Microcytic hypochromic anaemia is very common hematological abnormality in the clinical practice ( Snakar et;al. 2016 ) . Iron deficiency anaemia and beta thalassemia trait are the most common causes of microcytic hypochromic anaemia. As mentioned by the World Health Organization ( WHO ) estimates in 2004 , there were 273000deaths due to iron deficiency anaemia along with 19.7 million disability . Approximately 1.3 % cases were recorded globally in developing countries ( Kasseban et;al.2014 ) . Iron deficiency anemia is the most common nutritional disorder . This type of anemia is the final phase of a process that begins with exhaustion of iron stores and continues with iron depletion from other compartments that contain it compromising normal haematopoesis ( Wharton et;al. 1994 ) Beta thalassemia trait is the second most common cause of microcytic hypochromic anaemia . It is genetically determined disorder in which the defect of b globin gene results in decreased production of hemoglobin A1 ( Sliman et;al. 2004 ) The differentiation between Iron deficiency anemia and Beta thalassemia trait is important because of two main reasons . First , because hemoglobin will not improve in beta thalassemia trait if it is misdiagnosed as Iron deficiency anemia and unnecessary iron being prescribed by the attending physician ( Vehapoglee et;al. 2014 ) . The second grave reason is that misdiagnosed beta thalassemia trait as Iron deficiency anemia may get married to a beta thalassemia trait , resulting in homozygous or thalassemia major in the offsprings ( Tripathi et;al. 2015) Ideally one needs a battery of tests including detailed peripheral blood picture , HBA2 estimation , serum iron , TIBC , serum ferritin and transferrin saturation to differentiate Iron deficiency anemia from beta thalassemia trait clearly (Bordbar et;al. 2015 ) . But all these investigations are either not available in all clinical setup or they are relatively time consuming and need expensive techniques ( Natios et;al. 2007 ). Derived indices showed that RDW is the first index of the routine blood count to become abnormal during the development of Iron deficiency anemia ( McCulre et;al. 1985 ) . It quantitatively measures red blood cells size.

variation computed directly from the RBCs histogram and is calculated as standard statistical value , the coefficient of variation of the volume distribution ( Verma et;al. 2015 and Plengsures et;al. 2015) . RDW is high in Iron deficiency anemia because there is a wide variation in red cell size . in beta thalassemia trait , the red cells are all the same size , there is virtually no variation ,so RDW is low ( Park et;al.2009 ) . Another red cell discriminate function , RDWI had been proven to be reliable discrimination index in the differentiation between Iron deficiency anemia and beta thalassemia trait ( Ismail et;al.2016 ) . It can be easily calculated as ( MCV in ( Fl) x RDW / RBCs in (million per microlitre ) ) quotient more than 220 suggest Iron deficiency anemia

, less than 220 suggest beta thalassemia trait. RDWI provide valiable help to the attending physician as all discriminating factors including RBCs count , MCV and RDW are incorporated in its formula ( Jayabose et;al. 1999 ) .