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The Meaning of a Shift to the Left

October 26, 2010
Physicians should stop confusing the presence of granulocytosis with a left shift.
One of the features that can be obtained in a complete blood count, or CBC, is a differential of the various cell forms making up the white blood count.  The primary white cell types include lymphocytes, polymorphonuclear neutrophils (PMNS, also call granulocytes), monocytes, basophils, and eosinophils.  Within the PMN cell line are several cell types that represent various degrees of immaturity.  These include promyelocytes, myelocytes, metamyelocytes, and band cells, in order of increasing maturity.  In the mid-1900s, clinical laboratories determined the different count manually, with technicians counting the percentage of cells of each type.  To assist with their counting, a standard form was used consisting of individual columns for the tallies the various cell forms.  At some point, a mechanical device was developed to facilitate the task further, tallying each cell type’s numbers as the technician depressed a specific button, stopping them when they’d pressed buttons for 100 cells.  In both systems, the columns and the buttons were oriented with the most immature PMN forms on the left and increasingly mature cells in the columns or buttons moving toward the right.  Thus, when the number of immature forms present exceeded their normally small or non-existent percentages, it was often defined as a “shift to the left”. 

In recent years, it seems that many clinicians have confused an increase in the total number of granulocytes, or a granulocytosis, with a left shift, presumably thinking that if the total granulocyte numbers are increased, it must be due to more immature forms.  However, that assumption is frequently incorrect.  For example, granulocytosis is often present after a dose of steroids is administered because the steroids cause the white blood cells to leave their stationary positions on the inner walls of blood vessels and spill into the circulating blood, a process known as demargination; despite the increased numbers of PMNs, there is no increase in immature forms.  The presence of an actual left shift indicates the presence of an overwhelming infectious process.  However, granulocytosis without a left shift indicates that the immune system is handling the infection without resorting to desperate measures.  These different degrees of immunologic responses should warrant different degrees clinical response.  The paradigm shift in medicine that must occur is for clinicians to understand their terminology and the implications it provides.

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