CLSI NBS06—Newborn Screening for Severe Combined Immunodeficiency and Other Related Severe T-cell and B-cell Immunodeficiencies provides an update on newborn screening (NBS) for severe combined immunodeficiencies (SCID). It discusses the importance of harmonizing T-cell receptor excision circle cutoffs and reporting in NBS of infants with SCID and non-SCID severe T-cell lymphopenia. CLSI NBS06 also discusses the relevance of NBS for severe B-cell deficiencies using κ-deleting recombination excision circles as a biomarker in dried blood spots. CLSI NBS06 is written in the context of a decade of SCID NBS testing in the United States and global implementation in several regions and countries.

CLSI NBS06-Ed2 replaces CLSI NBS06-Ed1, published in 2013. Several changes were made in this edition, including:

• Summarizing advances in the field of NBS for SCID since 2013, as well as the current status of global NBS for SCID
• Updating methodology for measuring TREC
• Updating TREC screening changes for multiplexing with other molecular assays, such as assays for survival motor neuron genes when screening for spinal muscular atrophy since its addition to the RUSP in 2018
• Adding methodology for measuring KREC for detecting severe B-cell deficiencies
• Adding recommendations for harmonizing results interpretation and reporting practices for SCID NBS
• Providing a more prescriptive approach to follow-up and confirmatory testing

Compared with the previous edition of CLSI NBS06, a more in-depth review of these topics, based on nearly a decade of screening experience in certain programs, lessons learned about limitations and successes, and practical application of SCID NBS in terms of identification of secondary target diseases and their adjudication for diagnostic confirmation and management, is provided.

CLSI NBS06 discusses the current global status of newborn screening (NBS) for severe combined immunodeficiency (SCID) and describes the methodology used to measure T-cell receptor excision circles (TREC), the extrachromosomal DNA fragments uniquely created during T-cell formation in the thymus in newborns. The relevance, clinical utility, and feasibility of including another marker, κ rearrangement excision circles (KREC), in routine NBS for identification of early-onset B-cell immunodeficiencies are also discussed. 

CLSI NBS06 covers key concepts in the biology of T- and B-cell development, immunologic and clinical features of SCID, and severe B-cell deficiencies, as well as the current state of SCID NBS and screening for early-onset B-cell disorders. It also includes information on diagnostic testing, treatment, and outcomes of current therapies for SCID and early B-cell defects and provides an update to the clinical and economic effects of SCID NBS. Detailed discussion of preanalytical, analytical, and postanalytical assessment of TREC-based NBS for SCID, as well as strategies and implications of potentially including a second marker for immunodeficiency, KREC, into the NBS program, are reviewed. Particular emphasis is placed on updating methodologies for TREC and KREC analysis, multiplexing of analytes in the screening laboratory, follow-up testing, and harmonization, interpretation, and reporting of SCID NBS results. Detailed discussion on clinical interpretation of SCID NBS data, short-term followup (STFU), long-term follow-up (LTFU), and coordination of results between the screening laboratory, clinician, and follow-up diagnostic laboratory is included. 

CLSI NBS06 is intended for use by public health laboratories currently performing SCID NBS; laboratories, countries, and regions contemplating the use of NBS for SCID and/or diagnosis of inborn errors of immunity (IEI); diagnostic immunology laboratories; and health care providers (HCPs) for patients with these diseases. 

CLSI NBS06 is restricted solely to NBS for SCID and the potential inclusion of severe B-cell immunodeficiencies. It does not cover basic aspects of NBS follow-up, which are covered in CLSI NBS02 nor does it provide detailed information on the analytical and interpretive nuances of diagnostic testing. Details on validation of flow cytometric assays in the medical laboratory are covered in CLSI H62.