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Three neglected numbers in the CBC: The RDW, MPV, and NRBC count

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Release date: March 1, 2019
Expiration date: February 29, 2020
Estimated time of completion: 1 hour

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ABSTRACT

The complete blood cell count (CBC) is one of the most frequently ordered laboratory tests, but some values included in the test may be overlooked. This brief review discusses 3 potentially underutilized components of the CBC: the red blood cell distribution width (RDW), the mean platelet volume (MPV), and the nucleated red blood cell (NRBC) count. These results have unique diagnostic applications and prognostic implications that can be incorporated into clinical practice. By understanding all components of the CBC, providers can learn more about the patient’s condition.

KEY POINTS

  • The RDW can help differentiate the cause of anemia: eg, a high RDW suggests iron-deficiency anemia, while a normal RDW suggests thalassemia. Studies also suggest that a high RDW may be associated with an increased rate of all-cause mortality and may predict a poor prognosis in several cardiac diseases.
  • The MPV can be used in the evaluation of thrombocytopenia. Furthermore, emerging evidence suggests that high MPV is associated with worse outcomes in cardiovascular disorders.
  • An elevated NRBC count may predict poor outcomes in a number of critical care settings. It can also indicate a serious underlying hematologic disorder.


 

References

The complete blood cell count (CBC) is one of the most frequently ordered laboratory tests in both the inpatient and outpatient settings. Not long ago, the CBC required peering through a microscope and counting the red blood cells, white blood cells, and platelets. These 3 numbers are still the primary purpose of the test.

Now, with automated counters, the CBC report also contains other numbers that delineate characteristics of each cell type. For example:

The mean corpuscular volume is the average volume of red blood cells. Providers use it to classify anemia as either microcytic, normocytic, or macrocytic, each with its own differential diagnosis.

The differential white blood cell count provides absolute counts and relative percentages of each type of leukocyte. For example, the absolute neutrophil count is an important measure of immunocompetence.

But other values in the CBC may be overlooked, even though they can provide important information. Here, we highlight 3 of them:

  • The red blood cell distribution width (RDW)
  • The mean platelet volume (MPV)
  • The nucleated red blood cell (NRBC) count.

In addition to describing their diagnostic utility, we also discuss emerging evidence of their potential prognostic significance in hematologic and nonhematologic disorders. By incorporating an awareness of their value in clinical practice, providers can maximize the usefulness of the CBC.

RED BLOOD CELL DISTRIBUTION WIDTH

Example of normal red blood cell distribution width (RDW) of 13.5% (red line) in a patient with a normal complete blood cell count. B: Example of an increased RDW of 28.8% in a patient with iron deficiency shortly after initiation of iron supplementation.

Figure 1. A: Example of a normal red blood cell distribution width (RDW) of 13.5% (red line) in a patient with a normal complete blood cell count. B: Example of an increased RDW of 28.8% in a patient with iron deficiency shortly after initiation of iron supplementation.

The RDW is a measure of variation (anisocytosis) in the size of the circulating red cells. The term “width” is misleading, as the value is not derived from the width of the red blood cell, but rather from the width of the distribution curve of the corpuscular volume (Figure 1). Therefore, a normal RDW means that the cells are all about the same size, while a high RDW means they vary widely in size.

The RDW can be calculated either as a coefficient of variation, with a reference range of 11% to 16% depending on the laboratory, or, less often, as a standard deviation, with a reference range of 39 to 46 fL.

The RDW can differentiate between causes of anemia

A high RDW is often found in nutritional deficiencies of iron, vitamin B12, and folate. This information is helpful in differentiating the cause of microcytic anemia, as a high RDW suggests iron-deficiency anemia while a normal RDW suggests thalassemia.1 In iron deficiency, the RDW often rises before the mean corpuscular volume falls, serving as an early diagnostic clue.

The RDW can also be high after recent hemorrhage or rapid hemolysis, as the acute drop in hemoglobin results in increased production of reticulocytes, which are larger than mature erythrocytes.

Because a range of disorders can elevate the RDW, reviewing the peripheral blood smear is an important next step in the diagnostic evaluation, specifically looking for reticulocytes, microspherocytes, and other abnormal red blood cells contributing to the RDW elevation.

A normal RDW is less diagnostically useful. It indicates the red blood cells are of uniform size, but they may be uniformly small or large depending on how long the anemia has persisted. Since red cells circulate for only about 120 days, patients who have severe iron-deficiency anemia for months to years are expected to have a normal rather than a high RDW, as their red cells of normal size have all been replaced by microcytes.

A low RDW is not consistently associated with any hematologic disorder.

RDW may have prognostic value

Emerging data suggest that the RDW may also have prognostic value in nonhematologic diseases. In a retrospective study of 15,852 adult participants in the Third National Health and Nutrition Examination Survey (1988–1994), a higher RDW was associated with a higher risk of death, with the all-cause mortality rate increasing by 23% for every 1% increment in RDW.2

This correlation is particularly prominent in cardiac disorders. In 2 large retrospective studies of patients with symptomatic heart failure, a higher RDW was a strong predictor of morbidity and death (hazard ratio 1.17 per 1-standard deviation increase, P < .001), even stronger than more commonly used variables such as ejection fraction, New York Heart Association functional class, and renal function.3

In a retrospective analysis of 4,111 patients with myocardial infarction, the degree of RDW elevation correlated with the risk of repeat nonfatal myocardial infarction, coronary death, new symptomatic heart failure, and stroke.4

It is hypothesized that high RDW may reflect poor cell membrane integrity from altered cholesterol content, which in turn has deleterious effects on multiple organ systems and is therefore associated with adverse outcomes.5

Currently, using the RDW to assess prognosis remains investigational, and how best to interpret it in daily practice requires further study.

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