  We make use of the chiral two-loop representation of the pi K scattering
amplitude [J. Bijnens, P. Dhonte and P. Talavera, JHEP 0405 (2004) 036] to
investigate the isospin odd scattering length at next-to-next-to-leading order
in the SU(3) expansion. This scattering length is protected against
contributions of m_s in the chiral expansion, in the sense that the corrections
to the current algebra result are of order M_pi^2. In view of the planned
lifetime measurement on pi K atoms at CERN it is important to understand the
size of these corrections.
