Amgen Inc. said late Tuesday that it will challenge a portion of the new coverage policy issued by the agency that runs Medicare.

The Thousand Oaks-based biotech giant plans to challenge the Centers for Medicare and Medicaid Services’ decision to deny payment for Aranesp and Epogen as well as Johnson & Johnson’s Procrit if a patient’s hemoglobin level is greater than 10 grams per deciliter.

Amgen said it was concerned that “inappropriately” limiting coverage in such a way would increase the need for blood transfusions and compromise the quality of care.

“In our view, restricting coverage in this way is unreasonable, impractical and unworkable,” said Amgen’s spokesman, Dr. Roger Perlmutter, in a statement.

The drugs are commonly used to battle anemia that results from chemotherapy or chronic kidney disease.

Aranesp and Epogen accounted for nearly half of Amgen’s sales in 2006.

Anemia drugs are Medicare’s single biggest expense and have come under fire for being overused or improperly used. Lawmakers are also investigating whether Amgen’s incentive plan for doctors is paying them to prescribe the drug in larger doses.

Shares in Amgen were down $2.34, or 4.4 percent to $51.39. Shares have lost 11 percent since the policy change was announced.