Aims A recently proposed classification of sudden unexpected infant death incorporates consideration of possibly asphyxia. This depends on an adequate postmortem, scene investigation and history. A reliable marker of asphyxia has yet to be identified. Such a marker could assist in classifying these deaths. Our aim was to determine if the level of nucleated red blood cells (nRBCs) in the peripheral blood could help identify those possibly asphyxia-related deaths and if risk factors could influence this level in the peripheral blood.

Methods Cases of sudden unexpected deaths in infancy and sudden infant death syndrome (SIDS) which occurred over a period of 6 years (2007–2013) and were autopsied at Sheffield Children's Hospital were reviewed and categorised according to a new classification proposed by Randall et al. The cases were then correlated with the blood level of nRBCs determined at the time of post mortem examination. The study was approved by the Clinical Governance Department, number SE331.

Results 139 deaths were classified into Group A (true SIDS, 67 cases), Group B (possible asphyxia related, 24 cases), Group C (non-asphyxia-related, 6 cases), Group D (no crime scene investigation, 0 cases) and Group E (identifiable cause, 42 cases). The levels were significantly increased in ex-premature babies, in infants with an underlying condition (Group C) and in deaths related to illness or trauma (Group E). There was a trend towards higher levels of nRBCs in younger age groups and in babies born to smoking mothers.

Conclusions SIDS remains a difficult diagnosis to make despite the current medical technological advances where no marker of hypoxia has yet been identified.