$$W=I_{\nu }\left(z\right)K_{\nu }^{\prime }\left(z\right)-K_{\nu }\left(z\right)I_{\nu }^{\prime }\left(z\right)=-\left[I_{\nu }\left(z\right)K_{\nu +1}\left(z\right)+K_{\nu }\left(z\right)I_{\nu +1}\left(z\right)\right]=-\frac{1}{z}$$