During erythropoietic stress (e.g., acute anaemia) the reticulocyte count in peripheral blood normally increases as the bone marrow responds to increased erythropoietin stimulation of erythroid precursors. The efficiency of this process is an indicator of the patient’s bone marrow response. This study assesses the utility of the immature reticulocyte fraction (IRF) as a useful parameter of anaemia type, which may inform the decision to treat with red cell transfusion. Moreover, it investigates the value of using IRF as an inexpensive, non-invasive and objective indicator of a patient’s bone marrow response. EDTA-treated venous blood specimens were collected from in-patients with a haemoglobin value <100 g/L and analysed to establish the absolute reticulocyte count and IRF using the ABX Pentra 120 Retic analyser. Statistical analysis of results showed that there was a significant negative correlation between IRF and haemoglobin level. Importantly, IRF was also found to show a more significant correlation with haemoglobin level than did the absolute reticulocyte count.