Screening for severe combined immune deficiency

Severe combined immune deficiency (SCID) is a rare inherited disorder caused by a deficiency or absence of T cells, a type of lymphocyte that plays a central role in immunity. This results in babies having little or no immune response. Babies born with SCID appear healthy at first, but are extremely vulnerable to recurrent infections.

The symptoms of SCID include recurrent life-threatening infections, failure to thrive, diarrhea and thrush. The average age of diagnosis is approximately six months. If not treated, most babies with SCID do not survive past one year of age.

Very rare. The incidence of SCID is estimated to be 1 in 50,000 to 1 in 60,000 live births. So only about one baby a year in New Zealand is expected to be identified with the classic form of SCID.

There are more than 15 known genetic causes of SCID.

While the incidence is low, the severity of the condition and the impact on families and health services is considerable.

Detection of SCID in the neonatal period enables treatment in the first few months of life before significant infections set in. Affected babies can receive life-saving treatment including antibiotics, isolation precautions and stem cell transplant. Treatment before infections take hold is extremely effective and averts long hospitalisation in intensive care.

Babies found through screening, compared with those discovered after infections have occurred, are significantly more likely to survive and have better health outcomes.

The screening laboratory will perform the test for SCID on the same blood spot card as used for the other screened disorders.

The screening test for SCID involves taking a punch from the dried blood spot and testing for T-cell receptor excision circles (TRECs). TRECs are small circles of DNA that indicate maturation of T cells; TRECs are reduced in SCID. Although the TREC assay is designed to detect SCID, in some instances it can also detect other T-cell disorders, for example DiGeorge anomaly.

Blood spot specimen collection will not change from the current procedure.

Notification of results will be as for other disorders screened for on the Guthrie card. Medical consultants will be available to assist with all abnormal SCID screening results. The testing regime includes genetic testing as confirmation. Treatment for severe combined immunodeficiency is coordinated through the Starship specialist paediatric immunology service.

Inclusion of screening for severe combined immunodeficiency within the NMSP has been comprehensively assessed and was endorsed by the National Screening Advisory Committee. 

The National Screening Unit (NSU) initially received a request from immunology clinicians to consider the addition of testing for SCID. The NMSP Policy Framework has a rigorous process for assessing nominations for new disorders for screening. Expert advisors to the NSU (including paediatricians, laboratory scientists, geneticists, economists) reviewed New Zealand cases, international practice, literature, and cost-effectiveness. Views of relevant consumer groups were sought.

Documents commissioned for the assessment process:

• Screening for SCID Literature Review (DOCX, 611KB)
• Screening for SCID Literature Review (PDF, 869KB)
• Cost-effectiveness of newborn screening for Severe Combined Immune Deficiency (PDF, 696KB)

The decision to add testing for SCID to the NMSP is in line with a number of countries that are considering or have started SCID screening. SCID is tested for in most states in the USA and is being implemented or piloted in several European countries.