Small U.S. businesses with less than $5 million in annual sales are showing multi-year-high profit margins, even as their sales growth has significantly dwindled this year, according to a study by the financial information company Sageworks.

So far this year, the increase in sales for such companies is only 0.6 percent, compared to an average of 7 percent sales growth in 2012 and 2011. Meanwhile, the increase in net profit margin is 10.6 percent, up from 6.9 percent in 2012 and 4.6 percent in 2011, and the highest net profit margin in at least six years.

Despite the encouraging increase in profits across all industries, low sales growth in such key areas as manufacturing and the wholesale and retail trades could lead to moneymaking challenges ahead for small business owners, Sageworks analyst Regan Camp said in a release.

“Private companies are going to be hard-pressed to maintain those net profit margins, simply because one of the primary drivers of net profit margin is sales,” Camp said.

The Sageworks study cited a striking deceleration in sales among small, private manufacturers, with sales falling almost 3 percent so far this year, compared with an average 8.5 percent sales increase in 2012 and double-digit expansion the prior two years.

Still, not all businesses with less than $5 million in annual sales are experiencing tough sales, with construction industry sales showing a 2.2 percent increase so far this year, although that’s down considerably from 8.9 percent sales growth in 2012.

A reason for continued profit-making this year, despite the general sales slowdown, could be because “margins are benefitting from that sales growth from previous years,” Camp said.

It remains unclear what’s behind the current lethargy of most small-business sales, Camp said. But he offered some informed speculation regarding the impact of continuing economic uncertainty on spending by consumers.

“When you’ve got these cuts in federal spending, this political impasse in Washington over fiscal matters and this uncertainty over whether the market can sustain job growth, all of these things contribute to uncertainty,” Camp said. “I just think that people who had been optimistic the last year or two all of a sudden are tempering those expectations and are consequently cutting back on spending.”