Carrier screening is a form of genetic testing that can detect if an individual or couple are carriers of an autosomal recessive and/or X-linked genetic condition, when there is no a priori increased risk based on either partner’s personal or family history.^1,10 Involving a simple mouth swab or blood test, it allows a woman or couple to understand their risk of passing an inherited condition on to their children.

Despite being individually rare, inherited rare diseases are collectively common. Approximately 1–2% of non-consanguineous couples have a one in four chance of having a child with a severe autosomal recessive or X-linked recessive genetic condition with each pregnancy.³ This means around one in 200 babies are born with an autosomal recessive or X-linked recessive condition, which represents approximately four times the frequency of pregnancies affected by Down syndrome (one in 800).⁴ Rare diseases are often complex and can cause severe disability, impaired quality of life and premature death.¹¹ Around 20% of infant mortality and 10% of paediatric hospital admissions are associated with inherited disorders.⁵

Carrier screening enables women and couples who are planning a family, or in early pregnancy, to make informed reproductive choices in line with their personal wishes and values. Clinicians involved in family planning and antenatal care, such as general practitioners, fertility specialists, obstetricians and genetic health professionals, are ideally placed to have these conversations with patients.

Options available for 'carrier women' and ‘carrier couples’ following preconception screening include:²

• proceeding to a pregnancy but accepting the possibility that they may give birth to an affected child – in which case, they may be better placed to optimise the child’s health and wellbeing (through early diagnosis and intervention)
• undergoing fetal diagnostic testing (via amniocentesis or chorionic villus sampling) and terminating affected pregnancies or preparing for the possibility that they may give birth to an affected child
• undergoing in vitro fertilisation (IVF) with preimplantation genetic testing (PGT) to screen embryos before they are implanted in the uterus
• undergoing IVF with donor eggs, sperm or embryos
• choosing to adopt or not have children.

Options available for 'carrier women' and ‘carrier couples’ identified during early pregnancy include:²

• undergoing fetal diagnostic testing (via amniocentesis or chorionic villus sampling) and terminating affected pregnancies or preparing for the possibility that they may give birth to an affected child
• proceeding without fetal diagnostic testing and preparing for the possibility that they may give birth to an affected child.

Carrier women = female carriers of an X-linked recessive condition
Carrier couples = couples in which both partners are carriers for the same autosomal recessive condition

Traditionally, carrier testing was offered to adults with a family history of a recessive condition; to the partners and relatives of identified carriers; to the partners of people with the condition and/or to people of a particular ethnicity.¹⁰ However, this approach only identifies a minority of carriers, since the majority of affected children are born to parents with no previously known family history, and only a minority of relatives in high-risk families request carrier testing. Thus, it is now recommended that carrier screening be offered to all women planning a pregnancy or in the first trimester of pregnancy, regardless of family history or geographic origin.²

Ideally, carrier screening should be offered to women or couples who are planning a family as early as possible to allow them time to consider a wider range of reproductive options.³ As such, preconception carrier screening should be considered the ‘gold standard’ of care. When carrier status is determined before pregnancy, carriers should be referred for genetic counselling to review their options.² Carriers then have the option of taking steps to avoid having an affected child if they so choose.

If a woman or couple presents for the first time in pregnancy, they should be offered carrier screening if they are still within the first trimester. Those found to have a higher risk of having an affected child following screening should then be referred for genetic counselling to understand their options.

Carrier screening does not replace non-invasive prenatal testing (NIPT) and combined first trimester screening (CFTS). These tests should still be offered to all pregnant women to screen for an increased risk of having a child with Down syndrome or other chromosomal disorder, as carrier screening does not detect chromosomal anomalies and NIPT and CFTS do not identify carrier status of single-gene disorders.^1,9

Partners can be screened sequentially or simultaneously:^2,3,12

• Sequential screening involves testing one partner first – usually the woman, as carrier status is more relevant for X-linked conditions – and the male partner only if the female partner is found to be a carrier of an autosomal recessive condition. Sequential screening is generally more cost-efficient when screening for one or few conditions given that many couples will find they do not need to test the second partner. This method may be preferred where time is not limited (preconception screening).
• Couple screening involves testing both partners simultaneously to determine a combined ‘low probability’ or ‘high probability’ result. A high probability is given when results identify both partners as carriers for the same autosomal recessive disorder or the woman is identified as a carrier for an X-linked disorder. Couple screening is the preferred test in early pregnancy, when time is of the essence due to laws around timing of termination (which differ by state and territory). It may also be preferred with expanded carrier screening due to the high chance of any individual being a carrier of one or more conditions.

Medicare provides rebates for some genetic tests.* Patients can expect to pay fees in the range of:^†

• single-condition screening: $100–$200
• three-condition screening: $350–$400
• expanded carrier screening: $580–$900.

*The cost of screening for haemoglobinopathies is generally covered by state/territory government funding. Relatives of affected individuals/carriers may be eligible to access funded testing via Medicare or state/territory government funding. There are MBS items available for carrier screening, fragile X syndrome, SMA and cystic fibrosis. ^8
†Fee estimates are per person. Actual fees vary by provider.

A list of providers can be found at: www.mackenziesmission.org.au/other-options-for-carrier-screening

A list of major genetic centres where patients can be referred for diagnosis and management of genetic conditions is available at: www.genetics.edu.au/genetic-services/general-genetics-clinics