The team of defending champion Vincenzo Nibali were plunged into uncertainty on the eve of the Tour de France after Dutch rider Lars Boom returned cortisol levels under the normal level.
Boom, 29, has not failed a drugs test, but as Astana are members of the Movement for Credible Cycling (MPCC) he should, under the organisation's voluntary code, be removed from competition for a period of eight days.
However, news of the test came after the deadline for Astana to replace Boom in their nine-man Tour squad.
The team said they planned to appeal to the UCI for a late change to their squad, calling in reserve rider Alessandro Vanotti, but more realistically now face a choice of leaving Nibali without a key lieutenant or withdrawing from the MPCC and allowing Boom to race.
"Alessandro Vanotti will arrive in the Netherlands Saturday morning to undergo physical and blood tests and await a UCI decision on the matter," Astana said in a statement.
The situation is delicate not only as Nibali is the defending champion, but also because Astana risked having their WorldTour licence revoked earlier this year when the UCI asked its Licence Commission to take action after a number of anti-doping violations in 2014.
UPDATE: The latest reports on the morning of the race suggest Boom will be allowed to start today's individual time trial.