FOAMcastini – The “Left Shift”

(ITUNES OR Listen Here)

The Free Open Access Medical Education (FOAM)

The blog Brown Coat Nation (University of Illinois, Chicago) has a new series entitled “Inconceivable.” The idea is to expose medical terminology that we tend to use incorrectly. The first installment is focused on the misuse of the term “left shift,” and it’s the focus of this FOAMcastini.

The Core Content

The correct use of the term “left shift” refers to the presence of banded (immature) neutrophils in the blood. It does not refer to an elevated white blood cell count with a high percentage of neutrophils. An elevated white blood cell count with an abnormally high percentage of neutrophils should be called “neutrophillic leukocytosis.” Only the presence of immature neutrophils in the periphery (including bands) can accurately be called a “left shift.”

The term “left shift” is derived from the diagrams of the six stages of neutrophil development in the bone marrow. On the far left, you see the most basic precursor: the myeloblast. On the far right of the diagram one finds the mature segmented neutrophil (also known as the “polymorphonuclear leukocyte, or PMN). But just to the left of that is the “banded” neutrophil (the 5th stage of neutrophil development in which the large band of nuclear material has not yet “disbanded” into segments).

When an infection runs rampant, sometimes the bone marrow runs out of mature neutrophils to send to the periphery. So, the marrow panics and releases immature banded neutrophils that normally would not be considered “ready for prime time.”

Here’s some relevant spaced repetition on SIRS: Along with leukocytosis (>12,000) and leukopenia (<4,000), Bandemia >10% is one of the SIRS criteria (Systemic Inflammatory Response Syndrome). Temperature (>38C or <36C), Heart rate >90 (“sirs-ycardia”), tachypnea (RR>20) or PCO2 <32) are the other SIRS criteria. The presence of at least two these categories constitutes “positive SIRS”. In pediatrics, the temperature, heart rate, and respiratory rates should be age-adjusted.
Check out our previous episode on appendicitis for a reminder on whether leukocytosis (with or without neutrophillic predominance) is useful in risk stratification.

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