U.S. Medicare spending on doctors’ visits and related services rose by 15 percent in 2004 as patients saw physicians more often and underwent more tests and minor procedures, health officials said on Thursday.

The sharp spike in spending concerns Medicare officials, said Mark McClellan, administrator of the Centers for Medicare and Medicaid Services. The agency is taking a close look at why some services are being used more frequently and if they are leading to better health care, McClellan said.

“This is a big increase in expenditures…We are looking into this closely to find out exactly why these…costs are going up,” McClellan told reporters during a conference call.

Officials want to “get a better understanding of which of these are related to better improvement in care, and which may be more questionable,” he said.

Medicare is the federal health insurance program for the elderly and disabled.

The program’s spending for “Part B” services such as doctors’ visits and related tests and in-office procedures totaled $88.3 billion in 2004, compared with $76.7 billion in 2003, according to Richard Foster, Medicare’s chief actuary.

“Continued rapid spending growth strains both beneficiaries’ incomes and the federal budget. For this reason, understanding why Part B expenditures are rising so rapidly is of great concern,” the Centers for Medicare and Medicaid Services said in a letter to a Medicare commission that advises Congress.

The increase in the number of Medicare beneficiaries had little impact on spending growth, McClellan said.

The biggest factor was more frequent, and longer, doctor visits by Medicare patients, the agency said. Many visits were for follow-up treatment for chronic illnesses.

Use of physical therapy services and other minor procedures also went up, as did the number of laboratory tests and complex imaging scans such as MRIs.

Administration of drugs such as chemotherapy in a doctor’s office also was more frequent.

The use of services varied in different parts of the country, something the agency is investigating, McClellan said.

The Centers for Medicare and Medicaid Services estimated physician fee rates would be cut by 4.3 percent in 2006, based on a funding formula mandated by law. The American Medical Association urged Congress to intervene to raise the payments.

“If these steep cuts go into effect, physicians may have to make hard choices about taking new Medicare patients,” AMA Board Chair J. James Rohack said.

Monthly premiums for Medicare beneficiaries are projected to increase to $89.20 next year from $78.20 this year, according to preliminary estimates from the agency. Officials said patients would receive enhanced benefits, including prescription drug coverage, which would yield enough savings to make up for the higher premiums.