Jk(a-b-) red blood cells resist urea lysis

Falsely high automated platelet counts in a patient with aplastic anemia were found to be due to increased resistance of the red blood cells to urea lysis. The patient's blood group Jk(a-b-). Further investigation revealed that this phenomenon occurred with all of eight bloods of the phenotype Jk(a-b-) but not with red blood cells of other phenotypes tested. We therefore report an association of a rare blood group phenotype with unusual red blood cell behavior in vitro.