An investigation into why blood doesn’t always behave as doctors expect has revealed a super-rare mutation in an extremely uncommon variation of blood.

Testing more than 544,000 blood samples in a hospital in Thailand revealed three people carrying a never-before-seen version of the B(A) phenotype – a genetic quirk estimated to occur in about 0.00055 percent of people, or roughly one in 180,000.

This discovery, says a team led by hematologist Janejira Kittivorapart of Mahidol University in Thailand, suggests that there may be more rare blood variants out there, too subtle for standard testing to detect.