  We consider the critical temperature in strongly anisotropic
antiferromagnetic materials, with weak coupling between stacked planes, in
order to determine the interplane coupling constant from experimentally
measured susceptibilities. We present theoretical arguments for a universal
relation between interplane coupling and susceptibility shown numerically by
Yasuda et. al., Phys. Rev. Lett. \textbf{94}, 217201 (2005). We predict a more
general scaling function if the system is close to a quantum critical point, a
similar relation for other susceptibilities than considered in Yasuda et. al.,
and the validity of these relations for more general phase transitions.
