  We have studied the applications of the anomaly sum rule to the transition
form factors of light pseudoscalar mesons: $\pi^0$, $\eta$ and $\eta'$. This
nonperturbative QCD approach can be used even if the QCD factorization is
broken. The possibility of small non-OPE corrections to the spectral density in
view of the recent BaBar and Belle data is explored. The accuracy of the
applied method and the role of the quark mass contributions are discussed.
