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The following is part of the complete blood count (CBC) when you first see the patient:

WBC (109/L)Hb (g/dL)MCV (fL)PLT (109/L)
8.88.155516

What’s what: WBC, white blood cell count; Hb, hemoglobin; MCV, mean cell volume; MCHC, mean cellular hemoglobin concentration; RDW-SD, red cell distribution width-standard deviation; platelets, PLT; Normal values: WBC 5-10 x 109/L, RBC 4-6 x 1012/L, Hb 12-16 g/dL, Hct 35-47%, MCV 80-100 fL, MCHC 32-36 g/dL, RDW-SD < 45 fL, platelets (PLT) 150-450 x 109/L

Which value is more helpful:

a
Mean corpuscular hemoglobin (MCH)
The MCH is rarely helpful because it merely tracks with the mean cell volume (MCV).
b
Mean corpuscular hemoglobin concentration (MCHC)
Yes! The MCHC helps to discriminate between causes of microcytic anemia.

You expect the patient’s mean corpuscular hemoglobin (MCH) to be:

a
Low
Correct. The MCH tracks with the MCV. Since the MCV is low, the MCH will be low.
b
Normal
c
High

Mean corpuscular hemoglobin (MCH)

Think of the MCH this way: Take a single red cell, break it open, and pour out all the Hb (like a little bag of sugar or Splenda) onto a weighing scale, and then average the weight across many individual red cells. That gives you the MCH!
If we plot the mean cell volume (MCV) against the mean cell hemoglobin (MCH), we see a direct correlation between the two (straight line).

Mean corpuscular hemoglobin (MCH)

Think of the MCH this way: Take a single red cell, break it open, and pour out all the Hb (like a little bag of sugar or Splenda) onto a weighing scale, and then average the weight that across many red cells. That gives you the MCH!
The only other way to change the MCH (besides altering the red cell size) is to adjust the cellular hemoglobin concentration. But because the hemoglobin concentration is already at near-saturating levels, any such change – for example, as seen in a hyperchromic cell or spherocyte – will have a small effect on the MCH.

Mean corpuscular hemoglobin concentration

While the mean corpuscular hemoglobin is the average weight of the red cell in hemoglobin, the mean corpuscular hemoglobin concentration describes the average concentration of hemoglobin inside red cells. A lower-than-normal hemoglobin concentration manifests as hypochromia on the peripheral smear (increased area of central pallor). A higher-than-normal hemoglobin concentration manifests as hyperchromia (loss of central pallor, which is indicative of a spherocyte).

Consider the following thought experiment: Planet earth is one giant red cell. It’s inner hemoglobin (Hb)-free core comprises 1/3 of the volume of the planet. The rest is made of hemoglobin (Hb).

What is the “MCH” and what is the “MCHC” of this insanely large cell?

a
MCH is low
b
MCH is normal
c
MCH is high
“Astronomically” high!
d
MCHC is normal
The concentration (weight hemoglobin/volume) is maintained as long as the fractional volume of hemoglobin stays the same.
e
MCHC is high

MCH, mean cell hemoglobin; MCHC, mean cell hemoglobin concentration

Let’s return to the CBC and fill in the MCH and MCHC:

WBC (109/L)Hb (g/dL)MCV (fL)MCH (pg)MCHC (g/dL)PLT (109/L)
8.88.15513.624.8516

What’s what: WBC, white blood cell count; Hb, hemoglobin; MCV, mean cell volume; MCHC, mean cellular hemoglobin concentration; RDW-SD, red cell distribution width-standard deviation; platelets, PLT; Normal values: WBC 5-10 x 109/L, RBC 4-6 x 1012/L, Hb 12-16 g/dL, Hct 35-47%, MCV 80-100 fL, MCHC 32-36 g/dL, RDW-SD < 45 fL, platelets (PLT) 150-450 x 109/L

Which of the following schematic of peripheral blood smear best fits the patient?

a
A
b
B
Correct. Small red cells (their diameter is smaller than that of the lymphocyte nucleus; they have a low mean cell volume [MCV]) that are poorly hemoglobinized (low mean corpuscular hemoglobin concentration [MCHC]).
c
C

The mean cell volume (MCV) and mean corpuscular hemoglobin concentration (MCHC) are both incorporated in the traditional morphological classification of anemia:

Note that the MCH is nowhere to be found!

