Hematologist ordered ultrasound of spleen. what are typical reasons for this test?

To check size: The spleen is tucked under the left rib cage, & so examining it is rather difficult. There are blood disorders & problems with the immune system which cause the spleen to enlarge, so I imagine your hematologist is trying to figure out whether he should be treating these or not. A good ultrasound tech will measure the exact size of the spleen, & also tell if there's a cyst or infarct in the spleen.