Abstract
Sickle cell disease (SCD) is an autosomal recessive disorder characterized by rigid, poorly deformable red blood cells (RBC); worsening with hypoxia induced polymerization of sickle hemoglobin (HbS), contributing to potentially life-threatening acute chest syndrome (ACS) and painful vaso-occlusive events (VOE). Currently, fetal hemoglobin (HbF) levels and ACS history are commonly used predictors for VOE and ACS risk. However, some individuals experience frequent VOE or ACS despite HbF>20%, and history is only helpful after the first event. Thus, there is a need for additional VOE/ACS predictors. LoRRca (oxygen-gradient ektacytometry) uses shear stress across a range of oxygen tensions and reports RBC deformability as elongation index at normoxia (EImax), elongation index at hypoxia (EImin), and point of sickling (PoS), the oxygen tension at which RBC deformability changes. LoRRca biomarkers may be predictive of VOE/ACS risk; they were shown to be associated with prior VOE/ACS episodes. However, these biomarkers need to be evaluated using a prospective study design to be considered predictive.
Methods:
Peripheral blood samples (N=992) from 442 adults with SCD who received care at Grady hospitals from 2011 to 2025 were collected in EDTA under an IRB approved protocol. RBC deformability (LoRRca, RR Mechatronics); whole blood viscosity (Brookfield viscometer) at shear rates of 45 and 225 s-1; complete blood count, absolute reticulocyte count (ARC) (Advia cell counter); and hemoglobin phenotype (Agilent HPLC instrument) tests were performed. Demographic and clinical data were obtained from electronic medical records over two years prior to the first blood sample until the last health care visit. The number of ACS/VOE after each sample analysis was recorded separately. This prospective design ensured temporality (exposure preceded outcome). Associations between candidate biomarkers (EImin, EImax, PoS) and subsequent ACS or VOE were determined using logistic mixed models. ARC, platelet count, sex, hematocrit to viscosity ratio at 45 and 225 s-1, %dense RBC, hemoglobin, absolute neutrophil counts (ANC), smoking status, genotype, asthma, and prior ACS episodes (for ACS outcome) were used as covariates. Tests were excluded from the analysis if there was a VOE or ACS in the last 30 days or a transfusion in the last 90 days. StataNow 19.5 was used for analyses, and p<0.05 was considered significant.
Results:
Our cohort consisted of 442 adults with SCD; 292 with HbSS/HbSβ0, 94 with HbSC, 56 with HbSβ+or other disease-causing variants; 54.9% were female. Median values: age 31 years, follow-up period 2.28 years, and 7 healthcare visits per individual. ACS and VOE incidence rates were 0.13/year (135 events in 90 patients) and 0.87/year (886 VOEs in 223 patients), respectively. The rates were comparable across sex (p>0.05) and lower in HbSC compared to other genotypes (ACS: 0.02 vs 0.15, p<0.001; VOE: 0.16 vs 0.98, p<0.0001). EImin was associated with ACS (p=0.001) and VOE (p=0.03) in multivariate logistic regression analysis after adjusting for covariates including EImax and PoS. Each 0.1-unit higher EImin (indicating better RBC deformability) was associated with 48% and 23% lower odds of ACS and VOE, respectively. ACS history was also associated with future ACS (p<0.0001), but EImin was a better ACS predictor based on the lower Akaike (317.5 vs 380.1) and Bayesian (331.1 vs 389.8) information criteria. The median duration between the RBC deformability test and subsequent ACS or VOE was 115 (range 11-609) and 91 (4-572) days, respectively.
Conclusions:
Here we demonstrate the predictive value of EImin for future VOE and ACS using longitudinal data obtained after rheology measurement; previous studies reported an association between rheology and past events. EImin was the only statistically significant predictor in the multivariate model, indicating a better predictive power compared to other candidate biomarkers, including history of ACS. EImin captures changes in RBC deformability under hypoxia, a key aspect of VOE and ACS pathophysiology. Strengths of our study include large sample size-442 patients, 992 samples, and extensive median follow-up of 2.28 years. Our study shows that EImin can predict ACS/VOE irrespective of number of prior ACS and VOEs.
Annual measurement of RBC deformability at routine clinic visits will allow providers to predict future ACS/VOE risk and proactively modify SCD therapy if unfavorable EImin is noted.
