  The van de Hulst formula provides an expression for the effective refractive
index or effective propagation constant of a suspension of particles of
arbitrary shape, size and refractive index in an optically homogeneous medium.
However, its validity for biological matter, which often consists of very dense
suspensions of cells, is unclear because existing derivations of the formula or
similar results rely on far-field scattering and/or on the suspension in
question being dilute. We present a derivation of the van de Hulst formula
valid for suspensions of large, tenuous scatterers -- the type biological
suspensions are typically made of -- which does not rely on these conditions,
showing that they are not strictly necessary for the formula to be valid. We
apply these results specifically to blood and epithelial tissue. Furthermore,
we determine the true condition for the formula to be valid for these types of
tissues. We finally provide a simple way to estimate -- and, more importantly,
correct -- the error incurred by the van de Hulst formula when this condition
is not met.
