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Definitions of high and low prevalence
Low prevalence trusts are those where less than 1% of the booking bloods received by the laboratory are screen positive. High prevalence trusts are those where this figure is greater than or equal to 2%. Trusts that are between these 2 cut-offs are considered borderline and should continue to use their current algorithm.
Low prevalence trusts
In low prevalence trusts, a family origin questionnaire (FOQ) is used. This assesses the risk of either the woman or the baby’s biological father being a carrier for sickle cell or other haemoglobin variants, or severe alpha thalassaemia. This identifies those who need testing for haemoglobin variants. The FOQ should be completed and sent to the laboratory with the sample.
High prevalence trusts
In high prevalence trusts all pregnant women are offered screening by a blood test for sickle cell and other haemoglobin variants. The FOQ should also be completed and sent to the laboratory with the sample. This helps laboratory staff to interpret the results and identify those at risk of severe alpha thalassaemia.
The high prevalence algorithm is viewed as the gold standard. In the event of trust mergers, low prevalence trusts should move to the high prevalence algorithm rather than high prevalence trusts moving to the low prevalence algorithm.
There is no need to offer screening by a blood test for sickle cell and other haemoglobin variants again in the same pregnancy if a woman has already been screened in a low prevalence area but chooses to give birth in a maternity unit in a high prevalence area.
Each laboratory should run only one screening algorithm, either the high or low prevalence.
For more information on undertaking the screening test and the FOQ, please see the Sickle Cell and Thalassaemia Screening e-learning module on e-learning for Health.