The small capsid antigen p23 is a tegument protein. Although p23 has IgG sensitivity and intensity of reaction similar to those of p18, its IgM reactivity is a bit restricted. With respect to detection of IgG antibodies, p23 and p18 have similar sensitivities. While p18 is more sensitive in detecting previous infections, p23 detects more primary infections (Färber, et al., 2001, J. Med. Virol, 63: 271-6).