RDW - Kresser Institute Membership Site

Marker Name: RDW

REFERENCE RANGES FOR RDW:

Laboratory reference range: 12.3–15.4%

Functional reference range: 11.5–15%

DESCRIPTION:

Red cell distribution width (RDW) describes the variation in sizes of red blood cells in the bloodstream. Red cell distribution width is measured with mean corpuscular volume (MCV), since each of the two values can only be fully interpreted with knowledge of the other. MCV represents the mean volume across all red blood cells, but it does not provide information about red blood cell volume variability in a sample. RDW, on the other hand, indicates whether red blood cells are roughly the same volume or have a wide range of volumes. It is expressed as the coefficient of variation, that is, as a percentage.¹

Modern laboratories measure RDW using automated hematology instruments. These instruments only report RDW, but they have the ability to generate a histogram of red blood cell widths. Typically, the distribution of red blood cell widths is symmetrical, similar to a normal distribution.² Most red blood cells will have a width near the mean with fewer blood cells on either end of the distribution (small widths and large widths). However, abnormalities in this distribution curve could suggest the presence of certain disease states.²

If there is a “shoulder” on the right side of the curve, this indicates the presence of very large red blood cells in the sample. These red blood cells could be macrocytes or reticulocytes. Conversely, a “shoulder” on the left side of the curve indicates the presence of very small red blood cells such as microspherocytes, schistocytes, large platelets (macrothrombocytocytes), or platelet clumps.

The results of this histogram are not reported on standard lab results. Instead, a single mean value, the RDW, is reported. As such, RDW can be either normal or elevated; there is no medical condition that causes a consistently low RDW.¹ A normal RDW generally means that the red blood cells in a sample have a similar volume, while a high RDW indicates red blood cells with varying volumes. Importantly, a normal RDW does not necessarily mean that the red blood cells are themselves a normal size. A normal RDW could simply mean that a large portion of red blood cells are abnormally large or small.¹ For this reason, mean corpuscular volume (MCV) must be measured concurrently.

The pattern of elevated RDW and low MCV is associated with iron deficiency and sickle cell-β-thalassemia. If MCV is normal but RDW is high, it could indicate early iron, vitamin B12, or folate deficiency. This pattern may also occur in sickle cell disease, chronic liver disease, and myelodysplastic syndrome. When both RDW and MCV are high, it could also indicate folate or vitamin B12 deficiency, chronic liver disease, or myelodysplastic syndrome, but it may also reflect the administration of cytotoxic chemotherapeutic agents.

RDW and MCV are always measured together, as are the other red blood cell indices, mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC).³ These tests are virtually always performed along with a total red blood cell count, hemoglobin, and hematocrit, and, in some cases with a reticulocyte count. If it was not measured initially, abnormalities in RDW are usually confirmed and elaborated with a peripheral blood smear test.⁵

PATHOLOGICAL/CONVENTIONAL RANGE INDICATIONS: High in:^5-8

Nutrient imbalances
- Iron deficiency (especially early deficiency)
- Folate deficiency (especially early deficiency)
- Vitamin B12 deficiency (especially early deficiency)
Immune hemolytic anemia
Cytotoxic chemotherapy
Chronic liver disease
Myelodysplastic syndrome
Sickle cell disease
Sickle cell-β-thalassemia
Chronic liver disease
Cardiovascular diseases
- Peripheral artery disease
- Atrial fibrillation
- Heart failure
- Hypertension
- Acute myocardial infarction
- Acute stroke

Low in:¹

Laboratory error
Error during blood withdrawal or blood handling

FUNCTIONAL RANGE INDICATIONS:

High in:

Same as conventional indications

Low in:

Same as conventional indications

References: