(Medco, Express Scripts story, updated for Wednesday market close)NEW YORK ( TheStreet) -- Forward-looking guidance is often more important to investors than the printed earnings number, and that was the case on Thursday for the small group of health care stocks in the pharmacy benefits manager (PBM) sector. Medco Health Solutions ( MHS) provided no visibility on the 2011 sales environment in its earnings, a surprise for investors. The PBM stocks, including Express Scripts ( ESRX), SXC Health Solutions ( SXCI) and Catalyst Health Solutions ( CHSI) slipped in trading on Thursday as a result of the Medco commentary. Health care investors knew that the generic pipeline was weak for 2011, but the lack of any visibility from Medco -- which said it would wait until the third quarter earnings to provide an update on the 2011 sales environment -- seemed to spook investors in the PBM stocks, and lead to some profit-taking. Medco slumped by as much as 10% early on Thursday, before its losses were pared back to a 8% at the close on Thursday afternoon. Express Scripts ended down 6.8% as a derivative trade on the Medco news. Both PBM stocks were trading at four times their average daily share volume, with more than 20 million shares traded on Thursday in shares of each prescription benefit manager. SXC Health Solutions and Catalyst Health Solutions, the small cap players in the space, were both down on Thursday afternoon by roughly 3% on elevated trading volume. There were more than one million shares of SXC Health Solutions traded on Thursday. The PBM stock has only experienced three days previous to Thursday of more than one million shares traded in 2010, and in each case, the trading was bullish. The frank comments from Medco management about the 2011 outlook makes the upcoming Express Scripts earnings -- on July 28, after the market close -- that much more important as a read-through of the sales environment for these stocks, though it most likely will provide a similar outlook as Medco did on Thursday. Analysts are expecting 59 cents in earnings from Express Scripts in next week's report, on revenue of $11.4 billion. The Express Scripts estimate hasn't changed in the past three months. In the June 2009 quarter, Express Scripts beat the Street by a penny, with earnings of 44 cents. Medco beat in its earnings, reporting 83 cents per share versus a Street consensus of 79 cents. Yet the lack of visibility on 2011, and lower profit margin expectations for prescription sales for the rest of the year, led to the downward pressure on Medco shares.