Carrier testing (genetic screening): genetic information for family planning
For a long time, genetic risk only became apparent once it had already materialised in the form of a diagnosis. Clinical genetics came into the picture too late, at a time when decisions had already been influenced by urgency, uncertainty or, quite simply, a lack of prior information.
Today, that paradigm has changed.
The possibility of incorporating genetic information before pregnancy introduces a new way of understanding family planning: more proactive, more thoughtful and, above all, more informed. In this context, carrier testing — also known as genetic carrier screening or genetic compatibility testing — makes it possible to identify whether a person is a carrier of pathogenic variants associated with hereditary diseases that could be passed on to their offspring, even where there is no prior suspicion.
This distinction is essential. Many couples begin trying for a baby with the conviction that, if there is no family history, the genetic risk is virtually non-existent. However, a significant proportion of hereditary conditions are passed on silently across generations, leaving no clinical trace in those who carry them.
“In our practice, we frequently see couples with no known family history who nevertheless share variants of reproductive significance,” explains Dr Raluca Oancea Ionescu, head of the Clinical Genetics Unit at the Publio Cordón Memorial Hospital. “That is the starting point: understanding that the absence of a family history does not mean there is no risk.”
What is carrier testing and what does it really mean to be a carrier?
Carrier testing is, in essence, a tool for anticipation.
Unlike other genetic tests, it is not designed to confirm a current condition, but to explore a future possibility: that of passing on certain hereditary conditions to one’s offspring.
To understand its significance, it is necessary to consider a key concept: what it means to be a carrier.
In many hereditary conditions — particularly those known as autosomal recessive disorders — each person has two copies of the same gene with a mutation. When one of these copies is mutated and the other remains functional, that functional copy is usually sufficient to maintain biological normality. The result is that the person does not develop the disease and, in most cases, shows no symptoms. There are some conditions in which carriers may exhibit some clinical manifestations.
It is precisely this apparent normality that causes the risk to go unnoticed. “That is why this study makes sense,” explains Dr Oancea. “The risk may exist even when there is no previous family history.”
However, reducing the concept of a carrier to a ‘healthy person without symptoms’ would be to oversimplify a more complex reality.
There are X-linked disorders in which some female carriers may present with mild or variable symptoms. Furthermore, the development of broader genetic panels has shown that certain findings may have clinical implications beyond the reproductive sphere.
“In genetics, it is not only important to detect a variant, but to understand what it means in each individual case,” adds Dr Oancea. “And that always requires an appropriate clinical interpretation.”
What is the real purpose of carrier testing?
The value of carrier testing lies not in the individual, but in the relationship between two individuals.
This is probably the most difficult shift in perspective to grasp. In medicine, we are used to assessing a person’s risk. In reproductive genetics, however, the risk is relational: it arises from the combination of two genetic profiles.
Two completely healthy people may share alterations in the same gene without realising it. And it is that coincidence — invisible in everyday life — that can be significant for their offspring.
“The risk does not lie with the individual in isolation, but in how the variants combine within the couple,” points out Dr Oancea.
When this coincidence occurs, the test makes it possible to identify a scenario that would otherwise remain hidden: the possibility that a child might inherit two altered copies of the same gene.
What risk does the carrier test detect in a couple?
In autosomal recessive disorders, when both partners are carriers of the same genetic alteration or of different alterations but within the same gene, each pregnancy carries a 25 per cent probability that the child will be affected, a 50 per cent probability that they will be a healthy carrier like their parents, and a 25 per cent probability that they will not inherit any mutation.
This seemingly simple fact contains an aspect that is important to understand clearly: the risk is neither cumulative nor influenced by the outcome of previous pregnancies. Each pregnancy is an independent event, in which the probability is recalculated from the outset.
This conceptual framework underpins the current recommendations of leading scientific societies, such as the American College of Medical Genetics (ACMG), which promotes genetic screening in the preconception setting.
Which conditions are covered by carrier testing?
Current technical capabilities allow hundreds of genes to be analysed simultaneously. However, the value of the test lies not in the breadth of the analysis, but in its clinical relevance.
Genetic panels are designed to identify hereditary conditions with a significant impact, generally with early onset and a major effect on quality of life.
Among the most common are cystic fibrosis, spinal muscular atrophy, fragile X syndrome and various inherited metabolic disorders. Many of these conditions first appear in childhood and require complex medical management from an early stage.
However, not all panels are the same.
In clinical practice, these tests can be tailored to each couple’s specific circumstances, with the scope expanded in situations such as consanguinity or a relevant family history.
