Can I convert one into the other? Which one is currently in the software? What is the reference range for RDW-SD?
The red cell distribution width (RDW) may be expressed as a percentage (RDW-CV) or in femtoliters representing a standard deviation from the mean (RDW-SD). The numbers are not interchangeable.
The red cell distribution width (RDW) is a measurement of the variability between red blood cells in circulation. It is calculated from RBC and Mean Corpuscular Volume (MCV) values, both of which are represented in the CBC.
Red blood cells are usually uniform in size with little variation. However, when some cells are small (microcytic) or large (macrocytic) the RDW will reflect the degree of difference in size. The RDW will increase with various anemias including those caused by iron, folate, or B12 deficiency. It will also increase with hemolytic anemia and hemoglobinopathies such as sickle cell anemia (Pagana 2019). Increasing RDW is associated with an increased risk of diabetes, hypertension, cardiovascular disease, and inflammation (Fava 2019, Gang 2016).
When RDW is expressed as a coefficient of variation (CV) it is called RDW-CV and is reported as a percentage: RDW-CV (%) = 1 SD (femtoliters [fl])/MCV (fl) × 100. The RDW-SD is expressed in femtoliters (fL) and is based on the width of RBC volume distribution curve at 20% above baseline (Caporal 2013).
The standard lab reference range for RDW-CV is 11-15% and for RDW-SD it is 38-49 fL (Quest). The numbers are not interchangeable.
The software uses RDW-CV (shortened to the much-used RDW), so results are expressed as a percentage. An optimal range between 11 and 12.6% will help identify the early risk of anemia and metabolic disease. We do not use RDW-SD in the software.
References
Caporal, Fernando Augusto, and Samuel Ricardo Comar. "Evaluation of RDW-CV, RDW-SD, and MATH-1SD for the detection of erythrocyte anisocytosis observed by optical microscopy." Jornal Brasileiro de Patologia e Medicina Laboratorial 49 (2013): 324-331.
Fava, Cristiano et al. “The role of red blood cell distribution width (RDW) in cardiovascular risk assessment: useful or hype?.” Annals of translational medicine vol. 7,20 (2019): 581. doi:10.21037/atm.2019.09.58
Gang, Li, and Wan Lifang. “Association of the Elevated Red Blood Cell Distribution Width with the Risk of Developing Diabetes Mellitus.” Internal medicine (Tokyo, Japan) vol. 55,15 (2016): 1959-65. doi:10.2169/internalmedicine.55.5956
Pagana, Kathleen Deska; Pagana, Timothy J.; Pagana, Theresa N. Mosby's Diagnostic and Laboratory Test Reference. Elsevier Health Sciences. 2019.