  Entropy production is the hallmark of nonequilibrium physics, quantifying
irreversibility, dissipation, and the efficiency of energy transduction
processes. Despite many efforts, its measurement at the nanoscale remains
challenging. We introduce a variance sum rule for displacement and force
variances that permits us to measure the entropy production rate $\sigma$ in
nonequilibrium steady states. We first illustrate it for directly measurable
forces, such as an active Brownian particle in an optical trap. We then apply
the variance sum rule to flickering experiments in human red blood cells. We
find that $\sigma$ is spatially heterogeneous with a finite correlation length
and its average value agrees with calorimetry measurements. The VSR paves the
way to derive $\sigma$ using force spectroscopy and time-resolved imaging in
living and active matter.
