While this seems a fairly obvious answer, we’ve had issues lately. Two weeks ago, we had a cord sample give a ‘?’Reaction in Rh D in the newborn card. As per our sop, this was rerun and came up negative and to be sure, the bms washed the cells and checked the group in test tubes. It was weakly positive in one clone of Rh D, only detectable under the microscope. This was later confirmed as weak D by the reference lab and the mother was dosed appropriately with Anti-D.
Historically, we’ve always checked negative test tube Rh D under the microscope and this rarely caused problems. Since this incident though, we’ve had problems with weak (+/-) reactions which are confirmed as negative by the reference lab. Anti sera instructions state observation is macroscopic which would overrule our historic habit of checking microscopically BUT how else do you detect weak Rh D? If we had followed the macroscopic only rule, we would have missed the weak Rh-D and the mother could have been sensitised. Is weak Rh D in a foetus less likely to cause sensitisation?