Genetic Testing

More Information

The information below is produced by the CHI at Crumlin Department of Clinical Genetics  and is for educational purposes only. For more information on any of the content below, please contact:

The National Centre for Medical Genetics
CHI at Crumlin

Dublin 12
T: 00 353 1 409 6739

What types of genetic disorders can be tested?

There are two main types of genetic disorders:

  • Chromosome disorders Chromosome testing involves looking at all 46 chromosomes in the human cell
  • Single gene disorders Single gene testing involves targeting the DNA specific to that gene to look for alterations in the DNA sequence that might lead to a genetic disorder

When would we do chromosome testing?

Chromosome testing is done on newborn babies with birth defects or on couples who have had recurrent (≥3) miscarriages. Chromosome testing of at-risk relatives is available if something is picked up in a family member. If their relatives are at risk of carrying the same chromosome finding, that might put them or their children at risk of having a child with significant health problems.

When would we do single gene testing?

Single gene testing is done on children and adults who present with features typical of a specific genetic disorder (e.g. Cystic Fibrosis; Tuberous Sclerosis). The test is performed to confirm the clinical diagnosis. Once confirmed, we can offer genetic testing to other at-risk members of the same family. Their relatives could carry the same DNA alteration that might put them, or their children, at risk of having a child with health problems.

What is carrier testing?

Carrier testing is done to determine:

  1. If someone carries a chromosome abnormality that is causing health problems in their family or
  2. If someone carriers an alteration in a gene that might cause health problems for them or their (future) children

When would we ask a GP to take a blood sample for us?

As we are a National service we see families from all over the Republic of Ireland. Sometimes we need to offer cascade screening to elderly or sick relatives living far from Dublin. The result on this relative could have important implications for the wider family. We will ask GPs or practice nurses to help us with sample collection under these circumstances.

Sometimes we get referrals from GPs asking us to advise a family member of an affected individual. Depending on the condition (e.g. Cystic fibrosis), there are times when we, in turn, will ask the GP to help by arranging the genetic test, and we will offer the advice once the results are back.

The main reason we do this is to minimise appointments. Our staffing levels are between 10-15% equivalent European centres and our waiting times are > 1 year. We have started a strict triage process to attempt to manage the waiting list and reduce waiting times. Part of this triage is to arrange genetic testing for certain conditions with the GPs help.

For certain genetic conditions, where the test is a predictive test, all relatives must be seen by a member of NCMG before genetic testing can take place (e.g. Huntington’s Disease, cancer genetic tests).
Those that test negative do not require an appointment and are sent a letter of reassurance, thereby freeing up appointments for others.

What sort of blood samples do we request?

Sometimes we ask you to take blood to do carrier testing on an at-risk individual. We would send you details on how to go about this.

  • If the disorder in the family is a chromosome problem then blood needs to be taken in lithium heparin tubes and sent to the chromosome laboratory at The National Centre for Medical Genetics, Our Lady’s Children’s Hospital Crumlin, Dublin 12
  • If the disorder in the family is a single gene disorder then blood needs to be taken in EDTA tubes (the same as full blood count tubes) and sent to the molecular laboratory at NCMG, OLCHC, Crumlin, Dublin 12.
  • In both cases, we need to know the name of the affected relative and date of birth (if possible), which must be included on the form with the sample.

How do I go about arranging genetic testing?

Genetic testing is available for most genetic conditions, either locally at NCMG or abroad (arranged through NCMG). However, for many conditions it is advised that they see someone from the Clinical Genetics Department prior to testing.

For example, Part IV of the disability act 2005 states that genetic testing (processing of data in legal terms) should not be done without providing information or without consent being obtained. NCMG tries to provide information to patients and clinicians regarding if, when and how to pursue genetic testing.

Please note that there are times when we cannot offer testing. For example, we always need a sample from the affected individual before being able to advise any at-risk relatives. If the affected relative has died, then we may not be able to identify the family’s risk.

For families with whom we have worked previously, we do request that GPs and other health care professionals help us by taking bloods from at-risk family members. This is because many families cannot travel to us to have blood taken.

Why are the genetics laboratories so strict about documentation on blood samples?

The genetic laboratories adhere to the blood transfusion guidelines. They insist that names and dates of birth are exactly the same on blood bottles and the paperwork attached to the sample.

This is because a genetic test result not only has implications for the patient but also their relatives. If a sample mix up occurs the wrong risk could be given not only to the patient but also their relatives. Any samples that do not meet their guidelines are discarded.



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