After doping cases last year that nearly stopped the team, Astana faces more problems on the eve of the Tour de France.
Low cortisol levels may indicate use of the steroid cortisone.
However, General Manager Alexander Vinokourov is considering pulling Astana out of the MPCC to allow Boom to take part in the race. The team could make the decision this evening so that Boom can begin stage one on home Dutch roads in Utrecht on Saturday.
"Vino is thinking about what to do next," Team Manager Giuseppe Martinelli told Cycling Weekly.
"We only found out about the test at 2pm, one hour after we named our team in the meeting. We could leave the MPCC or we could race with eight riders — nothing's sure yet."
Dutch media has suggested that Vinokourov has already made the decision to leave the MPCC.
Eight WorldTour teams, including Sky, are not part of the MPCC. Membership entails following a different, stricter set of rules above those of cycling's governing body.
"As per UCI Regulations, teams can't change their starting list after the Directeur Sportif meeting has taken place," the UCI said in a statement.
"The UCI is committed to apply its regulations consistently and will therefore not authorise any team to change its starting list now."
"According to article 9 of MPCC regulation, in case of abnormally low cortisone levels, competition will resume after an additional eight-day rest minimum, and back-to-normal cortisone levels," said the MPCC in a statement.
"The MPCC would like to remind that cortisone level tests are carried out within the framework of health, not anti-doping."
If Astana left the movement, it would be the third in recent times after Lampre and LottoNL. LottoNL were not able to start Theo Bos in the Vuelta a España and in May, George Bennett in the Giro d'Italia due to the same test.
Leaving the MPCC would draw a darker cloud over team Astana, which had five positive doping tests from its teams last year – two from the WorldTour team and three from the feeder team. It fought, and succeeded, to keep its racing licence.
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