Problems with implementing a standardised transcranial Doppler screening programme: impact of instrumentation variation on STOP classification

Background
The Stroke Prevention Trial in Sickle Cell Anaemia (STOP) demonstrated the value of selective transfusion based on transcranial Doppler (TCD) US screening. This facilitated widespread surveillance, but due to reported differences with non-imaging TCD, imaging velocity thresholds have been reduced in some centres.

Objective
(1) Retrospectively review velocity measurements obtained by non-imaging and imaging TCD, using a standardised protocol. (2) Determine the impact on STOP classification of different velocity thresholds.

Materials and Methods
TCD data from 23 children (2-19 years of age) were reviewed. The TCD protocol focused on obtaining the velocity corresponding to the highest audible Doppler frequency. STOP velocity thresholds were the recommended for non-imaging TCD and values reduced by 5-15%.

Results
Non-imaging and imaging TCD velocities were correlated closely with little overall bias. Reducing imaging TCD velocity thresholds increased the number of abnormal and conditional classifications. Abnormal TCD imaging classifications ranged from 1.9% to 37% depending on the degree of correction applied to the velocity data.

Conclusions
The current approach for applying STOP thresholds to imaging TCD data may not be required. Centres need to validate their imaging TCD practice to avoid inappropriate selection of patients for transfusion therapy.