  It is known that factorization of the $\beta$-function loop integrals into
integrals of double total derivatives is an important ingredient needed for
deriving the NSVZ relation by direct perturbative calculations in ${\cal N}=1$
SQED regularized by the higher derivatives. It allows to relate the
$\beta$-function and the anomalous dimension of the matter superfields defined
in terms of the bare coupling constant. In this work we find the analog of this
result in the case of using dimensional reduction regularization in the lowest
orders. However, we demonstrate that in this case the NSVZ relation is not
satisfied for the RG functions defined in terms of the bare coupling constant.
Nevertheless, it is possible to impose boundary conditions to the
renormalization constants determining the NSVZ scheme in the three-loop order
for the RG functions defined in terms of the renormalized coupling constant.
