***Not a medical student; just your friendly med lab tech!
🙂***
First off, the CBC only refers to the WBC and RBC counts, HgB, Hct, Plt count, MPV, MCV, MCH, MCHC, RDW-CV, and RDW-SD. A CBC with Diff includes all the previous tests and a WBC differential, which may be an automatic result or a manual diff (depending on the performing laboratory's protocols with performing differentials).
A left shift indicates that in the granulocytic series, there's an increase in the presence of immature cells. Since granulocytes mature from left (immature) --> right (mature), a left shift is indicated. On a peripheral slide review, the lab tech will make a peripheral blood slide and stain it. The lab tech will then perform the differential (100 cell count) and indicate on the review that a left shift is present. In the results section, you'll see an increased count in band neutrophils, metamyelocytes, myelocytes, promyelocytes, and maybe blasts. If you're wondering how one is performed, ask your attending if you can see how the hospital lab at your rotation site reads slides. Most cases, the instrument will flag that a left shift is present and will require the lab tech to make a slide and review it.
Check out some of these links below if you want a little bit more info:
https://www.sysmex.com/US/en/Education/Webinars/Webinar Handouts/W41Handout.pdf
Left shift | Pathology Student