Assessment of Bone Marrow Function in Sickle Cell Anaemia Patients Using Corrected Reticulocyte Counts

Introduction: Chronic hemolysis occurs in sickle cell anemia as a result of recurrent sickling and other abnormalities of the red blood cells including eryptosis. Exuberant reticulocytosis is anticipated to partially compensate for the resultant anemia. Sickle cell anemia patients may also have aplastic crisis, bone marrow (BM) infarction and erythropoietin deficiency which could lead to reticulocytopenia despite the anemia. High degree of reticulocytosis among asymptomatic infants with sickle cell anemia has been associated with an increased risk of death or stroke during childhood. Assessment of BM function in sickle cell anemia is important due to potential complications associated with both under-activity and hyperactivity. This study aimed at evaluating the erythropoietic function of the BM in steady state sickle cell anemia using corrected reticulocyte counts.

Methods: This study was carried out at the hematology clinic in the University College Hospital, Ibadan. HbSS patients in steady state were recruited from the hematology clinic. Local ethical committee approval was obtained and all participants gave written informed consent. Patients with M. tuberculosis, Hepatitis B, HIV and P. falciparum infection were excluded. Peripheral blood samples were analyzed using Sysmex Ki-X21 for complete blood count (CBC) and standard point of care for serum electrolytes and liver function tests. The glomerular filtration rates were calculated using the Cockcroft-Gault formula. Reticulocyte counts were determined manually using fresh samples from K₂ EDTA bottles and methylene blue stain. Two drops of stain were mixed with two to four volumes of anticoagulated blood and incubated at 37ºC for 15 minutes. Afterwards, the cells were re-suspended and blood films were made. Corrected reticulocyte count and reticulocyte production index were calculated. Participants were categorized according to corrected reticulocyte counts of greater than or less than 2.5%. Univariate and multivariate analyses were performed to determine variables associated with corrected reticulocyte count <2.5%.

Results: 92 HbSS patients were recruited with a mean (SD) age of 19.6 (5.8) years. There was no correlation between age and eGFR (p-value: 0.227). Median (range) reticulocyte count, corrected reticulocyte count and reticulocyte production index were 5.5 (0.5 - 29.9), 3.3 (0.1 - 17.1) and 1.7 (0.2 - 8.6) respectively. 40 (43.5%) patients had corrected reticulocyte count <2.5% and 52 (56.5%) had a corrected count >2.5%. Those corrected reticulocyte count <2.5% were older (p: 0.013), taller (p: 0.041) and had higher aspartate transaminase (AST) levels (p: 0.006) than those with corrected counts >2.5% (Table 1). CBC parameters were not different when compared between both groups. Results of multivariate logistic regression analysis carried out showed that only AST was independently linked with corrected reticulocyte count <2.5% (R²: 0.172, p-value: 0.001) (Table 2).

Conclusion: Despite corrected reticulocyte count <2.5% in about half of the patients, there were similar hematological parameters and eGFR in both groups of patients. AST is a marker of hemolysis and low ALT rules out hepatic involvement. Since only 17.2% of the variability in BM response as assessed by corrected reticulocyte count could be accounted for by variables included in this study, there is a need to further evaluate the BM function of sickle cell patients to establish the causes of corrected reticulocyte count <2.5% in the setting of anemia, having ruled out erythropoietin as well as iron, folate or cobalamin deficiencies. This will aid the development of a functional algorithm for the individualized management of sickle cell disease patients with anemia.