RDW Blood Test: Red Cell Size Variation Explained

CV means coefficient of variation, a statistical description of spread relative to the average. It is not the percentage of red cells that are abnormal. The manufacturer’s analyser manual distinguishes this percentage measure from RDW-SD, a separate measure of distribution spread expressed in femtolitres (fL).^{See reference 4}

Before interpreting the number, check the full label and unit: RDW-CV (%) and RDW-SD (fL) are not interchangeable. Keep each with its matching laboratory interval. This guide primarily explains the percentage result commonly called RDW or RDW-CV.^{See reference 1,See reference 4}

Imagine one group of objects clustered around a similar size and another containing both small and large objects. Their averages could be similar while their spread differs. That is an illustration of average versus variation, not a simulated blood report. The red cells in the artwork likewise illustrate size variation, without a diagnostic cut-off.

In a blood count, this distinction is why RDW and MCV are read together. A normal average volume cannot describe every cell, and neither measure alone diagnoses iron deficiency, vitamin deficiency or another cause of anaemia.^{See reference 1,See reference 2}

A high RDW flag indicates greater size variation than the reporting laboratory’s interval. A normal result indicates variation within that interval, but the cells can still be uniformly smaller or larger than expected. A low flag indicates less variation and is not usually concerning on its own. These statements describe spread, not a diagnosis.^{See reference 1}

The complete blood count adds red-cell count, haemoglobin, haematocrit and other measurements. A clinician combines the pattern with symptoms and history and may order additional testing. RDW does not select an iron supplement or establish that a nutrient is deficient.^{See reference 2,See reference 3}