Knowledge Check

Sort the conditions (top) according to the red blood cell (RBC) phenotype (below)

Low testosterone
Vitamin B12 deficiency
Hydroxyurea
Thalassemia
Folate deficiency
Iron deficiency
Chronic kidney disease
Microcytic RBCs
Normocytic RBCs
Macrocytic RBCs

True or false: anemia of inflammation is usually macrocytic.

a
True
b
False
About 80% of cases are normocytic, 20% microcytic.

Notes on reticulocytes:

  • Reticulocytes are a heterogeneous population of cells at various stages of maturation, characterized by progressive loss of RNA, organelles and membrane proteins as well as changes in size, morphology and metabolic properties.
  • About 2 million reticulocytes are released by the bone marrow per second.
  • As they mature, reticulocytes lose about 20% volume (they become smaller) and they acquire a biconcave shape.
  • The mean cell volume (MCV) of reticulocytes averages about 115 fL.

There is normally a spectrum of sizes of reticulocytes, which reflect their maturation stage. Depending on how young the reticulocytes are when they are released from the marrow and how fast they mature in the circulation, their size can vary considerably and therefore their numbers will affect the MCV in ways that are difficult to predict. Some have proposed a correlation between reticulocyte count and MCV. Unfortunately, it’s not that tidy. The reticulocyte count does not always map to the MCV in predictable ways.

The effect of an absolute reticulocyte count on the MCV is more pronounced in patients with severe anemia because of their disproportionate representation among the total red cell population.

Which of the following is a clue that macrocytosis is caused by elevated reticulocyte count (one answer)?

a
Anemia
b
Elevated mean corpuscular hemoglobin (MCH)
c
Elevated mean corpuscular hemoglobin concentration (MCHC)
d
Reduced mean corpuscular hemoglobin concentration (MCHC)
Correct! Reticulocytes get smaller as they mature into red blood cells. They do not lose or gain hemoglobin during this process. Thus, the MCHC must get progressively higher.

True or false: anemia of non-alcoholic chronic liver disease is almost always macrocytic.

a
True
b
False
More cases are normocytic.

True or false: macrocytosis of chronic liver disease is almost always associated with anemia.

a
True
b
False
Most cases are unaccompanied by anemia (see next slide).

Data from 303 alcoholics with liver disease (95 females: 208 males), 60 non-alcoholics with chronic liver disease (44 females: 16 males) and 35 control subjects (15 females: 20 males). Clin Lab Haematol. 1981;3:35-44.

Wait, don’t hyperglycemia and hypernatremia cause water to shift out of cells? Shouldn’t that cause microcytosis?

Measuring mean cell volume (MCV) from a patient with hypernatremia or hyperglycemia. MCV is measured after blood dilution (1/50,000) by a solution of Isoton (330 mOsm/kg H20 ). The red blood cells (RBCs), which were adapted to a hyperosmolar environment in the patient’s bloodstream are now exposed to a relatively “hypotonic” environment inside the diluent. This causes water to move into the red blood cells.

To summarize, in patients with hyperglycemia of hypernatremia, red blood cells are adapted to a hyperosmolar environment in vivo (in the patient’s circulation). When performing the complete blood count, the red cells are diluted in isotonic diluent, which they experience/perceive as hypotonic. Water then moves into the cells, causing cell swelling and increased cell volume. This leads to:

  • Decreased mean corpuscular hemoglobin concentration (MCHC)
  • Elevated hematocrit (Hct)
  • Unchanged mean corpuscular hemoglobin (MCH)

We will return to other causes of spurious macrocytosis in another module.

Which descriptions apply to the term megaloblastic (more than one answer may apply)?

a
Impaired DNA synthesis
b
Discordance or asynchrony in nuclear-cytoplasmic maturation in red cell precursors in the bone marrow.
c
Hypersegmented neutrophils on the peripheral smear

True or false: macrocytosis is always associated with anemia.

a
True
b
False
Overall, about 50% of macrocytic cases occur without anemia (see next slide)

Percentage of cases of macrocytosis that are associated with anemia. PMID refers to the PubMed ID for the study of interest.

