drop table ab;
create table ab(x varchar2(100));
Insert into ab values ('not on dialysis');
Insert into ab values ('negative not on dialysis');
Insert into ab values ('negative on dialysis');
Insert into ab values ('or dialysis');

create   index a_idx on ab (x) indextype
  is ctxsys.context
  parallel 32;

Select * from ab where instr (lower(x),'dialysis') > 0 
and  (contains(x,'near((dialysis,negative={not}),50)') > 0);

'not on dialysis'
'negative not on dialysis'
'negative on dialysis'

create   index a_idx on ab (x) indextype
  is ctxsys.context parameters ('stoplist ctxsys.empty_stoplist');