  Oxygen is transported throughout the body by hemoglobin in red blood cells.
While the oxygen affinity of blood is well understood and is routinely assessed
in patients by pulse oximetry, variability at the single-cell level has not
been previously measured. In contrast, single-cell measurements of red blood
cell volume and hemoglobin concentration are taken millions of times per day by
clinical hematology analyzers and are important factors in determining the
health of the hematologic system. To better understand the variability and
determinants of oxygen affinity on a cellular level, we have developed a system
that quantifies the oxygen saturation, cell volume and hemoglobin concentration
for individual red blood cells in high-throughput. We find that the variability
in single-cell saturation peaks at an oxygen partial pressure of 2.5%, which
corresponds to the maximum slope of the oxygen-hemoglobin dissociation curve.
In addition, single-cell oxygen affinity is positively correlated with
hemoglobin concentration, but independent of osmolarity, which suggests
variation in the hemoglobin to 2-3 DPG ratio on a cellular level. By
quantifying the functional behavior of a cellular population, our system adds a
new dimension to blood cell analysis and other measurements of single-cell
variability.
