For reference, the official Des Moines metro area includes six counties: Polk, Dallas, Warren, Jasper, Madison, and Guthrie. The Des Moines CSA (combined statistical area) includes those six and two more: Story and Boone (which comprise the Ames metropolitan area).
Definition of a "metropolitan area": These are areas with at least one urban area of 50,000 or more inhabitants, plus adjacent territory with a high degree of social and economic integration.
Definition of a "combined statistical area": The U.S. Office of Management and Budget (OMB) defines a CSA by various combinations of adjacent metropolitan and micropolitan areas with economic ties measured by commuting patterns. CSAs retain their own designations as metropolitan or micropolitan statistical areas in their respective larger combined statistical areas.