An international reference reagent for the detection of RHD and SRY DNA in plasma

The determination of foetal RHD genotype using foetal DNA contained in the maternal circulation is increasingly used to manage pregnancies at risk of haemolytic disease of the foetus and newborn (HDFN) caused by maternal anti-D. The test is becoming increasingly reliable, and routine clinical services have been established in some centres. However, laboratories currently have no reference materials with which to determine the performance of their tests. This report describes the production and evaluation of a freeze-dried preparation of human plasma, code 07/222, containing RHD and SRY sequences which can be used as a minimum sensitivity reagent.

RhD-positive male plasma was diluted in an excess of RhD-negative female plasma, and 1 ml aliquots were freeze-dried in glass ampoules. To characterise the material, 19 laboratories took part in an international collaborative study. The participants evaluated dilutions of the material using their in-house routine assays and recorded the highest dilution where the genes could be detected.

When diluted 1 in 2, most laboratories were able to detect the presence of RHD and SRY sequences in the material and the participants agreed that this was an appropriate level to set as the minimum sensitivity required.

In October 2010, the WHO Expert Committee on Biological Standardization approved the material 07/222 as an International Reference Reagent for the detection of RHD and SRY DNA in plasma.