At this point, the two major conditions on the differential are:

  • Iron deficiency
  • Thalassemia trait/minor

In favor of iron deficiency: the patient’s symptoms and the low MCHC. Also his platelet count was elevated, which is consistent with iron deficiency-associated secondary thrombocytosis.

In favor of thalassemia: this degree of microcytosis is much more common in thalassemia than in iron deficiency.

If you could have one test result, what would it be?

a
Serum haptoglobin
Likely to be normal in both iron deficiency anemia and thalassemia trait/minor (though it would be decreased in more serious forms of thalassemia).
b
Reticulocyte count
Might be in appropriately low in iron deficiency, normal in thalassemia.
c
Hemoglobin electrophoresis
Might how elevated HbA2 levels in beta thalassemia, but would be normal in alpha thalassemia.
d
Serum ferritin
This is the highest yield test at this point.

Here is the serum ferritin result:

Which of the following would you predict:

a
Low total iron binding capacity (TIBC)
b
High total iron binding capacity (TIBC)
c
Low transferrin saturation (TSAT)
d
Normal transferrin saturation (TSAT)
e
High transferrin saturation (TSAT)

Here is the full iron panel:

TSAT, transferrin saturation

Are these results diagnostic of iron deficiency?

a
Yes
The low ferritin level itself (independent of the serum iron or TSAT) is diagnostic of iron deficiency.
b
No

Is it possible that the patient has thalassemia trait/minor in addition to iron deficiency?

a
Yes
b
No

Here are the results of the hemoglobin (Hb) electrophoresis (the numbers are reported as % of total Hb):

Normal HbA2 levels are 2.2% – 3.3%.

Relation between HbA2 and MCV in 150 patients with iron deficiency:

Scand J Clin Lab Invest 1999; 59: 65-70

An explanation of why/how iron deficiency leads to low HbA2 levels:

Iron insufficiency leads to the formation of a repressor of initiation of protein synthesis which preferentially affects α globin chain initiation, resulting in a relative deficiency of α chains. With the β chains being more competitive than δ chains in heme binding, HbA is formed preferentially, compared to HbA2. The lack of iron, interfering with δ chain synthesis and decreased transcription or translation of the δ globin gene in iron deficiency anemia (IDA) have also been put forth as mechanisms for low HbA2 levels in IDA.

Mutation Research-Reviews in Mutation Research 788 (2021) 108387

Because iron deficiency suppresses HbA2 expression and therefore masks the effect of beta thalassemia on HbA2 levels, the results of the hemoglobin electrophoresis are not interpretable in the setting of iron deficiency. This difficulty is articulated in an addendum to the patient’s results:

The hemoglobin electrophoresis is unchanged in alpha thalassemia. A normal hemoglobin electrophoresis in a patient with microcytosis of unclear etiology and normal iron stores is generally assumed to be caused by alpha thalassemia.

Here is a timeline of the patient’s prior labs:

Hb (g/dL)MCV (fL)
Day of clinic visit8.155
4 months earlier10.665
3 years earlier10.165

Which of the following is/are likely to be true?

a
The patient has chronic iron deficiency
b
The patient has a background history of thalassemia

Let’s add the hematocrit (Hct) to the mix:

Hb (g/dL)Hct (%)MCV (fL)
Day of clinic visit8.132.755
4 months earlier10.638.565
3 years earlier10.134.965

True or false: the patient’s MCHC is significantly lower on the day you see him compared to the past?

a
True
MCHC = Hb/Hct (see next slide for values).
b
False

Here we have added mean corpuscular hemoglobin concentration (MCHC) results:

Hb (g/dL)Hct (%)MCV (fL)MCHC (g/dL))
Day of clinic visit8.132.75524.8
4 months earlier10.638.56527.5
3 years earlier10.134.96528.9

MCHC = Hb/Hct

A quick way to determine whether the MCHC is normal based on the hemoglobin (Hb) and Hct is to apply the “3:1 rule”:

  • Normally the Hct is 3 x the Hb; so for a Hb of 15 g/dL the expected Hct is 45%.
  • If we take the patient’s Hb of 8.1 g/dL, the expected Hct is about 24%. However, his Hct was 31.5%; that is a violation of the 3:1 rule and is indicative of hypochromia, and virtually diagnostic of iron deficiency.
  • By contrast, earlier Hbs and Hcts, especially from 3 years ago, conform more closely to the 3:1 rule. In other words, the MCHC is closer to normal.

