A systematic review of the literature for severity predictors in children with sickle cell anemia

Is Severity Score Associated With Indication for Hematopoietic Stem Cell Transplantation in Individuals With Sickle Cell Anemia?

Manifestations of sickle cell disease (SCD) begin early in childhood and cause morbidity and decreased life expectancy. Hematopoietic stem cell transplantation (HSCT) is curative but associated with risk of mortality attributable to the transplant. This risk should be counterbalanced with SCD morbidity and mortality. A severity score using a Bayesian network model was previously validated to predict the risk of death in adult individuals with SCD. The objective of this study is to calculate the severity scores of participants in a multicenter cohort of Brazilians with SCD, using a previously published Bayesian network-derived score, associated with risk of death and then compare the severity scores between participants with and without an indication for HSCT as defined by the Brazilian Ministry of Health (MoH) criteria. This is an observational, retrospective study. We analyzed 2063 individuals with sickle cell anemia from the Recipient Epidemiology and Donor Evaluation Study-III Brazil SCD cohort and applied a Bayesian network-derived score to compare candidates and non-candidates for HSCT according to the Brazilian MoH transplant criteria. Classical statistical methods were used to analyze data and make comparisons. We compared severity scores between cohort members with (n = 431) and without (n = 1632) HSCT indications according to Brazilian MoH. Scores were not different in adult participants with ≥1 HSCT indication when compared to those with no indication (mean 0.342 versus 0.292; median 0.194 versus 0.183, P = .354) and receiver operating characteristic curves did not demonstrate an obvious threshold to differentiate participants with or without HSCT indications. Severity score may predict risk of death but does not differentiate HSCT candidates. Current indications should be evaluated to ensure that patients with more severe disease who might benefit from HSCT are appropriately identified.

How Hydroxyurea Alters the Gut Microbiome: A Longitudinal Study Involving Angolan Children with Sickle Cell Anemia

Sickle cell anemia (SCA) is an inherited hematological disorder and a serious global health problem, especially in Sub-Saharan Africa. Although hydroxyurea (HU) is the leading treatment for patients with SCA, its effects on the gut microbiome have not yet been explored. In this context, the aim of this study was to investigate this association by characterizing the gut microbiome of an Angolan SCA pediatric population before and after 6 months of HU treatment. A total of 66 stool samples were obtained and sequenced for the 16S rRNA gene (V3-V4 regions). Significant associations were observed in alpha and beta-diversity, with higher values of species richness for the children naïve for HU. We also noticed that children after HU had higher proportions of several beneficial bacteria, mostly short-chain fatty acids (SCFAs) producing species, such as Blautia luti, Roseburia inulinivorans, Eubacterium halli, Faecalibacterium, Ruminococcus, Lactobacillus rogosae, among others. In addition, before HU there was a higher abundance of Clostridium_g24, which includes C. bolteae and C. clostridioforme, both considered pathogenic. This study provides the first evidence of the HU effect on the gut microbiome and unravels several microorganisms that could be considered candidate biomarkers for disease severity and HU efficacy.

Follow-up of children with sickle cell anemia screened with transcranial Doppler and enrolled in a primary prevention program of ischemic stroke

Background

Stroke is a serious complication of sickle cell anemia (SCA). The transcranial Doppler (TCD) is the risk-screening tool for ischemic strokes. The objective of the study was to describe the clinical progression of children with SCA who presented with high risk for stroke by TCD or relevant changes by magnetic resonance angiography (MRA) and underwent the regular transfusion program (RTP) and/or hydroxyurea (HU) treatment between 2007 and 2018.

Method

This was a neonatal retrospective/prospective cohort study with children born between 1999 and 2014 with the homozygotic form (HbSS) or Sβ⁰-thalassemia who underwent TCD at least once.

Results

Of the 718 children screened during this period, 675 had HbSS and 43 Sβ⁰-thalassemia. In 54 children (7.5%), all with HbSS, a high-risk TCD (n = 45) or, when the TCD was inconclusive, an MRA with cerebral vasculopathy (n = 9) was used for detection. Of these, 51 started the RTP and the families of three refused treatment. Of the 43 children with a high-risk TCD who initiated the RTP, 29 (67.4%) reverted to low risk. In 18 of them (62%), HU was started at the maximum tolerated dose (MTD) before transfusion discontinuation. None of these 29 patients had a stroke. Eight children (18.6%) maintained a high-risk TCD, even using the RTP/HU and two had a stroke.

