Does higher hematocrit increase blood viscosity and do microcytosis and RDW affect microvascular flow?

Higher hematocrit has a well-established rheologic effect, whereas the implications of microcytosis and elevated red-cell distribution width (RDW) are more context-dependent. In a 77-year-old man, these indices should be interpreted alongside the cause of any hematologic abnormality rather than as stand-alone measures of vascular risk.

Clinical and rheologic evidence

• Higher hematocrit reliably increases whole-blood viscosity across shear rates. The increase is nonlinear: one human study found a relatively linear rise from hematocrit 20% to 40%, followed by a steeper increase above approximately 40%, especially under low-shear conditions.
• The magnitude is modified by plasma viscosity, temperature, red-cell aggregation, and red-cell mechanical properties. At low shear, rouleaux/aggregation is a major contributor to apparent viscosity; at higher shear, impaired deformability tends to raise viscosity.

Red-cell morphology, deformability, and microvascular flow

• Microcytosis is plausibly associated with impaired deformability, based on small ektacytometry studies in iron-deficiency anemia and β-thalassemia trait. However, these disorders show different deformability patterns despite similarly low mean cell volume, indicating that membrane characteristics, hemoglobin composition, cellular hydration, and disease-specific biology—not cell size alone—are important.
• Greater RDW was associated with lower deformability in a microfluidic ektacytometry study of 293 adults, including analyses excluding anemia. RDW is nonetheless a marker of anisocytosis, not a direct measurement of cell mechanics.
• Poorer deformability increases resistance to capillary transit because erythrocytes must deform to traverse vessels narrower than their resting diameter. Experimental volume heterogeneity has also been linked to slow-transiting red-cell subpopulations.

Bottom line

• Higher hematocrit directly increases blood viscosity; microcytosis and high RDW are plausible markers or contributors to reduced deformability and impaired microvascular transit, but their independent causal effects depend strongly on the underlying hematologic condition.