Hypochromia is more characteristic of iron deficiency than thalassemia trait/minor.

Overall, the data are consistent with a diagnosis of iron deficiency anemia superimposed on underlying thalassemia trait/minor.

Several discriminant formulas have been published for distinguishing thalassemia from iron deficiency in patients with microcytic anemia:

Lab Medicine 2020;51;628–634

It is interesting to note that despite the observation that iron deficiency is associated with a lower mean corpuscular hemoglobin concentration compared to thalassemia, the mean corpuscular hemoglobin concentration (MCHC) is conspicuously absent from these formulas.

Among the more popular formulas is the Mentzer index, first described in 1973. The index is very simple: MCV/RBC.

Mentzer Index > 13 indicates a greater likelihood of iron deficiency anemia diagnosis.

Let’s apply the Mentzer index (MCV/RBC count) to this patient over time:

Red cell countMCV (fL)Mentzer index
Day of clinic visit5.94559.3
4 months earlier5.916511
3 years earlier5.346512.2

Mentzer Index > 13 indicates a greater likelihood of iron deficiency anemia diagnosis

We can see from this analysis that while the Mentzer index is consistent with a diagnosis of thalassemia trait/minor, it does not reveal superimposed iron deficiency on the day you see the patient.

What is the Mentzer index really saying?


The Mentzer index reflects the fact that in iron deficiency, the loss of the essential nutrient, iron, causes a block in erythropoiesis and decreased production of red blood cells (RBCs), while in thalassemia, no such block occurs. In thalassemia, the bone marrow works overtime to compensate for the small RBC size by producing and releasing many more cells. The index also takes into consideration that the MCV tends to be lower in thalassemia.

Before moving on to treatment, let’s consider a couple more lab results from the complete blood count. As we noted earlier, the platelet count is elevated:

WBC (109/L)Hb (g/dL)MCV (fL)PLT (109/L)
8.88.155516

What’s what: WBC, white blood cell count; Hb, hemoglobin; MCV, mean cell volume; MCHC, mean cellular hemoglobin concentration; RDW-SD, red cell distribution width-standard deviation; platelets, PLT; Normal values: WBC 5-10 x 109/L, RBC 4-6 x 1012/L, Hb 12-16 g/dL, Hct 35-47%, MCV 80-100 fL, MCHC 32-36 g/dL, RDW-SD < 45 fL, platelets (PLT) 150-450 x 109/L

Reactive thrombocytosis in iron deficiency:

  • Occurs in about 15% of patients with iron deficiency anemia.
  • Pathogenesis poorly understood:
    • Cytokines typically elevated in secondary (reactive) thrombocytosis (interleukin-6 [IL-6], interleukin-11 [IL-11], and thrombopoietin [TPO]) are not elevated in patients with iron deficiency and thrombocytosis.
    • Erythropoietin (EPO) is elevated in iron deficiency, but its role in mediating platelet counts is unclear.

Now let’s consider the red cell distribution width (RDW). There are two ways to express the RDW

  • RDW coefficient of variation (RDW-CV, labeled simply as RDW in the results below)
  • RDW standard deviation (RDW-SD)
WBC (109/L)Hb (g/dL)MCV (fL)RDW-CVRWD-SDPLT (109/L)
8.88.15524.543.8516

What’s what: WBC, white blood cell count; Hb, hemoglobin; MCV, mean cell volume; MCHC, mean cellular hemoglobin concentration; RDW-SD, red cell distribution width-standard deviation; platelets, PLT; Normal values: WBC 5-10 x 109/L, RBC 4-6 x 1012/L, Hb 12-16 g/dL, Hct 35-47%, MCV 80-100 fL, MCHC 32-36 g/dL, RDW-SD < 45 fL, platelets (PLT) 150-450 x 109/L

The RDW-CV is significantly increased, whereas the RDW-SD is essentially normal.

We will return to the RDW in another case study.

For now, let’s move on to treatment.

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