The effect of extended pre-analytical time on reported chance as part of the foetal anomaly screening programme

BackgroundThe foetal anomaly screening (FAS) programme in England screens for trisomies 21, 18 and 13 using a combination of maternal age and biochemical and ultrasound markers in a multivariate Gaussian approach to calculate the chance of a foetal trisomy. The screened population is divided into higher and lower chance categories using a 1 in 150 cut-off. A truncation of 1 in 5000 is used for the lowest chance results.A series of postal strikes resulted in delay to specimen delivery and necessitated rebleeding of hundreds of affected clients across England. The effect of delay in processing specimens was investigated based on the final chance reported, rather than the effect on the individual biomarkers used in the chance calculation.MethodsThe FAS chances from samples delayed in postal strikes were compared against those from repeat samples from the same patients to determine the absolute effect on the final chance reported rather than the individual biomarkers.ResultsA total of 119 of 120 chances reported remained in the same high/low chance category.ConclusionMarker concentrations are affected by delayed separation, but the effect on calculated chance is only significant in those close to the 1 in 150 cut-off. There is little benefit in rebleeding clients at very low chance or at very high chance as this leads to a delayed result and the possibility of missed screening for some affected pregnancies.