In veterinary medicine, a peripheral blood smear is often used to complement a complete blood count (CBC) or hematology panel, especially when the CBC is done on a machine where interpretation may be lacking or even unavailable.

A blood smear is used to diagnose and/or monitor numerous conditions including appropriate number and morphology of white blood cells (WBCs), red blood cells (RBCs), and/or platelets (PLT).

To make a good blood smear, very little equipment is needed (aside from practice in how to create a good smear and a good microscope!).

The equipment you will need includes:

1)     2 glass slides
2)     Immersion oil
3)     Blood sample
4)     Microscope

Blood Smear Technique

1)    On one slide (the sample slide), place a small drop of blood












2)    Use the second slide as a “spreader” slide.

3)    Place the end of the spreader slide on the sample slide so that the short sided edge of the spreader is below the drop of blood.

4)    Hold the spreader slide at an angle of 30o to 45o (relative to the sample slide) and  bring the spreader slide back against the drop of blood so that the blood spreads in a thin line via capillary action.












5)    Rapidly – but gently – drag the spreader slide along the entire length of the sample slide in one fluid motion.

6)    If the technique was performed correctly, the smear should end before the end of the sample slide in a “feathered edge.”












7)    Air dry the sample slide. Fix and stain the slide.

Check out some “how to” videos at:


Once the blood smear is made, we then need to evaluate the sample for abnormalities.

 1)    VetGirl first starts at low power (10X ocular/objective). On this low power you can:

  • Evaluate the quality of the smear
  • Evaluate the number of WBCs, RBCs, cell distribution and morphology (e.g., rouleaux formation of RBCs, platelet clumps, leukocyte clumps, agglutination, etc.)
  • Evaluate the appropriate section on the slide where high power should be used, avoiding any thick or non-representative areas of the slide.

2)    VetGirl then moves to the 40X objective.  On this power you can:

  • Count the number of leukocytes in at least 3 fields and calculate the average number per field.
  • Multiply this number by 1500 to get the approximate total WBC count.

3)    VetGirl then moves to 100x (oil) looking for morphology of RBCs, WBCs, and platelets

  • RBC morphology
    • Size
    • Shape
    • Color
    • Hemoparasites
  • Platelet Estimate
    • Count the number of platelets in at least 5 fields and calculate the average number.  Multiply this by 10,000-15,000 to get the approximate total number of platelets.
  • White Blood Cell morphology
    • Size
    • Shape
    • Type (neutrophil, lymphocyte, eosinophil, etc)
    • Left shift

The more you practice, the better you become at making and interpreting blood smears. More importantly, this is a great clinical option for pet owners who have financial constraints and may not be able to afford a CBC. We use blood smears commonly in the emergency room on three main patient populations:

  • A patient who we suspect may have parvovirus but tests negative on a fecal ELISA (to evaluate if they are concurrent leukopenic)
  • A patient undergoing chemotherapy who presents “ADR” (ain’t doing right) (to evaluate if the use of antibiotic therapy is warranted if their WBC count is < 1,000-2,000)
  • A patient presenting with epistaxis or clinical signs consistent with thrombocytopenia

Anyone else that we missed that you like doing blood smears on?


  1. Help! I stained my blood smear in class but I forgot to wash or put water after the darkest stain. Will it affect something or is it okay what happened?

  2. Hi, I have been working in a laboratory for 11.5 years and I still cant seem to master slides. It’s really embarrassing because one of my colleagues has only been working for 1.5 years and his blood smears are infinitely better than mine. How can I improve???

  3. Hi, it just so happens that I work with a guy called Paul O’Rourke, but there’s no way yous can be related as he is a master at slides. Ill ask him to get in touch, he taught me and I’m still trying to master his technique :@

  4. I am working at a clinic where they heat fix blood smears. I am an RVT and have never heard of this before. Is there reasoning behind this or are they mistaken???

Leave a Reply

Your email address will not be published.