Separation from P. coarctata is by several characters, in workers and queens focusing especially on the petiole node, which seen in profile is shorter and thicker in testacea, usually not extending above the height of the thorax and gaster, and parallel-sided instead of slightly tapering. In dorsal view, the petiole tends to a semi-circular cross-section, rather than the elliptical one of coarctata. The subpetiolar process is more concave ventrally, with a slightly larger circular thin spot (fenestrum), and a more or less well-developed posterior tooth-like projection. Morphometric indices give certain diagnosis. Colour is usually a pale brown, instead of the dark brown to black of coarctata. These and other characters are described and figured by Csösz & Seifert (2003). Comparative images of GB specimens are given in Attewell, Collingwood & Godfrey (2010). Males have been less well defined; Csösz & Seifert cite differences in average body length, (testacea <3.0mm< coarctata), a head with sparse short hairs vs. dense long hairs of coarctata, and a less robust alitrunk. Boer (2015) additionally gives hind tibial pilosity, which in testacea is largely of appressed pubescence, without decumbent/subdecumbent hairs.

They are somewhat smaller, paler and more slender than the very common black ant Lasius niger. The posterior of the gaster has a sideways slit and not an acidipore. The single petiole is large.