MANDATORY drug testing in prison is failing, according to research at the University of Central England in Birmingham. Prisons must test between 5 and 10 per cent of their inmates each month. A positive test or refusal by a prisoner to be tested leads to loss of remission. But studies at a Birmingham prison by Morag MacDonald, a lecturer in sociology at UCE, showed prisoners were tempted to switch from soft to hard drugs because the latter were more difficult to detect in the body