Newborn blood spot screening programmes are designed to detect serious conditions affecting individuals, where early treatment can improve health. It is suggested that screening can improve the experience of diagnosis for parents. For example, without newborn screening, when a child with cystic fibrosis becomes symptomatic a period of uncertainty can arise prior to diagnosis. These potential advantages of screening need to be weighed against potential disadvantages of screening at individual and population levels. Some newborn screening programmes inadvertently identify newborn infants who, although not affected by the condition, carry a gene for it and can pass on that gene to their children; these are 'genetic carriers'. Knowledge of newborn carrier status can lead to: testing of parents and family members, and concern about possible affected future siblings should both parents be identified as carriers; the possibility of such testing revealing the putative father is not the biological father; concern about the child's future reproductive choices; and unjustified anxiety about the health of the carrier newborn.

There is an urgent need to develop clear guidance as to how to respond, with advances in technology fuelling the expansion of newborn blood spot screening and raised expectations of informed consent and disclosing test results. Depending on the condition for which screening is offered, options include: employing tests that do not identify carrier status, if available; identifying acceptable ways of disclosing carrier status; or identifying acceptable ways of not disclosing carrier status. These options are illustrated by screening programmes for sickle cell disorders and cystic fibrosis. Currently, there are no screening tests available for sickle cell disorders that do not identify carrier status. For cystic fibrosis, the policy choice is between an extended period of testing, and a screening result that is available sooner for most newborns, but inadvertently identifies carrier babies.