Evaluation of the validity of Hb A2 and mean corpuscular haemoglobin action values in antenatal screening for beta thalassaemia carriers in England.

National antenatal screening of all pregnant women in England is carried out using standards and guidelines produced by the National Health Service Sickle Cell and Thalassaemia Screening Programme. The algorithms for detection of beta thalassaemia carrier status rely on action criteria, which are set using the percentage Hb A2 and mean corpuscular haemoglobin (MCH) values. Three groups of samples: MCH <27 pg and Hb A2 3·5-3·9%, MCH ≥27 pg and Hb A2 4-4·3% and MCH ≥27 pg and Hb A2 3·5-3·9% were selected from a sample population of 59 500 to assess the validity and predictive value of the action criteria - 25 false positives (0·042% of total) and nine false negatives (0·015% of total) were detected. These findings support the continuation of the current action values.