Reading a CBC, Bands

Machines cannot reliably distinguish between band neutrophils and mature neutrophils yet (although they are very close), so this value is usually determined by a technician doing a manual differential or "manual diff".  This involves taking a drop of blood and making a smear on a slide.  It is allowed to dry, then stained with Diff Quick stain.  Once the stain has dried the slide is viewed under a microscope and evaluated for red and white blood cell morphology as well as a count of 100 white blood cells to determine what percent of cells are of each WBC type including bands.

Bands are immature neutrophils.  In the photo below, the white blood cell on the left is a band neutrophil while the one on the right is a "Seg" or normal, mature, segmented neutrophil:

Immature "band" neutrophils are released from the bone marrow when the body is running low on circulating, mature, neutrophils and decides that immature cells are better than none at all.  Neutrophils are the body's first line of defense against infection, but they are also involved in inflammation.  So, whichever the cause, band neutrophils indicate that the body's normal defenses aren't able to keep up.

When bands are present it's called a "left shift".  The next question to ask yourself is whether there are more bands or more mature neutrophils present.  If there are more mature neutrophils than bands it is a better prognosis and it's called a "regenerative left shift".  This means that bands are being produced, but the body is still putting up a good fight.  If there are more bands than segs, it's called a "degenerative left shift".  This is a bad prognosis because it indicates the body is not keeping up -it is losing the battle.

The presence of bands indicates:
-Inflammation:  this is the most common cause.
-Bone marrow disorders such as leukemia may cause early release of neutrophils.
-Bone marrow injury

The absence of bands indicates:-Normal, yay!