The term “left shift” means that a particular population of cells is “shifted” towards more immature precursors (meaning that there are more immature precursors present than you would normally see). Take the neutrophil series, for example. In normal blood, the neutrophils are virtually all mature (segmented). In a left shift, you see mature neutrophils but also immature neutrophils (bands, metamyelocytes, myelocytes, etc.). Check out the photo of a left shift, above: most of the cells are immature.
The term “left shift” almost always refers to the neutrophil series. It arose during the days when cells were counted by hand using a manual counting machine. The most mature cells (segmented neutrophils) were assigned to the right-most button, the least mature cells (myeloblasts) were assigned to the left-most button, and the other stages of cells were spread out in order in the buttons in between. In a normal blood smear, virtually all the neutrophils fell under the right-most counting button, but sometimes, it was noted that there were earlier precursors present (e.g., myelocytes, metamyelocytes, or promyelocytes). In these instances, the cells were “shifted” towards the left.
Most of the time, when you see a left shift, it means that the patient has an infection – often a bacterial one. Sometimes a left shift can occur when there is inflammation or necrosis. Beware, though, if you see nucleated red cells in addition to left-shifted neutrophils. This is called a leukoerythroblastotic reaction, and it may indicate a more serious problem. Sometimes, a leukoerythroblastotic reaction is physiologic. If the hemoglobin is very low (for whatever reason – severe iron deficiency, massive blood loss), the bone marrow tries very hard to make new red cells and send them out into the blood as fast as possible. Sometimes, it is a little overzealous, and it lets a few red cell precursors (nucleated red cells) slip out of the marrow too. And sometimes, it is so freaked out that it starts letting neutrophil precursors (metamyelocytes, myelocytes, promyelocytes) out too! This is a normal response to a severe anemia.
Sometimes, however, a leukoerythroblastotic reaction is pathologic. If the marrow is full of something besides hematopoietic tissue – say, for example, a carcinoma, or a leukemia – then the hematopoietic cells will not have enough room and space to mature properly. They will end up leaving home before they are ready, and you’ll see both nucleated red cells and neutrophil precursors in the blood. This is an ominous sign.
One way to determine whether a leukoerythroblastotic reaction is worrisome is to look at the hemoglobin. As mentioned above, if the hemoglobin is very low (say, below 6), then the leukoerythroblastotic reaction is probably physiologic. However, if the hemoglobin is normal, or only slightly decreased, then there is no good reason for the patient to have a leukoerythroblastotic reaction, and you’d better figure out what’s causing it.