Peripheral blood smear

Author: Mikael Häggström
Look at and comment separately on white blood cells, red blood cells and platelets. You generally don't need to aim for a perfect report, because for most purposes, a peripheral smear is a relatively simple screening test, and clinicians may opt to perform for example flow cytometry and/or a bone marrow biopsy if they want a better evaluation.

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Oil immersion microscopy

This is preferred for light microscopy of peripheral blood smears in order to achieve a very high magnification. First use low or medium power to center on suspicious cells, or where red or white blood cells are best appreciated. Put a drop of immersion oil on the location and switch to the immersion objective. Then, whenever there's oil on a slide, always think twice before switching between objectives so as to avoid getting oil on any of your dry objectives (which is a bit tedious to clean off).

Red blood cells

Automated values

When available, automatic quantification of mean corpuscular volume (MCV) and red blood cell (RBC) distribution width (RDW), usually as part of CBC panel, generally decides whether you will call the sample "normocytic" versus "microcytic"/"macrocytic" and/or "anisocytotic", even if it is not clearly visible in the microscope. If automated values are not available, compare RBC sizes to lymphocyte nuclei, which should normally be the same size. If mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) are normal, but you still see multiple RBCs with central pallor greater than 50% of the diameter, you can report it as "Increased central pallor", and you may add "indicating iron deficiency" if it is compatible with the clinical history.

Automated values can be graded as follows:

Interpretation	Mild	Moderate	Marked
Microcytosis	MCV : 70 - 79	MCV : 60 - 69	MCV <60
Macrocytosis	MCV : 100 - 115	MCV : 115 - 125	MCV >125
Hypochromasia	MCH : 23 - 26	MCH : 21 - 23	MCH <20
Anisocytosis	RDW: 14.5 or 16 - 18	RDW : 18 - 22 or 26	RDW > 22 or 26

Morphologic findings

Look for poikilocytosis (red blood cells of abnormal shapes). These are counted as a percentage of visible red blood cells:

	Image	Rare/Occasional	Moderate amount of	Many/Abundant
Polychromasia		3 - 5%	5 - 25%	>25%
Spherocytes		1 - 5%	5 - 25%	>25%
Schistocytes		up to 2%	2 - 25%	>25%
Target cells (codocytes)		up to 3%	3 - 25%	>25%
Tear drop cells		up to 2%	2 - 25%	>25%
Burr cells (echinocytes)		1 - 3%	3 - 10%	>10%
Sickle cells (drepanocytes)		3 - 5%	5 - 25%	>25%
Elliptocytes		1 - 5%	5 - 25%	>25%
Basophilic stipplings		up to 2%	2 - 25%	>25%
Howell Jolly bodies		up to 1%	2 - 3 %	>3%

Burr cells versus spur cells

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  Burr cell (echinocyte)
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  Spur cell (acanthocyte)

Burr cells are distinguished from spur cells by having more equally distributed and rounded projections. They are usually artifactual. However, they may also be caused by renal insufficiency, so if this is present, a report may include "Occasional/Multiple echinocytes, consistent with renal insufficiency".

Intraerythrocytic findings

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  When seeing what appears to a platelet overlying a red blood cells (as pictured), confirm that there is a halo.
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  Otherwise, make sure it is not babesia (pictured) or malaria (images below).

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  Malaria, showing appearance at different intraerythrocytic blood stages.
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  If malaria is suspected, also make a thick blood film.

Platelets

If CBC is performed, use count to determine whether platelets are "normal in number" or whether there is "thrombocytopenia" or "thrombocytosis". If no CBC, count platelets within a high power oil immersed field, which should normally be 8 to 20.

A giant platelet.

Large platelets are those with a diameter greater than 4 microns. Giant platelets are those with a diameter greater than 7 microns (larger than a normal red blood cell). Example report:

Numerous large and giant platelets(, suggesting an increased platelet turnover)(( such as in immune thrombocytopenic purpura. They may also be present in myeloproliferative neoplasms, myelodysplasia, and some congenital thrombocytopenia syndromes, including Bernard-Soulier syndrome and MYH9-related disorders.))

In thrombocytopenia from automatic counting, look in particular for:

• Clumping of platelets (which can cause a falsely low automatic platelet count). If present, check with the lab if it was sent in EDTA (which may cause artefactual clumping) and ask to have a blood sample sent in sodium citrate instead. Also, look for satellitosis (platelets attached around white blood cells).
• Schistocytes among red blood cells.

White blood cells

Comparison of monoblast, promonocyte and monocyte. Further information: Suspected blasts on peripheral blood smear

A common practice is to evaluate at least 100 white blood cells. Look for:

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  Blast cells, generally having large nucleus, immature chromatin, a prominent nucleolus, scant cytoplasm and few or no cytoplasmic granules. This example has an Auer rod (to the left of the nucleus). Further information: Suspected blasts on peripheral blood smear
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  Hypersegmented neutrophils. This is abnormal when more than half of neutrophils have at least 4 segments, or more than 5% of neutrophils have more than 5 segments.
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  In patients with known chronic lymphocytic leukemia, estimate the percentage of prolymphocytes, which are medium-sized lymphocytes with prominent nucleoli. A percentage of less than 5% can be reported as such.
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  When smudge cells constitute more than 10% of white blood cells, or in patients with CLL, a separate smear with a drop of serum albumin to every four or five drops of blood should be made to stabilize cell membranes. A semi-quantification of different cell lines should then be made on the albumin slide, whereas white and red blood cell morphology should still be made on the original slide, since it is altered by albumin.

Atypical/Reactive lymphocytes:

Morphological feature	Atypical lymphocyte — Downey's categorization —
	Type I (monocyte-like)	Type II (plasma cell-like) - Most common	Type III (lymphoblast-like)
Typical morphology	Type I (monocyte-like) atypical lymphocyte	Type II (plasma cell-like) atypical lymphocyte	Type III (lymphoblast-like) atypical lymphocyte
Size	≥16 µm	≥16 µm	≥16 µm
Nuclear features	Kidney-shaped or segmented	Fine nucleus Nucleolus (±)	Agglutinated chromatin Nucleolus (±)
Cytoplasm	Strongly basophilic Vacuoles (±)	Strongly basophilic	Strongly basophilic
Intracytoplasmic granules	—	Azurophilic granules (±)	—

Report

Example report:

Normochromic normocytic red blood cells. Red blood cells show <normal morphology / anisopoikilocytosis with occasional ___>. [[If thrombocytopenia, also add "Schistocytes are not significantly increased" if applicable.]] {{Leukocytosis with neutrophilia / lymphocytosis.}} White blood cells show no left shift or blasts. Platelets show no evidence of clumping, and show normal granularity. (Causes of the above findings include ___.)

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