Shares of American Express Co. fell Tuesday despite an analyst upgrading his outlook for the company's stock. "While we expect the news flow for [AmEx] to remain negative, the case for material downside to the shares is less compelling as sentiment is already very bearish and there are signs of potential credit market stabilization," Donald Fandetti, an analyst at Citigroup Global Markets Inc., wrote in a research note Tuesday. He raised his rating on the New York-based company to "hold" from "sell." AmEx shares have fallen more than 40% in the past four months. "There could be support for the stock due to the valuable processing business," Fandetti wrote, predicting "the shares could easily trade up to the high teens if the credit market stabilization gains momentum." Amex shares closed at $15.01 Tuesday, down 0.99% from Monday's close.