From MCV to clinical encounter

The patient has a mean cell volume (MCV) of 64 fL. Which one of the following questions would provide the most useful information?

a
Do you have craving for ice?
This symptom would suggest iron deficiency. It is present in up to 50% of such patients. It is hard to fault you for choosing this answer, thought it might be argued that a different question has greater diagnostic yield in this case.
b
Are you aware of whether your red cells have been abnormal all your life?
This gets at the question of whether the patient might have a type of hereditary hemoglobinopathy, e.g., thalassemia.
c
Do you have a rheumatological condition?
Anemia of inflammation is part of the differential diagnosis for microcytosis, but it does not cause an MCV this low.
d
Are your periods heavy?
A history of menorrhagia might be present in a patient with iron deficiency anemia but it not a high-yield question.

So, we are left with iron deficiency and/or thalassemia as possible etiologies. You ask the patient how long she has known about the condition. She says that someone told her many years ago that she has thalassemia, but she cannot remember the details. If could have one test to order to sort this out what would you choose?

a
Red cell distribution width
Tends to b elevated in iron deficiency, but not in thalassemia. However, it has poor discriminatory value.
b
Serum ferritin
Low serum ferritin would be diagnostic of iron deficiency while normal levels would point to thalassemia as the likely diagnosis.
c
Peripheral smear
The findings would differ between iron deficiency (hypochromic microcytes with pencil-shaped elliptocytes) and thalassemia (normochromic microcytes), but the discriminatory value is not as high as serum ferritin.
d
Mean corpuscular hemoglobin
We know this will be low because the mean cell volume is low.

If this patient has thalassemia minor, what would you expect to find?

a
Increased red cell count
b
Decreased mean corpuscular hemoglobin concentration
c
Increased red cell distribution width
d
Normal or near normal hemoglobin (Hb)
e
Normal or increased serum ferritin

If this patient has iron deficiency anemia, what would you expect to find?

a
Increased red cell count
b
Decreased mean corpuscular hemoglobin concentration
c
Increased red cell distribution width
d
Normal or near normal hemoglobin (Hb)
e
Normal or increased serum ferritin

The patient’s red cell count is, in fact, high at 5.65 x 1012/L (normal range is 3.9-5.2 x 1012). Does this mean the patient has polycythemia?

a
Yes
b
No
Polycythemia is not defined by the red cell count. Patients with thalassemia minor/trait have increased red cell counts, but normal or low-normal hemoglobin and hematocrit. Their red cell counts are high to compensate for the small size of the red cells, thereby allowing them reach a normal Hct (Hct = red cell count x mean cell volume).

Polycythemia is defined by a patient’s hemoglobin or hematocrit, which are surrogate markers for red cell mass.

The Mentzer index is one of many formulas that has been proposed to help distinguish between iron deficiency anemia and beta thalassemia based on parameters from the complete blood count.

A Mentzer index of < 13 is suggestive of thalassemia, > 13 is suggestive of iron deficiency. Thus, this patient’s Mentzer index of 11 indicates a likely diagnosis of thalassemia minor.

What is the Mentzer index really trying to say? That the patient with thalassemia minor is producing more red cells to compensate for the small red cell volume to reach a normal hematocrit.

The patient’s hemoglobin is 12.7 g/dL and hematocrit 35.6%.

Based on this information, which red cell schematic is likely to resemble the patient’s?

a
A
b
B
Central pallor correlates inversely with the mean corpuscular hemoglobin concentration (MCHC). The patient’s MCHC is in the normal range, so the central pallor is predicted to be normal.

This value for the mean corpuscular hemoglobin concentration (MCHC) is at the high end of normal (normal range 32-36 g/dL). In fact, previous labs for the patient show her MCHC ranging between 35.5 g/dL and 37.3 g/dL. In other words, the patient’s red cells are approaching the phenotype shown in panel C:

Does that provide any clues about the underlying diagnosis?

Here is the patient’s complete blood count when you see her:

Any time there is an abnormal white cell count, it is important to order a differential. Here are the results (accompanied by a cheat sheet for diagnosing lower- and higher-than-normal values):

Ideally, we would like a reticulocyte count, but one was nor ordered in this case. Based on what you know so far about the patient, what would you expect the reticulocyte count to be:

a
Inappropriately low
b
Appropriately high

The patient’s peripheral smear resembles the one shown below:

So far, everything is pointing towards thalassemia minor with two exceptions:

  • She has a high normal and at times elevated mean corpuscular hemoglobin concentration (MCHC).
  • She has many targets and some irregularly contracted red cells on the peripheral smear.

