A true "left shift" and checking a CBC
Hey guys, I was thinking maybe I should start doing a few short learning posts every now and then, especially as the studying hasn't stopped for me despite graduation from PA school and I continue to learn new things!
One thing that bugged me even as a student was the spouting of the term "left shift." I have a degree in immunology & microbiology and my undergrad professors really drilled this into us, but unfortunately in medical school it isn't always taught clearly.
To start in the beginning, let's say you have a sick patient. You check a simple CBC (complete blood count). It already starts to get tricky here... Not every lab/hospital will include a differential in your CBC. Mine doesn't. In that case, you would only get the top half of the information listed below.
The issue with this is that the WBCs listed here as 5.9 include ALL WBCs: neutrophils, lymphocytes, eosinophils, basophils, monocytes... And we have learned that elevations in different types of WBCs mean different things. An elevated percentage of neutrophils in the total WBCs usually raises bacterial infection on our differential diagnosis, BUT you need the differential on the CBC to get that information.
Now on to the second issue. Differentials on your CBC are usually done by machines using flow cytometry. This is how you get results back fast! However, a true left shift on a CBC is the presence of immature neutrophils (usually called banded neutrophils as their nuclei are not segmented like mature neutrophils) because the bone marrow is releasing these immature cells quickly in order to fight the infection. I don't believe there is a machine that can accurately identify a banded neutrophil yet. You need a real person to look at the stained blood film under a microscope to look for the bands, so you must order a manual differential (done via a peripheral blood smear) to be able to report a true "left shift."
(BTW, the term "left shift" was coined in the olden days when all CBCs were done by hand and some poor lab rat had to use a hand counter to count all the blood cells visualized, and the button for immature neutrophil was on the left side.)
Now, manual differentials are labor-intensive and prone to more errors in estimation and such versus the automated flow cytometry techniques. So unless it is really important that you know there is a true left shift or you need someone to look at the characteristics and shapes of the other blood cells (suspecting leukemia, hemolysis due to weird cause, etc) just having the CBC with automated differential will probably be enough information for you. Just don't go calling an elevated total % of neutrophils a left shift :-)
