It has been generally assumed that the asymptotic critical behavior of real fluids can be characterized in terms of the same physical variables as that of the lattice gas. This assumption implies that, below T-c, the second derivative of the pressure with respect to temperature should asymptotically diverge like the isochoric heat capacity C-V, while the second derivative of the chemical potential with respect to the temperature should remain finite at the critical point. The validity of this assumption has recently been questioned on the basis of an analysis of experimental two-phase C-V data in terms of the so-called Yang-Yang relation. In this paper we show how such an analysis may be affected by the presence of a small amount of impurity as well as by other nonasymptotic deviations from lattice-gas symmetry. When corrections for a small amount of impurity are applied and allowance is made for the leading asymmetric Wegner correction, the experimental C-V data are not inconsistent with previous treatments in which the second derivative of the chemical potential exhibits a cusplike singularity with a finite limiting value at the critical temperature.