We read with great interest the article ‘Red cell distribution width, haemoglobin A1c and incidence of diabetes mellitus’ by Engström et al.  in the recent issue of the Journal of Internal Medicine. The authors aimed to explore the relationships between red cell distribution width (RDW) and glucose, haemoglobin A1c (HbA1c) and incidence of diabetes mellitus (DM). They concluded that there was an association between low RDW and increased incidence of DM. However, we believe that this conclusion should be discussed.
RDW is an index of the variation in cell volume within the red cell population that is reported as an index of heterogeneity in size of circulating erythrocytes . It is measured by automated haematology analysers and reported as a component of the complete blood count. Levels of fasting plasma glucose, HbA1c, insulin, triglycerides and haemoglobin, RDW, mean corpuscular volume and erythrocyte and leucocyte counts were all analysed. Of note, variations in RDW may reflect pregnancy, inflammatory bowel disease, recent transfusion, neurohumoral activation, renal and hepatic dysfunction, thyroid disease, nutritional deficiencies of folate, vitamin B12 and iron, bone marrow dysfunction, chronic or acute systemic inflammation and use of some medications [3-7]. In general, patients with these conditions have been excluded from studies showing the prognostic and predictive value of RDW . It would have been useful if the authors had described the effect of these factors.
In conclusion, the low RDW was associated with a markedly increased risk of developing DM as presented in their study. However, RDW can be affected by many conditions, and further studies should be conducted to evaluate how they affect RDW in subjects with an increased risk of developing DM. This could provide clearer information regarding the relationship between RDW and the risk of developing DM. We thank the authors for their contribution to the literature.