You call sick. You don't risk your career.
Unfortunately, even in the 21st century, some people are still employed on a casual basis and don't get sick leave. This is not necessarily small operators either; the "national" carrier employs (via an agency) quite a few cabin crew on a casual basis, and they do not get sick leave.
If, as has been suggested, QF were so keen for this D & A testing to be introduced, might they not have reconsidered (in the interests of fairness) this no sick-leave policy for casuals?