Conclusions

The TCD was confirmed as a viable tool for tracking patients with a risk for stroke. The RTP was effective in preventing the primary event. New strategies are necessary to prevent stroke using HU and new drugs, in addition to bone marrow transplantation.

Development of a Severity Classification System for Sickle Cell Disease

Purpose

There is no well-accepted classification system of overall sickle cell disease (SCD) severity. We sought to develop a system that could be tested as a clinical outcome predictor.

Patients and Methods

Using validated methodology (RAND/UCLA modified Delphi panel), 10 multi-disciplinary expert clinicians collaboratively developed 180 simplified patient histories and rated each on multiple axes (estimated clinician follow-up frequency, risk of complications or death, quality of life, overall disease severity). Using ratings on overall disease severity, we developed a 3-level severity classification system ranging from Class I (least severe) to Class III (most severe).

Results

The system defines patients as Class I who are 8-40 years with no end organ damage, no chronic pain, and ≤4 unscheduled acute care visits due to vaso-occlusive crises (VOC) in the last year. Patients <8 or >40 years with no end organ damage, no chronic pain, and <2 unscheduled acute care visits are also considered Class I. Patients any age with ≥5 unscheduled acute care visits and/or with severe damage to bone, retina, heart, lung, kidney, or brain are classified as Class III (except patients ≥25 years with severe retinopathy, no chronic pain, and 0-1 unscheduled acute care visits, who are considered Class II). Patients not meeting these Class I or III definitions are classified as Class II.

Conclusion

This system consolidates patient characteristics into homogenous groups with respect to disease state to support clinical decision-making. The system is consistent with existing literature that increased unscheduled acute care visits and organ damage translate into clinically significant patient morbidity. Studies to further validate this system are planned.

Thrombin generation in children with sickle cell Anemia is Higher in the presence of platelets⋆ and ⋆

Cellular and plasma interactions underlie hypercoagulability in sickle cell anemia (SCA). In healthy adults, thrombin generation (TG), a biomarker of hypercoagulability, is similar in plasma with and without platelets. Studies investigating TG in SCA using platelet-poor plasma (PPP) show conflicting results. There are no studies in SCA simultaneously comparing TG using platelet rich plasma (PRP) and PPP. This prospective study compares TG in children with SCA, at steady state, in PPP versus PRP and investigates the association of predefined clinical variables with the difference between PRP and PPP. Our secondary aim was to investigate derangements in the protein C and S pathway measuring TG with and without thrombomodulin (TM). In forty-three paired samples from SCA patients, aged 2-15 years, TG in the presence of platelets was 5.9 % higher [1239 nmol/(min*L) (SD: 224.1) vs. 1151 nmol/(min*L) (SD 223.3); p = 0.026]. The difference was highest in the 6-10 year age group (9.5 %; SD 14.1) followed by the 2-5 year age group (5.4 %; SD 21.4). In a multiple linear regression model, age, gender, current use of hydroxyurea, degree of hemolysis and severity of pain crises were not predictive of the difference between PRP and PPP. In PPP, TG reduction after TM addition was 7.4 % (SD 16.8), signifying activated protein C resistance. In conclusion, TG in children with SCA aged 2-10 years is higher in the presence of platelets. TG using PRP along with TM addition may be a useful biomarker of hypercoagulability in this population.

Fetal Hemoglobin Modulators May Be Associated With Symptomology of Football Players with Sickle Cell Trait

OBJECTIVES

This study investigates whether genetic modifiers previously shown to influence adult fetal hemoglobin (HbF) levels and glucose-6-phosphate dehydrogenase deficiency were associated with variable symptomology in a small sample of collegiate football players with sickle cell trait.

METHODS

Survey data on self-assessed symptoms and genotype data from five single nucleotide polymorphisms (SNPs) related to HbF production and two SNPs that cause glucose-6-phosphate dehydrogenase deficiency were collected from current and former college football players.

RESULTS

In this sample, SNPs found within the β-globin gene cluster were found to be associated with a previous diagnosis of exertional sickling and experience of extreme heat during and after training. rs10189857 in the BCL11A gene was associated with body mass index and weight and with experiencing extreme thirst during and after training. No significant correlations were found between the other SNPs and symptoms within this sample.

CONCLUSIONS

These findings show that genetic variation known to affect sickle cell disease symptomology may partly explain why some football players with sickle cell trait experience adverse clinical outcomes during periods of extreme physical exertion and others do not.

Nucleated Red Blood Cells in Children With Sickle Cell Disease Hospitalized for Pain

Acute chest syndrome (ACS) and transfusion requirements are common and difficult to predict during hospitalizations for acute vaso-occlusive episodes (VOE) among individuals with sickle cell disease (SCD). This study examined the relationship between nucleated red blood cell (NRBC) counts during hospitalization for VOE and development of ACS or transfusion requirement among children with SCD. Retrospective chart review was performed for 264 encounters of patients with SCD hospitalized for uncomplicated VOE who had NRBC count data at admission during a 5-year period. Multivariable logistic regression analysis was conducted to determine the relationship of admission and change in NRBC ([INCREMENT]NRBC) to ACS/transfusion requirement. Overall, 44 of 264 (16.7%) encounters resulted in ACS, transfusion, or both. Admission NRBC was not associated with development of ACS/transfusion requirement. Among 125 of 264 (47.3%) encounters in which a subsequent CBC was obtained, greater increases in NRBCs and greater decrease in hemoglobin were significantly associated with ACS/transfusion requirement (OR, 2.72; 95% CI, 1.16, 6.35; P=0.02 and OR, 2.52; 95% CI, 1.08, 5.89; P=0.03, respectively). Our finding that an increase in NRBC counts was associated with development of ACS/transfusion requirement suggests that [INCREMENT]NRBCs may represent a useful biomarker for predicting complications in children with SCD hospitalized for VOE.

Prevalence and predictive factors of splenic sequestration crisis among 423 pediatric patients with sickle cell disease in Tunisia

This study was aimed to identify the predictors of splenic sequestration crisis (SSC) among pediatric patients with sickle cell disease (SCD). This prognosis study was carried out in the pediatric immuno-hematology unit, over 20 years (1998 to 2017), enrolling patients with SCD. The cox model was used in multivariate analysis. Among 423 patients with SCD (240 S/S phenotype, 128 S/B0, 30 S/B+, 14 S/O arab and 11 S/C), 150(35.4%) had at least one episode of SSC. The average age of patients at the first episode was 48.3 months ± 32.4(2-168). Recurrence of SSC was observed in 117 patients (78%). Spleen size ≥3 cm at baseline was the strongest predictor of SSC occurrence (HR = 7.27, CI: 4.01-13.20, p = 0.05) and recurrence (HR = 6.37, CI: 1,46-27.83, p = 0.01). Pallor revealing the disease, age at onset of symptoms <24 months and reticulocytosis ≥300,000/mm³ increased the risk of SSC. Pain crisis revealing the disease as well as neutrophilia was associated with a lower risk of SSC. In conclusion, this study confirmed the high prevalence of SSC in SCD and the high frequency of recurrence after a first episode. The SSC occurrence and recurrence were intimately linked to the presence of splenomegaly, chronic pallor revealing the disease as well as reticulocytosis.

[Study of - maximum systolic velocities and mean maximum velocities of skull base arteries recorded with transcranial Doppler in adult sickle cell patients without neurovascular complication]

INTRODUCTION

Sickle cell disease is the leading genetic disease in Île-de-France. Stroke is one of its most severe complications. In SS sickle cell children, transcranial Doppler (TDC) is, through the study of average speeds of the skull base arteries, the gold standard for screening and diagnosis of vasculopathy. To our knowledge, in adults with sickle cell disease, no standards have been established for the speed of the arteries at the base of the skull. It therefore seemed useful to us to establish an approach to brain speeds recorded in adults with sickle cell disorders without neurovascular complications.

MATERIAL AND METHODS

This was an observational, prospective, monocentric study conducted between February 2017 and June 2017. The main objective of the study was to determine the mean and standard deviation of maximum systolic velocities (MSS) and mean maximum velocities for all arteries recorded during the transcranial Doppler echo. The secondary objectives were to compare the mean maximum systolic velocities in sickle cell adults with those of healthy adults, to compare the mean maximum systolic velocities in sickle cell adults with those of sickle cell children, and to determine whether parameters could influence the speeds recorded at TCD.

RESULTS

Forty patients were included between February 1, 2017 and June 30, 2017, with an average age of 39.3 years. The mean maximum velocities recorded were: 78cm/s for the middle cerebral arteries; 59.6cm/s for the internal carotid arteries; 61cm/s for the anterior cerebral arteries; 44cm/s for the posterior cerebral arteries and 55cm/s for the basilar trunk.

DISCUSSION

The highest circulatory velocities are found in the middle cerebral arteries. The speeds found in the internal carotid arteries and anterior cerebral arteries are faster than in the vertebrobasilar system. Speeds in sickle cell adults are slower than those described in sickle cell children SS but significantly faster than those found in healthy adults.

CONCLUSION

To our knowledge, this study is the first to evaluate transcranial Doppler circulatory velocities in adult sickle cell patients. This work has limitations due to its small sample size, however, it provides a basis for further studies on transcranial Doppler in sickle cell adults.

Sickle Cell Disease and Stroke

Cerebral infarction is a common complication of sickle cell disease and may manifest as overt stroke or cognitive impairment associated with "silent" cerebral infarction on magnetic resonance imaging. Vasculopathy may be diagnosed on transcranial Doppler or magnetic resonance angiography. The risk factors in sickle cell disease for cognitive impairment, overt ischemic stroke, silent cerebral infarction, overt hemorrhagic stroke, and vasculopathy defined by transcranial Doppler or magnetic resonance angiography overlap, with severe acute and chronic anemia, acute chest crisis, reticulocytosis, and low oxygen saturation reported with the majority. However, there are differences reported in different cohorts, which may reflect age, geographic location, or neuroimaging techniques, for example, magnetic resonance imaging field strength. Regular blood transfusion reduces, but does not abolish, the risk of neurological complications in children with sickle cell disease and either previous overt stroke or silent cerebral infarction or abnormal transcranial Doppler. There are relatively few data on the use of hydroxyurea or other management strategies. Early assessment of the risk of neurocognitive complications is likely to become increasingly important in the management of sickle cell disease.

Association of fetal hemoglobin level with frequency of acute pain episodes in sickle cell disease (HbS-only phenotype) patients

BACKGROUND

Sickle cell disease (SCD) is a Mendelian single gene disorder with highly variable phenotypic expression. In the present study, we analyzed the influence of HbF, alpha thalassemia and other hematological indices to determine their association with acute pain episodes.

METHOD

This case control study consisted of SCD subjects with HbS phenotype experiencing three or more acute pain episodes in last twelve months (cases) and without any episode of acute pain during last twelve months (controls). Hematological parameters, HbF, and presence of alpha thalassemia were assessed in all subjects.

RESULTS

A statistically significant difference between HbF levels (P < 0.025, χ2 test) and alpha thalassemia (P < 0.008, χ2 test) was observed between controls and cases group. Univariate analysis indicated that increased HbF levels > 25% (OR: 0.37, 95% CI: 0.18-0.77, P < 0.008) and presence of alpha thalassemia (OR: 0.53, 95% CI: 0.33-0.85, P < 0.009) provided protection, while multivariate analysis revealed significant protection was attributable only by higher HbF levels (OR: 0.39, 95% CI: 0.17-0.88, P < 0.025). Significantly higher HbF levels were observed only in the 11-20 age group of cases in comparison to controls (Student's t-test, P < 0.001).

CONCLUSION

Higher concentrations of HbF are associated with protection against frequent episodes of acute pain crisis in SCD patients.

Measuring success: utility of biomarkers in sickle cell disease clinical trials and care

Progress in the care of sickle cell disease (SCD) has been hampered by the extreme complexity of the SCD phenotype despite its monogenic inheritance. While epidemiological studies have identified clinical biomarkers of disease severity, with a few exceptions, these have not been routinely incorporated in clinical care algorithms. Furthermore, existing biomarkers have been poorly apt at providing objective parameters to diagnose sickle cell crisis, the hallmark, acute complication of SCD. The repercussions of these diagnostic limitations are reflected in suboptimal care and scarcity of adequate outcome measures for clinical research. Recent progress in molecular and imaging diagnostics has heralded a new era of personalized medicine in SCD. Precision medicine strategies are particularly timely, since molecular therapeutics are finally on the horizon. This chapter will summarize the existing evidence and promising data on biomarkers for clinical care and research in SCD.

