Evaluating a blood smear

1.  Make a blood smear and stain it.  For a refresher on how to do this, check out this video:
http://www.youtube.com/watch?v=R1DU_N6eazg
2.  Put the slide on the microscope and focus using low power magnification, then move up to high dry.
3.  On high dry magnification, briefly scan the slide to find where your monolayer is.  The monolayer is where you'd do your cell counts -the area where the cells do not overlap, but cover the whole viewing area without many gaps.  Then find the feathered edge where the cells are more spread out at the end of your smear.  Evaluate whether there are more white blood cells at the edge than the monolayer or not.  If there are, this may erroneously decrease your white blood cell count and another smear should be made.
4.  Move up to oil power and then scan the feathered edge for platelets.  Again, if there seem to be a lot of platelets or clumped platelets at the feathered edge this will erroneously decrease your platelet count and another smear should be made.
5.  If everything looks okay, move to your monolayer and begin your evaluation.

Things to look for by type:

• White blood cells (in order of prevalence in dogs and cats):
• Neutrophils:  
• Segments:  Neutrophils should have 4-5 segments on average.  They may appear to have fewer due to their being viewed in 2D.  When a good percentage of neutrophils have more than 5 segments indicates hypersegmentation or older neutrophils that may be kept around longer due to decreased production as happens with steroid use.  Consistantly seeing hyposegmented neutrophils (or neutrophils with fewer than 4 segments) may indicate younger than average neutrophils as happens at the beginning or end of a left shift.  A neutrophil with a nucleus that has parallel sides for its entire length (no segments) is called a "band" and seeing many bands in a smear indicates a "left shift".  If under 50% of neutrophils are bands the left shift is regenerative.  If over 50% of the neutrophils are bands the left shift is degenerative.
• Signs of toxicity:  Dohle bodies appear as dark spots in the cytoplasm.  Vacuoles look like "bubbles" or white spots in the cytoplasm.  Blue or "basophilic" cytoplasm, or more noticeable granules in the cytoplasm (toxic granulation).
• Lymphocytes:
• Abnormal findings:  Red granules in the cytoplasm of more than a few cells can indicate Ehrlichia canis infection or cancer..  Lymphs with deeper blue cytoplasm are reactive lymphocytes and can be normal in young animals.
• Monocytes:
• Distinct from lymphocytes due to their large size and vacuoles as well as their less dense nucleus and larger amount of cytoplasm surrounding the nucleus.
• Eosinophils:
• Segments:  Should be evaluated the same as neutrophils although they may be difficult to see due to the visible granules.
• Basophils:
• Very rare to see.
• Segments:  Should be evaluated the same as neutrophils although they may be difficult to see due to the visible granules.  No link provided because Cornell's basophil photo is very poor. 
• Red Blood Cells:
• Cell size:  If the cells differ in size, this is called anisocytosis.
• Cell color:  Large cells that are more blue are called "polychromatophils" and are not quite mature.  Pale cells are called "hypochromic".  Clear cells are called "ghost cells" and indicate that the cell membrane had a leak.
• Shape:  If an RBC has a knob or an "ear" or "nose" growing out of it, this is likely a Heinz body.  RBC's that look like splats may be acanthocytes or echinocytes.  Acanthocytes have irregular protrusions that may be round on the ends while echinocytes have a pointier protrusions all over the cell.  Target cells look like targets in that they have an area of central pallor with a circle of red inside of it.  If it looks like a red blood cell was ripped and you are just seeing a piece of it the piece is called a schistocyte.  Teardrop shaped RBC's are dacryocytes and are usually an artifact from making the smear.  Spherocytes are smaller RBC's with no area of central pallor.  A general term for differently shaped red blood cells is poikilocytosis.
• Inclusions:  Large dark circles that make the RBC look like a googly eye or a fried egg are early stage cells that still have their nucleus, these are called metarubricytes.  Smaller dots in the red blood cell are Howell Jolly bodies if they are not refractile (shiny when the focus is moved up and down).  If they are refractile they are likely granules from the stain.  Real Howell Jolly bodies often occur between the center of the cell and the membrane rather than right in the center or touching the membrane.  Basophilic stippling appears as lots of granular looking dots and can mean resolving anemia or lead poisoning.  RBC parasites may also be seen.  Check the menu on the left of this page for more about these: https://ahdc.vet.cornell.edu/clinpath/modules/rbcmorph/para-f.htm
• Platelets:
• Platelets appear as irregularly shaped basophilic cells that are usually smaller than a red blood cell.  If you are in the monolayer and you are seeing 10-15 platelets per oil field you can assume the platelet count is average.  If you are unsure or are asked to do a platelet count than count the number of platelets per oil power field across 10 fields and then average them.  Multiply the average by 15,000 and you'll get an estimated platelet count per microliter.  Normal platelet counts should close to or above 200,000 per microliter.

Relevant information based on the patient:

• For basic manual differentials:  Evaluate as many fields in the monolayer as necessary to count 100 white blood cells by type.  At the end the number of each cell type gives you the percentage of that cell in the smear.  For example, 64 neutrophils, 26 lymphs, 7 monocytes, and 3 eosinophils means there are 64% neutrophils, 26% lymphs, 7% monocytes, and 3% eos.  But, you should also be taking note of any irregularities in the WBC's or the RBC's and should take a mental note of the platelet numbers you are seeing by field as well.  If nothing seems out of the ordinary just give the cell percentages, but if anything abnormal was found it should be reported.
• For patients who are anemic:  A platelet count is usually what is asked for.  In addition to the platelet count the RBC's should be evaluated fully.  This can give the doctor a hint as to what the problem might be or an idea of whether or not the anemia is regenerative or not (whether the animal is creating new RBC's and then losing them or if the issue might be that they have stopped producing them).
• Heartworm microfilaria:  Examine the smear on low or medium power.  The microfilaria are longer than 50 RBC's strung together, so they should be pretty obvious if present.