What causes a high number/percentage of MID cells in CBC when the overall WBC is in the normal range? ESR is also high (50mm/hr) and slight anaemia.

Increase in monocyte. Monocytes are increased frequently after a viral infection or after some bone marrow insult--again usually viral, but can be associated with exposures or other type of illness. The elevated sed rate represents some sort of inflammatory response to something--the slight anemia requires more information. Please visit with your physician to understand what is going on with you medically.
Old machine, many Qs. Older automatic differential counters figured out how many lymphocytes (small WBC), granulocytes (large WBC) and "mid-range" WBCs are in blood. The mid-range would include some (but not all) eo-&baso-phils, monocytes, & immature forms. (Larger eo-&basophils are in the granulocyte count). Modern diff counters separate each of the 5 main types directly. Without a manual count, it is hard to say.
Sorry, MID cells. are not ones that I have normally seen on CBC's in any lab I've seen. I suspect they are monocytes... raised in chronic inflammation and viral infection including mononucleosis. Numerous chronic infections including all chronic viral infections except HIV and chornic inflammmatory states like rheumatoid arthritis raise mononuclear cells and have high sed rates. cannot say in your particular case.
Pl clarify. HealthTap forward your question to me. I don't know what MID cells is in CBC. If you can clarify the question and re- direct the question to me, I will be more than happy to help. Good luck.
Several populations. of cells are in the mid range on an automated diff. Eosinophils are there and could go with a reactive or inflammatory process with an elevated ESR. Anemia alone can lead to ESR elevation.