Elevated red cell distribution width (RDW) means your red blood cells vary more than usual in size. When anemia is also present, this pattern can help healthcare professionals look for causes such as iron deficiency or low vitamin B12 or folate. RDW does not measure how much hemoglobin you have and cannot identify the cause by itself; it is interpreted alongside the rest of your blood count and other tests.
What is the connection between elevated RDW and anemia?
Red cell distribution width (RDW) is a measurement included in many complete blood count (CBC) tests. It describes how much the sizes of your red blood cells differ from one another. A high RDW means there is a wider-than-usual range of red blood cell sizes, a pattern sometimes called anisocytosis.
Anemia occurs when the blood has too little hemoglobin or an insufficient amount of healthy red blood cells to meet the body's needs. Hemoglobin is the protein in red blood cells that carries oxygen to tissues. RDW and anemia are related because several conditions that affect red blood cell production can produce cells of different sizes. According to MedlinePlus, RDW helps healthcare professionals investigate anemia and distinguish between possible causes when considered with other blood test results.
The two measurements answer different questions. Hemoglobin and related blood count measurements help establish whether anemia is present, while RDW shows how much red blood cell size varies. An elevated RDW can occur with anemia, but it does not automatically mean someone has anemia. Similarly, a normal RDW does not rule out anemia.
What can a high RDW reveal about the type of anemia?
Healthcare professionals often interpret RDW alongside mean corpuscular volume (MCV), which measures the average size of red blood cells. Looking at both measurements can provide more information than looking at either one alone. The Merck Manual Professional Edition explains that red blood cell indices help narrow the possible causes of anemia. However, these patterns are clues rather than diagnoses.
| Blood test pattern | What it may suggest |
|---|---|
| Low hemoglobin, low MCV, and high RDW | This pattern may occur with iron-deficiency anemia, in which red blood cells tend to be smaller and vary more in size. |
| Low hemoglobin, high MCV, and high RDW | This combination may occur with vitamin B12 or folate deficiency, which can produce unusually large red blood cells with differences in size. |
| Low hemoglobin, normal MCV, and high RDW | A mixture of smaller and larger red blood cells may produce a normal average size even when individual cells vary considerably. |
| Normal hemoglobin and high RDW | Red blood cell sizes vary, but the hemoglobin result does not meet the criteria for anemia. The finding still needs to be interpreted with the rest of the blood count and clinical context. |
For example, someone can have a combination of conditions that affect red blood cell size in opposite directions. Smaller cells and larger cells may average out to a seemingly normal MCV. An elevated RDW can help reveal that difference, although additional tests are needed to establish the underlying explanation.
Which conditions can cause both elevated RDW and anemia?
The relationship depends on why the red blood cells are varying in size. Some of the more relevant possibilities include the following:
- Iron deficiency: The body needs iron to make hemoglobin. When iron stores become low, red blood cells may become smaller and more varied in size. Iron deficiency can develop from blood loss, inadequate iron absorption, or insufficient iron intake.
- Vitamin B12 deficiency: Vitamin B12 is needed for normal red blood cell development. A deficiency can lead to anemia with larger red blood cells and variation in their size.
- Folate deficiency: Folate also supports normal red blood cell production. Insufficient folate can cause anemia with enlarged red blood cells.
- More than one underlying problem: When two conditions affect red blood cells differently, such as iron deficiency alongside a vitamin deficiency, the blood may contain a mixture of smaller and larger cells. This can raise RDW even if MCV remains within its reference range.
The National Heart, Lung, and Blood Institute (NHLBI) identifies blood loss and problems with iron absorption among the causes of iron-deficiency anemia. Blood loss may be associated with heavy menstrual periods, gastrointestinal bleeding, injury, or other medical problems. Determining the reason for iron deficiency is important because correcting the underlying cause may be necessary alongside replacing iron.
These causes do not account for every elevated RDW result. Other medical conditions and inherited blood disorders can also affect red blood cell measurements. The complete blood count, medical history, symptoms, and any additional findings help determine which explanations are worth investigating.
What symptoms may occur when anemia accompanies a high RDW?
Symptoms depend on whether anemia is present, its underlying cause, and how much it affects the body's oxygen supply. Some people with anemia have no noticeable symptoms, while others experience changes that affect everyday activities. Possible symptoms include:
- Unusual tiredness or low energy
- Weakness or reduced ability to exercise
- Dizziness or lightheadedness
- Shortness of breath, especially during physical activity
- Paler-than-usual skin
- Headaches or a noticeable heartbeat
These symptoms can have many explanations and do not indicate whether RDW is elevated. The RDW value itself does not measure symptom severity. A person may have an abnormal red blood cell measurement without feeling unwell, so laboratory findings need to be reviewed alongside the person's overall health.
How do healthcare professionals investigate elevated RDW and anemia?
Evaluation usually begins with a review of the complete blood count. In addition to RDW, the healthcare professional may examine hemoglobin, hematocrit, red blood cell count, MCV, and other blood cell measurements. The MedlinePlus guide to red blood cell indices explains how these measurements help distinguish between different types of anemia and guide further testing.
Depending on the results and clinical history, additional tests may include:
- Ferritin and iron studies: These help assess the body's iron stores and iron availability when iron deficiency is suspected.
- Vitamin B12 and folate measurements: These may be appropriate when a vitamin deficiency is a possible explanation.
- Reticulocyte count: Reticulocytes are young red blood cells. Their number can help show how the bone marrow is responding to anemia.
- Peripheral blood smear: A laboratory specialist examines the blood cells under a microscope to assess their size, shape, and appearance.
- Other targeted tests: Depending on the findings, further investigations may look for blood loss, chronic illness, or another underlying cause.
Not everyone needs every test. The choice depends on the combination of blood count results, medical history, examination findings, symptoms, and how the measurements have changed over time.
How should you interpret an elevated RDW result?
Start by checking the RDW result alongside hemoglobin and MCV rather than interpreting the RDW number in isolation. Laboratories use different methods and reference ranges, so a result flagged as high should be compared with the range printed on the report. According to the Cleveland Clinic, a high RDW may indicate anemia or another related condition, but additional tests are needed to determine what it means in an individual case.
If your report shows both elevated RDW and low hemoglobin, arrange a review with a healthcare professional to discuss the pattern and possible causes. Persistent fatigue, worsening breathlessness, unusual bleeding, or other concerning symptoms are also reasons to seek assessment. In children, blood count reference values vary with age, so adult reference ranges should not be applied to younger patients.
Avoid starting iron supplements solely because your RDW is high. Iron is appropriate when iron deficiency is established or when a healthcare professional recommends it. Vitamin deficiencies, blood loss, and other underlying conditions may require different approaches. Understanding the cause helps ensure that any treatment addresses the actual problem rather than only one laboratory result.
What is the main takeaway about elevated RDW and anemia?
Elevated RDW means red blood cells vary more than usual in size, and this finding can help explain the pattern seen in some forms of anemia. It is particularly useful when interpreted alongside hemoglobin and MCV, as well as iron, vitamin, and other tests when indicated. High RDW alone cannot establish that anemia is present or identify its cause. A review of the full blood count and relevant clinical information provides a clearer picture.