Corrections to the measuredn=1hyperfine interval of positronium due to annihilation effects

The existence of corrections, due to annihilation, to the usual expressions for the energy levels and decay rates of the $m=0\mathrm{}$ triplet and singlet states of positronium in a magnetic field are noted. The corrections cause the value of the hyperfine interval $\nu$, as obtained from wellknown resonance experiments, to increase by 1.8 standard deviations of the current experimental error to $\nu =203\mathrm{}387.5\left(1\right)\mathrm{}$ MHz where $\nu \left(\mathrm{theor}\right)=203\mathrm{}400\left(10\right)\mathrm{}$ MHz. This shift could be observable in the near future when calculations of $\nu$ to order ${\alpha }^2$ are completed.