Prognostic factors of disease severity in infants with sickle cell anemia: A comprehensive longitudinal cohort study

In order to identify very early prognostic factors that can provide insights into subsequent clinical complications, we performed a comprehensive longitudinal multi-center cohort study on 57 infants with sickle cell anemia (55 SS; 2 Sβ°) during the first 2 years of life (ClinicalTrials.gov: NCT01207037). Time to first occurrence of a severe clinical event-acute splenic sequestration (ASS), vaso-occlusive (VOC) event requiring hospitalization, transfusion requirement, conditional/ abnormal cerebral velocities, or death-was used as a composite endpoint. Infants were recruited at a mean age of 4.4 ±1 months. Median follow-up was 19.4 months. During the study period, 38.6% of infants experienced ≥1 severe event: 14% ASS, 22.8% ≥ 1 VOC (median age: 13.4 and 12.8 months, respectively) and 33.3% required transfusion. Of note, 77% of the cohort was hospitalized, with febrile illness being the leading cause for admission. Univariate analysis of various biomarkers measured at enrollment showed that fetal hemoglobin (HbF) was the strongest prognostic factor of subsequent severe outcome. Other biomarkers measured at enrolment including absolute neutrophil or reticulocyte counts, expression of erythroid adhesion markers, % of dense red cells, cellular deformability or ϒ-globin genetic variants, failed to be associated with severe clinical outcome. Multivariate analysis demonstrated that higher Hb concentration and HbF level are two independent protective factors (adjusted HRs (95% CI) 0.27 (0.11-0.73) and 0.16 (0.06-0.43), respectively). These findings imply that early measurement of HbF and Hb levels can identify infants at high risk for subsequent severe complications, who might maximally benefit from early disease modifying treatments.

Treatment Options for Sickle Cell Disease

Sickle cell disease (SCD) complications begin with the polymerization of sickle hemoglobin (HbS). Thus, SCD therapies are focused on preventing HbS production or reducing the circulating amount of HbS. Hydroxyurea treatment has become more widespread, whereas the number of evidence-based indications for erythrocyte transfusion is small. Hematopoietic stem cell transplant is a curative option for SCD but less than 25% of patients have a suitable donor. This article focuses on supportive and preventive care improvements and the benefits of hydroxyurea. Indications for erythrocyte transfusion, hematopoietic stem cell transplant, and gene therapy trials are also summarized.

The erythrocyte alloimmunisation in patients with sickle cell anaemia: a systematic review

Transfusion therapy is a common practice in the treatment of anaemia and can cause erythrocyte alloimmunisation. To systematise data related to erythrocyte alloimmunisation in patients with sickle cell disease (SCD), a bibliographic search was carried out in September 2017 to search for studies in four electronic databases. (i) Referring to the original work, (ii) being cohort or case-control, (iii) having been developed with individuals with SCD and (iv) having evaluated the erythrocyte alloimmunisation. Two reviewers identified the articles for inclusion in the study, extracted the predetermined data and carried out the evaluation of the methodological quality of the work. 21 studies were selected; the studies included data on 20 636 individuals (children and adults), were mostly published in the last 10 years, were developed in the United States and had high methodological quality. The occurrence of erythrocyte alloimmunisation ranged from 4·4 to 76%, and there was a higher rate of alloimmunisation against antigens of the Rh system. The risk factors for alloimmunisation were age; gender (female); red blood cell (RBC) units received; presence of ≥1 autoantibodies, TNF-α, interleukin (IL1B), human leukocyte antigens (HLA)-DRB1 gene polymorphisms; first blood transfusion (BT) after 5 years of age, transfusion episodic, multiple or during inflammatory events, acute chest syndrome (ACS) and vase-occlusive crisis (VOC); increased percentage of CD41 T memory cells; and positive direct antiglobulin test. Transfusion policies should be developed to protect the patient and his or her health based on the main factors associated with its incidence.

Early initiation of inhaled corticosteroids does not decrease acute chest syndrome morbidity in pediatric patients with sickle cell disease

Acute chest syndrome (ACS) is a leading cause of mortality in patients with sickle cell disease (SCD). Systemic corticosteroids decrease ACS severity, but the risk of readmission for vaso-occlusive crises (VOC) has limited their use. The efficacy of inhaled corticosteroids (ICS) as a safer alternative is currently unknown. An observational, historic cohort study compared patients with SCD with ACS who received ICS at admission (ICS) to those who did not (non-ICS). Outcome measures included rates of transfusion, oxygen requirement, BiPAP initiation, PICU transfer, intubation, readmission, hospital cost, and length of stay. One hundred twenty patients with SCD (55 non-ICS, 65 ICS) were included. A significantly higher proportion of the non-ICS group had bilateral infiltrates, but fewer had asthma. More children in the ICS group had BiPAP initiated, however transfer to the PICU, intubation, transfusion rates, oxygen requirement, hospital cost, length of stay, and readmission rates did not differ between groups. Regression analysis did not reveal any differences in outcomes, nor were outcomes changed when patients were separated based on the presence or absence of asthma. In this observational cohort study, ICS did not demonstrate a significant reduction in ACS morbidity, though ICS use should be studied in a prospective manner.

