The Architectural Billings Index for the nation was positive in September for the first time in two years, but billings in the western region that includes Colorado weren’t, according to the index released Wednesday.

The ABI, compiled by The American Institute of Architects, is a leading economic indicator of construction activity. It reflects the nine- to 12-month lag between when architecture firms bill clients and when funds for construction are spent.

September’s national ABI score was 50.4, up from 48.2 in August — and up for the fourth month in a row. A score of 50 and above represents an increase in billings.

“This is certainly encouraging news, but we will need to see consistent improvement over the next few months in order to feel comfortable about the state of the design and construction industry,” AIA Chief Economist Kermit Baker said in a statement.

The western area had the lowest regional ABI score for last month, at 44.5, a dip from 45.8 in August. But September’s western score was up significantly from 36.0 in September 2009.

The index breaks the country into four regions: Northeast, Midwest, South and West.

Both the northeastern and midwestern parts of the country had positive ABI scores for September, at 56.7 and 51, respectively. The South had a score of 47.

The ABI is based on a “work-on-the-boards” survey of AIA members by organization researchers. Members are asked each month whether their billings have increased, decreased or stayed the same for the month just ended.

The AIA is more than 150 years old, and based in Washington, D.C.

Hat tip to multiple sources including AIA, Denver Business Journal and Sam Armijos.