A simple alternative for Rh phenotyping red cells that have a persistently positive Rh control

Investigation of a patient’s red cell sample with a persistently positive Rh control revealed that if the patient’s red cells were treated with ZZAP, then incubated at 37°C with commercial Rh-typing reagents, then washed four times with saline, the positive Rh control could be circumvented and the Rh phenotype readily determined. One hundred red cell samples of known Rh phenotype were treated with ZZAP and coated with autoantibody to resemble the cells of the index case. Accurate results were obtained when these modified cell samples were tested against Rh typing reagents from three manufacturers using a 37°C incubation followed by four saline washes. The procedure, termed Z37W, appears to be a simple alternative that can assist in determination of Rh phenotypes when the Rh control is positive.