What color is cholesterol? Cash green or blood red, depending on which company you talked to at this year's American Heart Association scientific meeting.

The top story from the conference is a clinical trial pitting Abbott Labs' (NYSE:ABT) Niaspan against Merck's (NYSE:MRK) Zetia. The trial had its flaws, including that data was from just 208 patients, but Niaspan was the clear winner. When added to cholesterol-lowering statins, Niaspan was able to decrease the thickness of an artery wall -- a measurement of plaque buildup. Zetia plus a statin didn't have an effect on the thickness of an artery wall.

It's a clear win for Abbott. Niaspan works by increasing "good" HDL cholesterol, and the trial clearly shows that the changes in HDL cholesterol have a positive effect on artery-wall thickness.

How bad the results are for Merck remains to be seen. Like Pfizer's (NYSE:PFE) Lipitor, AstraZeneca's (NYSE:AZN) Crestor, and older statins -- Merck's Zocor and Bristol-Myers Squibb's (NYSE:BMY) Pravachol -- Zetia works by lowering bad LDL cholesterol. In fact, when used in combination with statins, Zetia can lower LDL levels below the level that statins can on their own.

The decrease in cholesterol levels didn't translate into a decrease in the thickness of an artery wall, but that shouldn't come as much of a surprise; last year's Enhance trial showed the same thing. Since that trial, sales of Zetia and Vytorin, which combines Zetia with Zocor, dropped 12% year over year in 2008 and an additional 14% so far this year.

This trial is another nail in Merck's now-fully owned cholesterol franchise's coffin, but we can't start the funeral procession just yet. Cholesterol levels and artery-wall thickness are secondary to what patients and doctors really care about: heart attacks. Merck has a large trial testing the effect of Zetia on heart attacks, but the results won't be available until 2012.

Until then, expect Zetia's and Vytorin's sales to bleed a little.