Rasmussen, who was subsequently fired by his Garmin-Sharp team, had been cleared by the Danish Olympic Committee after he missed two tests and failed to provide his whereabouts once in an 18-month period, but the International Cycling Union had appealed the decision.
"The UCI filed an appeal with the CAS to request that Alex Rasmussen be suspended for a period of two years following two missed anti-doping tests and one failure to file whereabouts information within a period of 18 months," CAS said in a statement.
"The CAS ruled that the procedural mistake (delay in communications) did not change the fact that an anti-doping test was missed by Alex Rasmussen after he had committed two other failures for which he had received proper notice.
"Accordingly, the CAS has partially upheld UCI's appeal and has imposed a suspension of eighteen months on the rider, commencing on 1 October 2011."
Garmin-Sharp, owned by Slipstream Sports, said in a statement to Reuters: "We have followed the lead of the authorities throughout this process. Although the missed whereabouts tests occurred on Alex's previous team, in light of the CAS verdict, Alex will no longer be with Slipstream Sports."