Scientific evidence supports this selective approach. An analysis based on more than 14,000 exomes highlights the importance of optimising the design of panels to prioritise clinically relevant conditions, rather than indiscriminately increasing the number of genes analysed:
When does it make sense to carry out carrier testing?
The timing of the test directly determines its usefulness.
From a clinical perspective, the ideal scenario is the pre-pregnancy period. It is at this stage that genetic information can be most usefully integrated into decision-making, without the time pressure introduced by an ongoing pregnancy.
Both the ACMG and the American College of Obstetricians and Gynaecologists (ACOG) recommend this pre-conception approach.
“That is when genetic information is truly useful,” says Dr Oancea. “It allows you to make decisions with more time and with more options available.”
A common genetic reality
From a biological perspective, being a carrier is not the exception, but the norm. Most people carry some form of recessive genetic variant, although in the majority of cases this has no clinical consequences.
The real problem is not being a carrier, but being unaware of it when that information may be relevant. “We are not looking for a current illness,” explains Dr Oancea, “but rather to understand a risk that may have implications for the future.”
The role of genetic counselling
Carrier testing is not a one-off test, but a process. It includes taking a medical history, pre-test counselling and the personalised interpretation of the results. This support is not incidental, but essential.
“Genetics is not just a diagnostic tool; it is a decision-making tool,” points out Dr Oancea. “And for that decision to be truly informed, the information must be understandable and properly contextualised.”
In this context, a second opinion takes on particular importance, especially in complex situations or when results are open to interpretation.
What happens if the result is positive?
A positive result does not imply disease, but rather the identification of a possibility.
The next step is to assess the test results for the couple. Only when both partners share a mutation in the same gene — or in certain scenarios linked to the X chromosome — does the risk become clinically significant.
From there, various reproductive options can be considered, always depending on the context and the couple’s preferences. “A positive result is the start of an assessment, not a conclusion,” explains Dr Oancea.
Limitations of carrier testing
Like any diagnostic tool, carrier testing has its limitations. A negative result reduces the risk, but does not eliminate it entirely. This may be due to technical limitations, variants not yet described, genes not included in the analysis, or future changes in genetic interpretation.
“Genetics helps us to better understand the risk, but not to eliminate it entirely,” notes Dr Oancea.
Information to help you make better decisions
Carrier testing does not eliminate uncertainty, but it transforms it.
It allows us to move from an unknown risk to an understood risk. And in medicine, that difference is crucial. “Genetics does not make the decision for the couple, but it does provide them with the information they need to make a better decision,” concludes Dr Raluca Oancea Ionescu.
Frequently asked questions about carrier testing
What exactly is carrier testing and what information does it provide?
Carrier testing is a genetic test that identifies whether a person is a healthy carrier of certain hereditary conditions.
It is not designed to diagnose a condition in the person being tested, but rather to detect whether there is a possibility of passing it on to their offspring. Its value lies in anticipating a risk which, in most cases, would not be apparent due to the absence of symptoms or a family history.
In this sense, the test does not affect the person’s current health, but it can influence the way a couple approaches their family planning.
Does it make sense to have carrier testing if there is no family history?
Yes. Many genetic conditions are passed on silently through generations. Carriers do not usually show any symptoms, so the risk can go completely unnoticed.
Therefore, the absence of a family history does not guarantee the absence of risk. The test allows these situations to be detected before pregnancy, when it is still possible to understand and weigh up different reproductive options.
What happens if both partners are carriers?
If both partners are carriers of a mutation in the same gene, there is a risk of passing it on to their children.
In autosomal recessive disorders, each pregnancy carries a 25 per cent chance that the child will develop the condition, a 50 per cent chance that they will be a healthy carrier, and a 25 per cent chance that they will not inherit the mutation.
This result does not imply an automatic decision, but it does initiate a process of clinical assessment in which different reproductive options can be considered on a case-by-case basis.
Does carrier testing detect all genetic conditions?
No. Although current technology allows a large number of genes to be analysed, no test can identify all genetic disorders.
A negative result reduces the risk, but does not eliminate it completely. This may be due to technical limitations, as yet unknown variants, or genes that are not included in the analysis.
For this reason, results must always be interpreted within the context of appropriate genetic counselling.
When is the best time to have carrier testing?
The most appropriate time is before pregnancy, during the preconception phase.
Having the test at this stage provides sufficient time to understand the genetic information and make decisions with greater flexibility and peace of mind. During pregnancy, turnaround times and the available options are more limited.
For this reason, leading scientific societies recommend incorporating this test into reproductive planning.
This content is for information and educational purposes only and is not a substitute for an individual medical assessment.
The information in this article is provided for general awareness and does not replace an individualised medical assessment.
