What causes a slightly low Red Cell Distribution Width (RDW)?

Causes of Low Red Cell Distribution Width (RDW)

A slightly low RDW (just below the normal range of 11.0-14.0%) typically indicates an unusually homogeneous population of red blood cells, which is rarely clinically significant but may occasionally be associated with certain hematological conditions. 1

Understanding RDW Values

• RDW is calculated by dividing the standard deviation of red blood cell volume by MCV and multiplying by 100 to express the result as a percentage 1
• Normal RDW is generally considered to be between 11.0-14.0%, with values below 11.0% considered low 1
• Low RDW indicates that red blood cells are very uniform in size, showing minimal variation 1

Potential Causes of Low RDW

• Certain hemoglobinopathies, particularly thalassemia minor, can present with low or low-normal RDW and microcytosis (low MCV) 1
• Polycythemia vera in early stages may occasionally present with low RDW as it produces a homogeneous population of red blood cells 2
• Recent blood transfusions can cause temporarily low RDW as transfused blood typically contains red cells of uniform size 1
• Some cases of early anemia of chronic disease before significant changes in red cell morphology occur 1
• Certain rare congenital conditions affecting red cell production 1

Clinical Significance and Approach

• Low RDW alone is rarely clinically significant and is much less commonly discussed in medical literature compared to elevated RDW 1, 3
• When evaluating low RDW, it should always be interpreted alongside other hematologic parameters including hemoglobin, MCV, and other red cell indices 1
• If other abnormal hematologic parameters are present, further investigation may be warranted despite the RDW being only slightly out of range 1

Diagnostic Considerations

• Complete blood count with review of other red cell indices (hemoglobin, MCV, MCH, MCHC) should be the first step in evaluation 1
• If microcytosis is present with low RDW, consider hemoglobinopathy evaluation, particularly for thalassemia 1
• If polycythemia is suspected (elevated red cell count), further workup may include JAK2 mutation testing 2
• Consider checking iron studies, vitamin B12, and folate levels to rule out early deficiency states 1

Important Caveats

• RDW values can vary slightly between different laboratory instruments, so values should be interpreted according to the specific laboratory's reference range 1
• A slightly low RDW that is just below the normal range is much less concerning than elevated RDW, which has been associated with increased mortality in multiple studies 4, 5
• Repeat testing may be warranted if the value is borderline low to confirm the finding before extensive workup 1
• Unlike elevated RDW, which has strong prognostic implications, slightly low RDW has not been consistently associated with adverse outcomes 4, 6