left shift 1024x683 Left shift

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.

35 Responses to Left shift

  1. Melissa,PA says:

    Great explanation! Very informative and actually makes sense. Thank you!

  2. jon dec says:

    Studying MLT work, first explanation, of shifting I can buy!!!!

  3. Carolyn, NC says:

    Exceptional site and explanation! Accolades to you for a well done depiction.

  4. whats the abnormalities occur in white blood cells of sickle cell anemia

  5. admin says:

    The abnormalities in white cells, when present, are caused by severe anemia and splenomegaly. Patients with mild sickle cell disease will usually have no WBC abnormalities. Patients with severe anemia (of any cause, for that matter) can show a left shift (immature neutrophils) and even a leukoerythroblastotic reaction (immature neutrophils and nucleated red cells) in the blood. As the marrow tries really hard to kick out new red cells, it can get a little sloppy and let out nucleated red cells and even some immature neutrophils in the process. Patients who have undergone autosplenectomy will have a slightly elevated white cell count as well as an elevated platelet count, because the spleen is home for these cells, and when you remove the spleen, the same cells will circulate in the blood.

  6. Kelly says:

    I have been in healthcare for 20+ years and this is in wonderful simple terms. Great explaination!!

  7. marko says:

    great explanation!

  8. emad says:

    Leucoerythroblastic reaction is also seen in septicemia.
    Also, look for tear-drop cells, they indicate bone-marrow infiltration rather than severe anemia or sepsis.

  9. admin says:

    Yes – you can see a leukoerythroblastotic reaction (immature neutrophils plus nucleated red cells) in sepsis. It may also mean something more ominous, though, so you can’t assume it’s just sepsis. Sometimes when there is tumor (or fibrosis, or whatever) filling up the marrow, and there is less room for the cells to mature, you’ll see young red cells and neutrophils in the blood. Teardrop red cells can be seen in cases of marrow fibrosis (as in idiopathic myelofibrosis) – you’re right!

  10. Luke, Mark or Sam says:

    Every morning from now on, I will visit these archives and have an educational peek rather than going on facebook.

    Accolades to you Professor

  11. Petra says:

    I have always had trouble with remembering this, mainly because the term ‘left shift’ was so abstract. Now that I know where it originated from, it’s so much easier to remember the definition!

  12. sumera amin says:

    wonderful job for the learners and clinicians and thanks…

  13. Kristine says:

    Thanks so much!

  14. sumera amin says:

    Dear DOC, PLS explain about atypical lymphocytes,virocytes and reactive lymphocytes.How far they are helpful during the reporting.

  15. Maisarah says:

    just what I wanted to know. Thank you very much! :)

  16. Kristine says:

    You’re welcome! Glad it helped.

  17. yeliana says:

    thanks a lot, really helpfull!!

  18. faisal says:

    thank you its very nice

  19. Tory says:

    I thought the term “left shift” also included when the percentage of neutrophils in the differential was greater than or equal to 75%. Is this incorrect? Thanks for clarifying.

  20. Kristine says:

    Nope – it just means that there are immature neutrophils in the blood. It has nothing to do with the number of neutrophils.

  21. Andrew says:

    Thats awesome thank youeasy breskdown

  22. ALI says:



  23. Kristine says:

    No – not unless they have a malignancy that is treated with bone marrow transplant. Chronic myeloid leukemia (which has a left shift) may be treated with bone marrow transplant – but there is also an excellent new drug called imatinib that is used with great success. Most left shifts are just due to infection.

  24. G.Vuckovic MD says:

    Thank you for posting this and google finds it near top. I teach medical students and residents and I am amazed that they universally don’t understand this phenomenon and its significance. Most are completely oblivious. Patients die because of it not being understood by many practitioners as they miss severe infections often.

  25. Denes says:

    Our teacher explained this phenomenon like that:
    Draw a diagram with a horizontal axis representing “number of segments in PMN’s nucleus” and a vertical axis representing cell rate(%). Then we get ‘left shifted’ when we have high count of poorly segmented (immature) neutrophils. Opposite, when most of the neutrophils are hypersegmented, we say ‘right shift’.
    Is it right? Thanks

  26. Kristine says:

    I don’t think that is the best way to approach the concept of left shift, primarily because the number of segments doesn’t change in a linear way as neutrophils age. Myeloblasts, promyelocytes, myelocytes, metamyelocytes and band cells all have one “segment” (though we don’t call it that – we just call it a nucleus), and segmented neutrophils have 3-5 segments – so there isn’t a progression of segmentation from 1 to 2 to 3 to 4, etc. It’s best to just think of the cells in a line, with the most immature (blasts) on the left, then promyelocytes, and so on, with mature, segmented neutrophils at the farthest right position. Then, normal blood would have all the cells under the segmented neutrophil category (on the right), and blood that is “left-shifted” would have cells in some or all of the other categories.

    We don’t use the term “right shift” because there is no more mature neutrophil than the segmented neutrophil – so there’s no way to shift the cells to the right. Hypersegmented neutrophils (with 6 or more segments) are seen in megaloblastic anemia. They occur due to a pathologic process (usually B12/folate deficiency) and are not part of the normal sequence of neutrophil development – so they are not included in the concept of a left shift.

  27. gourav says:


  28. Esmeranda says:

    If there is a let shift with normal neutrophil ans white cell count, does it still mean there is a bacterial infection present?

  29. Kristine says:

    It might mean that there is an infection, but it could be something else as well (the same is true when you see a left shift with an increased neutrophil count).

    In some cases of infection, the neutrophil count actually drops, because the neutrophils are exiting the blood so fast to get to the place where they are needed. It’s much more common to have a neutrophilia – but a neutropenia can occur too.

    You need to be careful to look for things that might indicate a non-infectious cause for the left shift, though. As mentioned above, if there are also nucleated red cells out in the blood, that is called a leukoerythroblastotic reaction, and it may represent something more ominous than infection.

  30. Tess says:

    You’re an absolute legend, I’m studying medicine in Australia and always refer to this blog whenever something becomes too confusing – you have saved me on multiple occasions!


  31. Danny says:

    I’m a dentistry student and without these concise and helpful explanations I wouldn’t have been able to pass my oral medicine and oncology exams :) thankyou!

  32. mary mcdermott says:

    Genius! This is so clearly explained. I am an NP student and really get it now, Thank you.

  33. Sahan says:

    thanks buddy.

  34. deborah says:

    My 2 year old Grandson has a 12,000 WBC with a left shift. He has been vomiting with some bile showing up. The MD is looking more toward virus, but I say more of a Bacterial infection. This is second time in two weeks that he started vomiting. What Bacterial infections will cause a left shift? I am very concern.

  35. Kristine says:

    Hi Deborah –
    Most any bacterial infection can cause a left shift. In general, as you allude to, bacterial infections cause a neutrophilia (with or without a left shift) and viral infections cause a lymphocytosis. You might also have the pathologist look for toxic changes as further evidence that there is a bacterial infection present. Unfortunately, neither the left shift nor the toxic changes point you towards a specific bacterium – but it should at least prompt a search for a bacterial source (bacterial blood cultures might be a good start). I hope he feels better soon and they find out what it is!

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