The Stat Lab


On the internet I am known as Slip. I am a 22 year old nerdface who practically lives and breathes laboratory medicine.

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Lab Tests

Lab test: CBC histograms

Anonymous asked: hematology histograms (how to read them/compare with a peripheral blood smear), por favor? 

Sure thing! A lot of people just glance at them since the numbers are what really drive the testing process (when to do a manual differential, when to look at the history, etc), but the histograms are pretty helpful in anticipating what to expect in a smear.


Complete blood counts are ordered on nearly every routine blood draw, so there is basically no escaping hematology. I won’t go into the technology of the coulter analyzer, since that is a whole post in itself for another day.

Three histograms generated in a CBC: the RBC, WBC, and platelets. Not only the histograms supply us with information about RBC’s, WBC’s, and platelets frequency, their distribution, and average sizes, but also depict the presence of cell subpopulations. They are important for helping the operator visualize numeric results, verify abnormal patterns, and help pick out interferent particles.

First, the cell size is determined (this actually happens at the same time as the count through impedence), and a graph is made to show the frequency of cells of each size. The X-axis is the fL size thresholds, and the Y axis is the relative number of cells in each size range. A mean raw data curve is drawn on top of this to smooth it out a bit. The mean cell volume is derived from this histogram as well.

Red Blood Cells

That’s our nice normal red cell histogram, which counts everything from 36fL to 360fL in size. The coincidence demarcated in that graphic is when more than one cell passes through the aperture at a time but is counted as one. But since there is a direct relationship between cell concentration and effective aperture volume, this is easily corrected by instrument in its report. A normal RBC histogram has the following features:

  • One main population
  • The curve begins at the baseline 36fL, goes up, and comes all the way down to baseline at around 200-250fL
  • The peak is fairly narrow
  • A small population typically occurs to the right of the main population.

It’s actually really hard to find pictures of these things on the net, so you’ll have to forgive me for not having good visuals….

But the histogram is probably the most handy when you have a dimorphic population such as this: 

Where the mean cell volume will look deceptively normal (your normal-large and very small cells will average out), but your histogram will become bimodal (two peaks). Bimodal curves may be seen in cold agglutinin disease (many red cells will agglutinate and the instrument will think it is one really big rbc), in iron deficiency anemia (microcytic) with recent blood transfusion, in sideroblastic anemia especially in the acquired forms, and in megaloblastic anemia (macrocytic anemia) with recent blood transfusion.

Shifts in the histogram will be observed in microcytic anemia (left shift) or macrocytic anemias (right shift). Depending on the cause, the width of the peak (red cell distribution width) will likely also increase. This means there are lots of cells in a whole host of different sizes.

When the curve does not start at baseline, suspect extremely small cells and cell fragments, giant platelets, white cell fragments, etc. When it does not come back down to baseline, suspect either agglutination. In some cases when the patient has leukemia and a very high white cell count, some of these will be counted as RBCs and you will also see an extended right peak.

White Blood Cell histograms:

A typical WBC histogram has the following points:

  • the counting threshold begins at 35 fL
  • the curve should be at or very close to baseline at this threshold
  • There should be nothing below the 35fL cut off point

This histogram has something called a high takeoff. Note that it doesn’t start at baseline and it looks like there are interfering particles at that size range. These can be nucleated red blood cells, clumped/aggregated platelets, red cells inclusion bodies, unlysed mature red blood cells (premature infants and in patients with higher osmotic resistance), intracellular parasite, or other less common interferents, so a blood film is made and a manual smear review is done.

As the normal histogram has valleys between the regions, the instrument will flag results where the valleys do not dip as low as they should. A peal in the region between the lymphocytes and mononuclear cells are typically blast cells, plasma cells, or increased eosinophils and basophils.

A peak between mononuclear cells and the granulocytes includes immature granulocytes, blasts, and eosinophils. A large peak or failure to come back down to baseline on the far right side of the curve usually indicates a high absolute granulocytic count and/or presence of toxic granulation, both of which can be observed in infection or burns.

Don’t forget that multple flags can exist on the same patient.

Platelet histograms:

A normal histogram (in black) has the following features:

  • The curve exists between 2-20fL.
  • A best fit line drawn overtop the data curve fits between 0-70fL
  • Both curves are parallel and start and stop at baseline.

The lower region between 0 to 2 fl can be expanded by air bubbles, dust, electrical and electronic noises, whereas the upper region (over 20 fl) can be made larger by microcytic red cells, red blood cell fragments (schistocytes), WBC fragments, giant platelets, clumped platelets, and platelets satellitism.

Because the histogram area is so small, the most the instrument tends to do with it is set an R flag and ask you to review it. It is pretty good at picking out giant platelets and will put a comment on the print out though. A lot of the smears we do are for platelet review only.

Mean platelet volume/platelet distribution width is actually a little more helpful than the histograms themselves. Large platelets(>10fL) suggests immature platelets, which can be normal, but can also be associated with sickle cell, following splenectomy, and Idiopathic thrombocytopenic purpura. Small (<7fL) platelets can be caused by several anemias (aplastic, megaloblastic, hemoglobinopathies). These conditions are also associated with an increase in distribution width.

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