Altered RBC aggregability in diabetes: a threshold for pathophysiological structure-function RBC changes

Vascular complications have been associated with pathophysiological RBC changes including reduced RBC deformability (RBCD): the ability of RBCs to change their shape in the microvasculature and increased red blood cell aggregability (RBCA): the tendency of RBCs to form aggregates (rouleaux). However, understanding of aberrant RBCA in diabetes and its relationship with RBCD is limited. We investigated clinical and RBC structural and functional changes associated with RBCA in comparison with RBCD. We conducted an outpatient cross-sectional study of participants with diabetes (n = 81) and nondiabetic controls (n = 78) at the National Institutes of Health. Clinical history was obtained using standardized forms, fasting blood and urine samples were collected for clinical laboratory measurements and ektacytometry studies of RBC physiological parameters: RBCA, RBCD and osmotic fragility. Functional RBC changes were assessed using hemoglobin-oxygen dissociation (p50). Major outcomes assessed differences in RBCA Aggregation Index (AI), with increased RBCA defined as higher AI. Exploratory outcomes assessed differences in subgroups with type 1 (T1D) and type 2 (T2D) diabetes, and associations with clinical, inflammatory, endocrine and RBC physiological parameters. Outcomes were assessed using both nonparametric and multivariable regression methods. Compared with controls, AI was significantly higher in the diabetes cohort (75±8 vs 71 ± 11, p = 0.007), T2D (77±7 vs 71 ± 11, p < 0.001), but not T1D (69±7 vs 71 ± 11, p = 0.242). While RBCD values were similar in T1D and T2D (p = 0.31), mean AI value was significantly higher in T2D vs T1D (p = 0.02). In contrast to RBCD, AI was associated with T2D (p < 0.001), BMI (p < 0.001), insulin resistance (HOMA-IR, p = 0.033), leptin (p < 0.001), C-peptide levels (p = 0.009), but not vascular complications (p = 0.156). While RBCA and RBCD were associated with CRP (p < 0.001 and p = 0.005 respectively), neither was associated with fibrinogen (p = 0.768 and p = 0.118 respectively). In contrast to RBCD, AI was associated with increased osmotic fragility (p < 0.001) and a threshold of altered hemoglobin-oxygen dissociation (p50) estimated at AI of 75 in both groups using piecewise linear statistical modeling. Thus, AI was positively correlated with p50 at AI < 75 (p = 0.033) and negatively correlated at AI ≥ 75 (p = 0.007). Findings show that in contrast to RBCD, RBCA is characterized by a RBC functional threshold beyond which oxygen release is reduced, and RBC structure-function relationship likely modulated by insulin resistance and pro-inflammatory endocrine factors. The distinct but complementary and synergistic changes in RBCD and RBCA provide a framework for strategies aimed at mitigation of vascular risk in diabetes.