The Mayo Clinic’s expansion plans are crimping its financial results and raising a note of concern from one of the nation’s top ratings agencies.

Audited financial documents filed this week showed that net income in 2012 dropped 35 percent to $395.4 million compared with a year earlier, as the Rochester-based medical organization purchased clinics in Red Wing, Minn., and Waycross, Ga., and took on $300 million in bonds to fund several construction projects.

Mayo remains a well-funded operation, however. Consolidated revenue ticked up 6.3 percent to $8.84 billion, results that also include a clinic system that covers more than 70 communities in the Midwest and hospitals in Scottsdale, Ariz., and Jacksonville, Fla.

Ratings agency Standard & Poor affirmed its AA rating for Mayo’s long-term debt, a step below its highest rating, and said the rating reflects S&P’s assessment of “Mayo Clinic’s excellent enterprise profile” and “international reputation.”

However, S&P also revised Mayo’s rating outlook to “negative” based on the additional debt, a slowdown in revenue growth and about $1.2 billion in unfunded pension obligations.

In determining a ratings outlook, S&P says it considers any changes in economic and fundamental business conditions.

A change in the outlook is not necessarily a precursor of a rating change, which is a measure of an organization’s creditworthiness, but it also is not uncommon for the agencies to adjust an outlook in advance of a forthcoming change.

“We revised the outlook to reflect our opinion of Mayo Clinic’s weaker operating performance, especially in the second half of 2012, and additional debt with this issue, which we did not expect and did not include in our last rating analysis,” S&P credit analyst Martin Arrick said in a statement issued Wednesday.

Mayo is charting what it believes could be a $6 billion remake of southeastern Minnesota and the downtown area around its flagship medical center over the coming decade.

The organization is lobbying state lawmakers for $585 million in taxpayer support to pay for infrastructure costs, as Mayo pays for $3.5 billion in capital projects.

Several major building projects, including a proton beam therapy center and expansion of a wellness facility, are underway and a few are waiting in the wings, including an overhauled emergency department at St. Marys Hospital.

With start-up costs for the projects and pension obligations, growth in expenditures has outstripped growth in revenue. Standard & Poor noted that the investments may not produce net improvements in operating income for a few years.

Mayo officials plan to offer a fuller explanation of its financial performance Wednesday. But a spokesman said Mayo is taking advantage of favorable interest rates as it seeks to expand, which may have a short-term impact on cash flow. The spokesman said the organization’s operating margins are in line with its annual target of 4 to 6 percent.