What are these data suggestive of:

a
HbE disease
b
HbC disease
The elevated MCHC and the co-existence of irregularly contracted cells and target cells are characteristic of this condition.
c
Autoimmune hemolytic anemia
d
Chronic liver disease
e
Lead poisoning

What would you expect from her history?

a
Headache
b
No symptoms
c
Palpitations
d
Hair loss and brittle nails
e
Weight loss and night sweats

This is a 38 year-old woman referred to you for evaluation of microcytic anemia. She has a long history of microcytosis. She recently transferred care to a new PCP, who ordered a hemoglobin electrophoresis. The results showed HbA 20.8%, HbC 69.2%, HbA2 6.8% and HbF 3.2%. She denies a history of anemia, iron deficiency, or blood transfusions. She denies symptoms of pica, headache, restless legs, alopecia, brittle nails, shortness of breath on exertion or chest pain. She has insulin-dependent diabetes. She has had breast reduction surgery and two Cesarian sections. There is no known family history of thalassemia or other hemoglobinopathy. She is from Trinidad, she is a non-smoker and she drinks socially. She works as a nurse’s aid. In addition to insulin, she takes metformin. She is allergic to sulfonamides.

The following describes this patient’s physical exam when you see her in the clinic:

General appearance: Looks well, in no distress

Vital signs: Heart rate 2/min, blood pressure 133/70 mmHg, respiratory rate 13/min, T 99.2oF

Head and neck: No scleral icterus, no lymphadenopathy

Chest: Normal to inspection, palpation, percussion, and auscultation

CVS: Heart sounds normal, no S3 or S4, no murmur

Abdomen: Non-tender, no hepatosplenomegaly

CNS: No focal changes

Extremities: Normal

Integument: No rash









Let’s take a closer look at her hemoglobin electrophoresis:

How would you interpret this?

a
HbCC
No, she would not have any HbA in that case.
b
HbAC
No, the HbA would be closer to 50% in the heterozygous state and the HbA2 would not be elevated.
c
HbC-beta0 thalassemia
This would explain the elevated HbA2, but not the presence of any HbA.
d
HbC-beta+ thalassemia
This would explain the presence of some HbA and the elevsated HbA2.

The following was the report of the patient’s hemoglobin electrophoresis:

Let’s see how Hb electrophoresis can used to discriminate between different hemoglobinopathies. Typically, samples are run on cellulose acetate at alkaline pH, and then on an agarose gel at acid pH. Different types of hemoglobin migrate in distinct patterns on each gel allowing for identification of the relevant hemoglobin:

JaypeeDigital | eBook Reader
Left, cellulose acetate at alkaline pH is the initial procedure. At this pH Hb is negatively charged and moves toward the positively charged anode. Right, next step is the Citrate agar or agarose gel at acid pH. Note how the position of HbC changes between the two conditions.

We began this exercise by discussing the differential diagnosis of microcytosis. The leading causes are iron deficiency anemia, thalassemia and inflammation. There are rarer causes such as hyperthyroidism, certain congenital anemias and conditions associated with increased schistocytes. Where does Hb C fit into all this? In the next two slides, we will consider hemoglobinopathies in general and will then place Hb C into context.

Hemoglobinopathies

Hemoglobinopathies are a group of recessively inherited genetic conditions affecting the alpha or beta globin chains of hemoglobin. Hemoglobinopathies are the most common autosomal recessive disorder worldwide, with 7% of the global population carrying an abnormal hemoglobin (Hb) alpha or beta globin chain allele.

Hemoglobinopathies may be split into 2 groups, one leading to quantitative changes in hemoglobin, the other to qualitative changes:

  • Thalassemia syndromes:
    • Quantitative defect caused by decreased expression of one of the two globin chains of the hemoglobin molecule, α (HBA) and β (HBB).
    • Decreased expression can result from:
      • Deletion of the structural gene(s).
      • Mutations that result in decreased RNA synthesis, processing, or stability.
      • Mutations resulting in decreased protein synthesis or stability.
    • The decrease in expression of one of the globin chains results in accumulation of excess polypeptides encoded by the unaffected gene. This chain imbalance causes abnormal RBC maturation, resulting in microcytosis as the characteristic laboratory abnormality.
    • Thalassemia syndromes include:
      • Alpha thalassemia
      • Beta thalassemia
      • Hb E (here’s the catch – Hb E actually has a thalassemia phenotype)
  • Structural hemoglobin variants:
    • The hemoglobin variants are caused by amino acid substitutions in either globin chain.
    • Clinical disease includes:
      • Thalassemia-like phenotype
      • Sickling
      • Hemolysis due to unstable hemoglobins
      • Hemoglobins associated with altered oxygen affinity
      • Hemoglobins in which iron cannot be maintained in the ferrous (Fe2+) state
    • The main Hb abnormalities worldwide are:
      • Hb S
      • Hb C
      • Hb E
    • Rarer abnormalities that may have clinical significance include:
      • Hb D
      • Hb OArab
      • Hb Lepore

As can be appreciated from the previous slides, there are many different kinds of hemoglobinopathy. In fact, more than 1,800 hemoglobin variants have been characterized! The good news is that only a small subset are associated with microcytosis. These include:

  • Those with a thalassemia phenotype
    • Beta thalassemia
    • Alpha thalassemia
    • Hb E
  • Hb C

All of this is to say, if you working up a patient with microcytosis who you think has thalassemia, you should consider Hb C and Hb E in the differential diagnosis, but not any of the other hundreds of hemoglobin variants.