The American Society of Hematology has developed new reference ranges for children and adults with Duffy-null status, a common genetic trait associated with naturally lower circulating white blood cell counts that affects more than 66% of people with African or Mediterranean ancestry in the U.S.. These personalized reference ranges are the first of their kind and will help clinicians accurately determine normal and abnormal white blood cell counts, preventing unnecessary testing. The reference ranges have been published in Blood Advances.