CF + paternal CF -    babe has cystic fibrosis.

 

Now you have the four possible combinations.

These determine the possible outcomes for the baby.

This means that a baby has:

     a 1 in 4 chance of getting two normal genes,

    a 1 in 4 chance of getting two CF genes and cystic fibrosis,

    and 2 chances in 4 of getting one normal and one CF gene and becoming a carrier.

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The woman whose sibling has CF.

A common question would relate to the counselling of a woman whose sibling has cystic fibrosis.

The affected sibling means that the parents must be carriers.

It could be that one has CF and the other is a carrier, but that is unlikely and they would need to tell you if it was an EMQ.

When she was conceived, all 4 of the above possibilities applied to her.

The EMQ would tell you that she does not have CF or "is healthy".

If she doesn’t have the disease, then only three of the above 4 possibilities remain: 

    one possibility that she has no CF genes,

    and two possibilities that she is a carrier.

So, it is 2 in 3 that she is a carrier and 1 in 3 that she is "normal". 

You would need to know the risk of the partner being a carrier.

Then you would work out the risk of a child having CF.

Usually they would tell you he is Caucasian or give you a figure.

Assuming he is Caucasian, his risk is 1/25.

The risk that both she and the partner are carriers is 2/3 (her risk) x 1/25 (his risk).

The risk of the baby having CF would be 2/3 x 1/25 x 1/4; i.e. 2/300 or 1 in 150.

 

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EMQ 1.

A woman is 8 weeks pregnant and known to be a carrier of cystic fibrosis.

Her husband is Caucasian.

What is the risk of the child having cystic fibrosis?

Click here for the option list.

 

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EMQ 2:

A healthy woman attends for pre-pregnancy counselling.

Her brother has cystic fibrosis.

Her husband is Caucasian.

He has been screened for cystic fibrosis.

The test was negative.

What is the risk of them having a child with cystic fibrosis?

Click here for the option list.

 

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EMQ3.

A healthy woman is a known carrier of cystic fibrosis.

She attends for pre-pregnancy counselling.

Her husband has cystic fibrosis.

What is the risk of a child being affected?

Click here for the option list.

 

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EMQ 4.

A healthy Caucasian woman is 10 weeks pregnant.

Her husband is a known carrier of cystic fibrosis.

Which test would you arrange.

Click here for the option list.

 

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Variants on the above EMQ themes.

Variants would be the woman who has cystic fibrosis, the woman whose sister had had an affected child etc.

If the woman has cystic fibrosis, she must hand on a CF gene to all of her children.

All of them must at least be carriers.

The risk of a child getting CF would be determined by the risk of the partner being a carrier too.

If her sister had an affected child, she (the sister) must be a carrier (assuming she does not have CF).

This means that one of her parents must be a carrier.

(It is possible that both are carriers or one has CF, but this is much less likely.)

If the sisters have the same parents, the risk of your “patient” being a carrier is 1 in 2.

It would obviously be higher if both parents were carriers or one had CF.

The other issue is screening.

Imagine that you had a 1 in 10 risk of a condition.

A test exists that can screen with an accuracy (sensitivity) of 50%.

If you have the test and it is negative, you reduce your risk by 50%.

I have dealt with mutation screening here.

As noted in the screening section, the test used in the NW screens for the 50 most common mutations.

The accuracy is about 90% overall for the UK Caucasian population.

So, if you are Caucasian and have a negative screen, your chance of having a mutation goes down to 10% of the risk you started with.

As a Caucasian, this risk is 1: 25.

So, the final risk is 1 in 250.

If you have to deal with this in the exam, you should be told the sensitivity of the test.

Otherwise, I would use 90% sensitivity as it makes the arithmetic easier.

In real life and a MRCOG OSCE you would offer referral to a geneticist for screening and further discussion.

You could drive yourself mad with all the permutations.

I suspect that this comes up frequently in some form in the exam.

So play around with the basics as outlined above.

I prefer Su Doku myself. 

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Meconium ileus.

Meconium ileus is diagnostic.

But only about 10% of babies with cystic fibrosis present in this way.

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Fertility.

Sadly, close to 100% of males are sterile due to absence of the vas and the problem is not yet remediable in most cases.

There is the possibility of some becoming fathers.

MESA, microsurgical epididymal sperm aspiration, is used followed by ICSI, intra-cytoplasmic sperm injection (see MCQ 7, question 16).

Partners obviously need to be screened for cystic fibrosis carrier status and there are other issues like the life-expectancy of the man. 

Fertility in women is also reduced.

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The pancreas is affected.

This results in malabsorption, particularly of fats and fat soluble vitamins, and a major risk of diabetes.

The woman with CF must be screened for gestational diabetes.

Usually in the first trimester and again around 24 weeks.

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Amniocentesis and CVS.

Both amniocentesis and CVS will permit the diagnosis to be made.

A negative test cannot exclude the diagnosis.

It simply gives a risk-assessment.

This is done by eliminating the 50 or so common mutations.

And reducing the risk to about 7% of the starting risk for NW Caucasians.

And to about 10% for UK Caucasians.

                       

 

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 Reasons for "l" and "m" being false.

“l” and "m" are false because it is not your business to encourage or discourage the woman from becoming or staying pregnant.

It is your job to ensure that she fully understands the risks.

These will suffice to discourage most women.

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Neonatal screening.

Routine screening of all neonates in England began in 2006, with all regions included by the spring of 2007.

Interestingly, the advice to the Minister of State for Health in 2003 was that screening should not be offered.

However, it was introduced in Scotland in 2003 and Northern Ireland in 2006.

In Wales it is offered but not routine.

This meant that 20% of UK babies were screened, so the Minister decided to introduce it everywhere on the grounds of “equity”.

The test is done on the same heelprick used for screening for phenylketonuria, hypothyroidism etc., so there is no extra hassle or risk for the baby.

The test is biochemical, based on looking for immuno-reactive trypsinogen.

So it doesn’t involve the costs of genetic screening looking for all the above mutations.

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Will early diagnosis improve outcomes?

The rationale of screening is that early diagnosis is beneficial to the baby.

Before universal neonatal screening, most affected babies were not diagnosed until they were more than a year old.

By then they have had multiple admissions with failure to thrive and chest infections.

Littlewood delivered a comprehensive review in a lecture to the European Cystic Fibrosis conference.in 2004.

We continue to hope that early intervention will have long term benefits, particularly on lung function.

But screening hasn’t been around long enough to see if this will be the case.

And there have been reports that it may not be of benefit to lung function.

There was a Cochrane Review in 2009. (Cochrane Database Syst Rev. 2009 Jan 21;(1):CD001402.)

The main conclusions were:

    "Nutritional benefits are apparent.

     Screening provides potential for better pulmonary outcomes, but confounding factors influenced long-term pulmonary prognosis".

Only time will tell!

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I am indebted to Martin J Schwarz, Consultant Clinical Molecular Geneticist, St Mary's Hospital, Manchester.

He provided me with expert guidance on this subject, added hugely to my knowledge and patiently corrected my misunderstandings.

 

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