American Express Co.'s credit card delinquencies are rising faster than expected, reaching the highest since the 2001 recession, and potentially pushing the stock lower, according to a research note from Friedman, Billings, Ramsey Group Inc.

Amex's delinquency rate rose 29 basis points, to 3.71% in August, while the default rate rose 51 basis points, to 6.14%, Friedman analysts Scott Valentin and Ram Gowrisankaran wrote in a research note that was published Monday.

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