Clinical and genetic factors are associated with pain and hospitalisation rates in sickle cell anaemia in Cameroon

We aimed to investigate the clinical and genetic predictors of painful vaso-occlusive crises (VOC) in sickle cell disease (SCD) in Cameroon. Socio-demographics, clinical variables/events and haematological indices were acquired. Genotyping was performed for 40 variants in 17 pain-related genes, three fetal haemoglobin (HbF)-promoting loci, two kidney dysfunctions-related genes, and HBA1/HBA2 genes. Statistical models using regression frameworks were performed in R^® . A total of 436 hydoxycarbamide- and opioid-naïve patients were studied; median age was 16 years. Female sex, body mass index, Hb/HbF, blood transfusions, leucocytosis and consultation or hospitalisation rates significantly correlated with VOC. Three pain-related genes variants correlated with VOC (CACNA2D3-rs6777055, P = 0·025; DRD2-rs4274224, P = 0·037; KCNS1-rs734784, P = 0·01). Five pain-related genes variants correlated with hospitalisation/consultation rates. (COMT-rs6269, P = 0·027; FAAH-rs4141964, P = 0·003; OPRM1-rs1799971, P = 0·031; ADRB2-rs1042713; P < 0·001; UGT2B7-rs7438135, P = 0·037). The 3·7 kb HBA1/HBA2 deletion correlated with increased VOC (P = 0·002). HbF-promoting loci variants correlated with decreased hospitalisation (BCL11A-rs4671393, P = 0·026; HBS1L-MYB-rs28384513, P = 0·01). APOL1 G1/G2 correlated with increased hospitalisation (P = 0·048). This first study from Africa has provided evidence supporting possible development of genetic risk model for pain in SCD.

Fetal Hemoglobin is Associated with Peripheral Oxygen Saturation in Sickle Cell Disease in Tanzania

Fetal hemoglobin (HbF) and peripheral hemoglobin oxygen saturation (SpO₂) both predict clinical severity in sickle cell disease (SCD), while reticulocytosis is associated with vasculopathy, but there are few data on mechanisms. HbF, SpO₂ and routine clinical and laboratory measures were available in a Tanzanian cohort of 1175 SCD individuals aged ≥ 5 years and the association with SpO₂ (as response variable transformed to a Poisson distribution) was assessed by negative binomial model with age and sex as covariates. Increase in HbF was associated with increased SpO₂ (rate ratio, RR = 1.19; 95% confidence intervals [CI] 1.04, 1.37 per natural log unit of HbF; p = 0.0004). In univariable analysis, SpO₂ was inversely associated with age, reticulocyte count, and log (total bilirubin) and directly with pulse, SBP, hemoglobin, and log(HbF). In multivariable regression log(HbF) (RR 1.191; 95%CI 1.04, 1.37; p = 0.013), pulse (RR 1.01; 95%CI 1.00, 1.01; p = 0.026), SBP (RR 1.008; 95%CI 1.00, 1.02; p = 0.014), and hemoglobin (1.120; 95%CI 1.05, 1.19; p = 0.001) were positively and independently associated with SpO₂ while reticulocyte count (RR 0.985; 95%CI 0.97, 0.99; p = 0.019) was independently inversely associated with SpO₂. In SCD, improving SpO₂, in part through cardiovascular compensation and associated with reduced reticulocytosis, may be a mechanism by which HbF reduces disease severity.

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Fetal hemoglobin may moderate sickle cell disease through increased oxygen saturation.

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Low oxygen saturation is associated with reticulocytosis which might moderate cerebral vasculopathy and stroke risk.

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Higher pulse rate and systolic blood pressure in those with higher SpO₂ suggests cardiovascular compensation for low SpO_2.

Fetal hemoglobin (HbF) is normally synthesized during intrauterine life and it starts to decline before birth being replaced by adult hemoglobin (HbA). However some individuals continue to synthesize HbF to adulthood and are relatively protected from severe sickle cell disease. The mechanism of HbF protection in SCD has not been entirely established. This study reports a positive association between HbF and oxygen saturation (SpO₂). Higher SpO₂ is associated with decreased reticulocytes but increased pulse rate and systolic blood pressure, suggesting SpO₂ is maintained in part through cardiovascular compensation. Increasing HbF may reduce disease severity partly through increasing SpO_2.