Anemia is:

  • More common in those with:
    • MCV > 110 fL
    • Vitamin B12 deficiency
    • Folate deficiency
    • Hemolytic anemia
    • Underlying bone marrow disease
    • Liver disease
  • Less common those with:
    • MCV < 110 fL
    • Alcoholism
    • Medications
    • Unexplained macrocytosis

True or false: we can safely ignore cases of macrocytosis without anemia.

a
True
b
False
Such cases may be clinically significant!

Notes on macrocytosis

  • Prevalence:
    • Macrocytosis occurs in about 3% of the general population.
  • About 50% of cases of macrocytosis occur without anemia (termed isolated macrocytosis):
    • The absence or presence of anemia does not predict a worse outcome.
    • There is a negative correlation between MCV and Hb/Hct.
    • Absence of anemia does not mean that the condition can be ignored!
  • About 30% of cases of macrocytosis remain unexplained despite extensive work up:
    • Unexplained cases are typically associated with MCV < 110 fL.
    • Patients with unexplained macrocytosis are less likely to be anemic.
  • Highest MCV (> 130 fL) seen in:
    • B12 deficiency
    • Medications
    • Liver disease
    • Folate deficiency

Notes on individual causes of macrocytosis

  • Vitamin B12 deficiency:
    • The higher the mean cell volume (MCV), the likelier the diagnosis of B12 deficiency.
  • Alcohol:
    • The most common cause of macrocytosis in the primary care setting.
    • Up to 70-80% of alcoholics have macrocytosis.
    • Rarely associated with MCV>110 fL.
    • The majority of cases are non-anemic (80-90%).
    • MCV is higher in female vs male drinkers.
    • MCV is a surrogate marker of alcohol intake.
    • Shows a dose response to alcohol and takes 8-16 weeks to correct following abstinence.
  • Liver disease:
    • Macrocytosis occurs in 1/3 – 2/3 of patients with liver cirrhosis.
    • Elevation in MCV is greater in those with alcoholic vs. non-alcoholic liver disease.
    • Degree of macrocytosis correlates with severity of liver disease.
    • The MCV in the cirrhotic patients is almost always < 120 fL.
    • Only a fraction of liver disease-associated macrocytosis is associated with anemia.
    • Non-alcoholic liver disease associated with lower rate of macrocytosis; macrocytic anemia occurs mostly in liver cirrhosis, especially in alcoholic cirrhosis.
  • Drugs that cause macrocytosis include:
    • HIV antiretroviral therapy:
      • Zidovudine
      • Stavudine
    • Azidothymidine
    • Hydroxyurea
    • Cancer chemotherapy
    • Azathioprine
    • Methotrexate
    • Metformin
    • Sulfasalazine
    • Dilantin
    • Valproic acid
    • Bactrim
  • Retics may increase the MCV above 110, but not above 120 fL.
  • Hypothyroidism:
    • Anemia present in about 7% of hypothyroid patients.
    • Anemia usually normocytic (>90% of cases), rarely macrocytic (about 6% of cases) .
    • Macrocytosis reported in up to 35-40% of hypothyroid patients; only one fifth of tese patients have anemia.
  • Myelodysplastic syndrome:
    • Vast majority of patients with MDS and anemia have macrocytosis.
    • Patients often have macrocytosis preceding dx of MDS by several years.

Other potential causes of macrocytosis reported in the literature include:

  • Smoking
  • Solid tumors
  • Splenectomy
  • Solid tumors (independent of chemotherapy)
  • Down syndrome

True or false: the mean corpuscular volume (MCV) in vitamin B12 and folate deficiency tends to be higher than most other causes of macrocytic anemia.

a
True
The higher the MCV, the more likely B12 and folate deficiency. If the MCV exceeded 120 fL, megaloblastic hematopoiesis (B12 and folate deficiency) is highly likely (>80 % in one study).
b
False

True or false: some patients with macrocytosis may have 2 or more etiologies.

a
True
b
False

True or false: the incidence of vitamin B12 and folate deficiency is lower today than 30 years ago.

a
True
b
False

True or false: vitamin B12 deficiency can present with macrocytosis without anemia.

a
True
Some patients with pernicious anemia may present with non-anemic macrocytosis for several months prior to diagnosis.
b
False

Acute change in mean cell volume.

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