Hematology is the branch of physiology which studies blood (hemoglobin) and related organs and diseases.
A medical practitioner who studies hematology is known as a hematologist. Dr. Phlox had several medical related degrees, including one in hematology. (ENT: "A Night in Sickbay") Denara Pel was a prominent Vidiian hematologist. (VOY: "Lifesigns")
Treatments
While attempting to cure the Teplan blight, Julian Bashir ran a hematology scan on Epran. He noticed an unaccountable change in the viral base pair sequence. It was later discovered that the EM fields from his medical instruments was causing a mutation in the virus. (DS9: "The Quickening")
The Doctor had Kes run a hematological scan on Neelix in order to regulate the dosage of nanoprobes for his next injection. (VOY: "Mortal Coil")