Costco Wholesale Corporation’s (COST) sales at stores open at least a year were flat in April. This comes in slightly below analysts' estimates, primarily because of the strong dollar and lower gas prices. Analysts surveyed by Thomson Reuters predicted a 0.3 percent increase in the figure. Sales at stores open at least a year are a key indicator of a retailer's health because they exclude results from stores recently opened or closed. The wholesale club operator said Thursday that sales at U.S. locations open at least a year rose 2 percent for the four weeks ended May 3. Internationally, the metric declined 4 percent. Stripping out lower gas prices and fluctuations in foreign currency, Costco's sales at stores open at least a year climbed 7 percent. The figure rose by that same amount at U.S. locations. The metric increased 9 percent overseas. Total revenue edged up 2 percent to $8.75 billion. Year to date, Costco Wholesale Corp.'s sales at stores open at least a year climbed 2 percent. In the U.S. the figure rose 4 percent. It slipped 2 percent internationally. Total revenue climbed 4 percent to $76.33 billion.