We read with great interest the review entitled, "Iron therapy in the management of anaemia in non-dialysis chronic kidney disease: the perspective of the S.E.N. [Sociedad Española de Nefrología (Spanish Society of Nephrology)] anaemia group"1. In this manuscript, the authors provide an update on the management of iron deficiency in patients with chronic kidney disease (CKD).

It is clearly shown that the diagnosis and treatment of absolute iron deficiency is simple and that there is a broad consensus on that matter1–3. The same does not apply to functional iron deficiency. In such clinical situation, caused in most cases by inflammation, there is an increase in hepcidin synthesis (due to IL-6) in the liver4. Hepcidin blocks ferroportin, the only cell channel that exists for exporting cellular iron into the bloodstream, thereby reducing suitable availability of iron in the bone marrow. This leads to deficient haemoglobin synthesis in the reticulocytes5. Unlike mean corpuscular haemoglobin, whose value diminishes after several weeks, bone marrow iron deficiency may be estimated in a few days based on reticulocyte haemoglobin content (CHr)5. Therefore, all the guidelines recommend the percentage of hypochromic red blood cells or CHr as the best laboratory parameters for the diagnosis of functional iron deficiency (1B)6–8.

Inexplicably, the authors1 state that we must continue to use the classic markers (serum ferritin and transferrin saturation - TSAT), suggesting that the new markers are less accessible, more expensive and somewhat unreliable. We cannot convey this concept, since following the widespread introduction of automated cell counters, most laboratories can now measure number, volume and CHr and thus detect iron deficiency at an early stage6. Moreover, not only are red blood cell markers not expensive, they are also the most rewarding option in comparison to the different tests that assess FID and response to treatment in patients with CKD on haemodialysis or not8. Finally, these markers are very reliable. Mast et al.9 demonstrated, in patients undergoing a bone marrow examination for other reasons, that their predictive value for iron deficiency is higher than the classic parameters (serum ferritin or TSAT). We have also seen the excellent correlation between CHr and the classic markers10, which is why we believe that at this point in time they are accessible, cost-effective and very reliable and that their use should be recommended in accordance with the guidelines6–8.