biology

Red blood cell distribution width

Red blood cell distribution width is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Red blood cell distribution width rather than just read about it. In short: Red blood cell distribution width (RDW), as well as various types thereof (RDW-CV or RCDW and RDW-SD), is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count. Red blood cells have an average volume of 80–100 femtoliters, but individual cell volumes vary even in healthy blood.

Key takeaways

Red blood cell distribution width belongs to biology; place it in that map before memorising details.
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Connect Red blood cell distribution width to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Red blood cell distribution width from memory before moving on to harder problems.

Reference excerpt

Red blood cell distribution width (RDW), as well as various types thereof (RDW-CV or RCDW and RDW-SD), is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count. Red blood cells have an average volume of 80–100 femtoliters, but individual cell volumes vary even in healthy blood. Certain disorders, however, cause a significantly increased variation in cell size. Higher RDW values indicate greater variation in size. Normal reference range of RDW-CV in human red blood cells is 11.5–15.4%. If anemia is observed, RDW test results are often used together with mean corpuscular volume (MCV) results to determine the possible causes of the anemia. It is mainly used to differentiate an anemia of mixed causes from an anemia of a single cause. Deficiencies of Vitamin B12 or folate produce a macrocytic anemia (large cell anemia) in which the RDW is elevated in roughly two-thirds of all cases. However, a varied size distribution of red blood cells is a hallmark of iron deficiency anemia, and as such shows an increased RDW in virtually all cases. In the case of both iron and B12 deficiencies, there will normally be a mix of both large cells and small cells, causing the RDW to be elevated. An elevated RDW (red blood cells of unequal sizes) is known as anisocytosis. An elevation in the RDW is not characteristic of all anemias. Anemia of chronic disease, hereditary spherocytosis, acute blood loss, aplastic anemia (anemia resulting from an inability of the bone marrow to produce red blood cells), and certain hereditary hemoglobinopathies (including some cases of thalassemia minor) may all present with a normal RDW.

Calculations The "width" in RDW is sometimes thought to be "misleading", since it in fact is a measure of deviation of the volume of RBCs, and not directly the diameter. RDW-CV "width" refers to the width of the volume curve (distribution width), not the width of the cells.

RDW-SD is calculated as the width (in fL) of the RBC size distribution histogram at the 20% height level. This parameter is, therefore, not influenced by the average RBC size (mean corpuscular volume, MCV). RDW-CV (expressed in %) is calculated with the following formula:

RDW-CV = (1 standard deviation of RBC volume ÷ MCV) × 100%. Since RDW-CV is mathematically derived from MCV, it is therefore affected by the average RBC size (MCV).

Pathological implications

Normal RDW Anemia in the presence of a normal RDW may suggest thalassemia. A low Mentzer Index, calculated from CBC data [MCV/RBC < 13], may suggest this disorder but a hemoglobin electrophoresis would be diagnostic. Anemia of chronic diseases show normal RDW.

High RDW High RDW may be a result of the presence of fragments, groups of agglutination, and/or abnormal shape of red blood cells.

Iron-deficiency anemia usually presents with high RDW and low MCV. Folate and vitamin B12 deficiency anemia usually presents with high RDW and high MCV. Mixed-deficiency (iron + B12 or folate) anemia usually presents with high RDW and variable MCV. Recent hemorrhages typically present with high RDW and normal MCV. A false high RDW reading can occur if EDTA anticoagulated blood is used instead of citrated blood. See Pseudothrombocytopenia. By severity, elevated RDW can be classified as follows:

References

Further reading RDW Blood Test - Red Blood Cell Distribution Width

Illustrations

Red blood cell distribution width illustration

Worked examples

Example 1 — a first encounter with Red blood cell distribution width

Start with the simplest possible case. Write down what Red blood cell distribution width claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Red blood cell distribution width before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Red blood cell distribution width ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Red blood cell distribution width

In research: Red blood cell distribution width appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Red blood cell distribution width in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Red blood cell distribution width is common in secondary-school and first-year university syllabi. It links to neighbouring topics Blood tests, so understanding it makes those chapters shorter.
In everyday life: Look for Red blood cell distribution width outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Red blood cell distribution width in 20 minutes

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Frequently asked questions

What is Red blood cell distribution width in simple terms?

Why does Red blood cell distribution width matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Red blood cell distribution width?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Red blood cell distribution width.