By Dan Fitzpatrick 

The Federal Reserve revised its stress-test results for 15 U.S. banks a day after releasing its annual report card, producing a weaker outcome for Bank of America Corp., American Express Co. and HSBC North America Holdings Inc.

Despite the changes, 29 of the 30 banks still were said to have enough capital to continue lending even if faced with a hypothetical jolt to the economy. Charlotte, N.C.-based Bank of America was already the worst performer among big U.S. banks in the test, and Friday's revision dropped its Tier-1 common capital to 5.9% from 6%.

The Fed views 5% as a minimum level allowance.

The Fed measured how 30 large institutions would perform when faced with a hypothetical shock to the U.S. economy lasting into 2015, including a severe drop in housing prices and a surge in unemployment.

Fed officials declined to comment Friday about the changes, which it attributed in a press release partly to "inconsistencies" in how regulators treated dividend increases or stock buybacks that occurred in the fourth quarter of 2013.

For 12 banks, including Citigroup Inc. and Goldman Sachs Group Inc., the revisions resulted in a slightly better test performance as their capital ratios rose by as much as 0.3%.

The largest rise went to Northern Trust Corp., which moved to 11.7% from 11.4%. The largest drop belonged to American Express Co., which dropped to 12.1% from 12.6%.

Friday's changes could stoke a continuing source of tension between banks and regulators. The two sides have clashed in recent years over the exams and the math used to produce the results, with banks arguing the Fed doesn't reveal enough about the metrics it is using.

The Fed also raised the capital ratio for Zions Bancorp by 0.1%, to 3.6%, but it wasn't enough to pull the Salt Lake City lender above the Fed's minimum of 5%. Zions has said it expects to resubmit its capital plan to the Fed.

Investors and analysts focused Friday on whether Bank of America would need to downsize its plans to return capital to shareholders to get approval from the Fed when the central bank releases the next round of its stress tests on Wednesday. Bank of America shares fell 2% Friday, as broader bank stocks were mixed.

Write to Dan Fitzpatrick at dan.fitzpatrick@wsj.com

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