  QED theory of the hyperfine splitting of the $1s$ and $2s$ state in hydrogen
isotopes and helium-3 ion is considered. We develop an accurate theory of a
specific difference 8E_{HFS}(2s)-E_{HFS}(1s). We take into account fourth order
corrections and nuclear structure effects. The theoretical prediction is now of
a higher accuracy than the experiment is. The study of the difference provides
the most accurate test (on a level of a part in 10^8) of the QED theory of $1s$
HFS up to date. The theory agrees with most of the experimental data.
