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Chest pain is one of the big reasons people go to the ER or to see their primary care doc. Often when people get there, it’s hard to tell who has serious heart disease or something less serious. Now Palo Alto, CA-based CardioDx is presenting study results that say its new blood test, in certain situations, can help doctors tell the difference.

CardioDx said today that its test, called Corus CAD, was more effective than standard imaging techniques in a study of 537 patients. The study found that the gene expression test, which relies on a blood sample, was better than standard myocardial perfusion imaging (MPI) at ruling out the patients who don’t have coronary artery disease. The results, from a study called Compass, are being presented today at the American Heart Association’s scientific sessions in Orlando, FL.

The company is hoping that studies like this one will help convince doctors to start sending in samples into its lab for a new kind of gene expression test, in hopes of avoiding other expensive and invasive procedures that end up being a waste. And there’s a huge amount of waste in cardiology that doctors, patients, and insurers would like to reduce. Of 400,000 people studied who underwent elective angiographic procedures, almost two-thirds (62 percent) were found to have no coronary artery blockage, according to a separate analysis published last year in the New England Journal of Medicine. The CardioDx test won’t be able to eliminate all the unnecessary procedures, but it should be able to reduce them significantly when used in combination with other diagnostic tools.

“We now have a blood test for coronary artery disease, which we never had before,” said Gregory Thomas, a professor of medicine at UC Irvine, who’s presenting the results today in Orlando. “It’s not perfect, but it can provide a very powerful opportunity for physicians to evaluate patients who may have heart disease.”

Gregory Thomas

CardioDx is backed by a cast of big name investors, who are betting that this test will work its way into mainstream medical diagnosis, and that insurers will figure it’s worth the $1,195 price tag. CardioDx raised $60 million last May, and has raised cash from the likes of J.P. Morgan, Kleiner Perkins Caufield & Byers, Mohr Davidow Ventures, TPG Biotech, Intel Capital, Pappas Ventures, and GE Capital, among others. At the time of the May investment, CardioDx said its test had been used for 13,000 patients, and a company spokeswoman now says that figure has grown to 20,000.

The potential market for a new coronary diagnostic like this is huge—an estimated 9.2 million people underwent the standard MPI tests in 2005, CardioDx says. Those tests typically cost about $800, and take about three hours to yield results, Thomas says.

Today’s test results, for sure, will go toward helping the company make its case for broader adoption. Patients in the study were considered stable, but had symptoms suggesting they might have coronary artery disease—a warning sign for future heart attack or stroke. They gave blood samples for the CardioDx test, which analyzed the extent to which RNA from 23 genes was active or inactive. The patients then got the usual myocardial perfusion imaging (MPI). Both of those readouts were compared later with the gold standard diagnostic findings from invasive angiography or CT angiography.

Researchers found that the new test was superior to MPI in what’s known as negative predictive value, which helps doctors rule out coronary artery disease. The CardioDx test was able to rule out coronary artery disease 96 percent of the time, compared with 88 percent of the time for MPI imaging. The difference was statistically significant.

The CardioDx test separated patients into two categories—those with high or low probability of having coronary artery disease. Researchers found that 46 percent of patients had a low score, and 54 percent had a high score. The results meant the most for patients who have the low score, because for them, it means that 96 percent of the time they be confident they don’t have coronary artery disease, says CardioDx chief medical officer Mark Monane.

Still, there’s a fair bit of uncertainty among those with a high score. CardioDx suggests that those with a high score go on to get a follow-up MPI test, Monane says. By layering those two tests on top of each other, doctors can hope to weed out many more patients who would otherwise get unnecessary invasive coronary procedures, he says. Even so, by adding in the CardioDx test, a majority of patients (53 percent) are still expected to get invasive procedures when they don’t have coronary blockages, Monane says.

The CardioDx test has its limits. It can’t be used in emergency situations, because it only provides an answer in a 72-hour turnaround time from the company’s Palo Alto lab. It’s an open question whether in the real world, physicians who get a low score will trust the data, and forgo further tests that add costs. And it isn’t for patients with diabetes, who already have higher coronary risks, Monane says.

But today’s findings could help CardioDx appeal to a broad group of primary care physicians and cardiologists who are looking for ways to better diagnose chest pain in a convenient way.

“We think our test is validated, it’s easy to use, it’s a blood draw, it can be done immediately, and it doesn’t require people to go out and buy an expensive piece of equipment,” Monane says.

Thomas, who is presenting the results today at the American Heart Association meeting, said there is a lot of interest among his peers in the data. But it will take time before people start actually incorporating it into their practice. He isn’t sure whether it will reduce healthcare costs, because some doctors may like the technology enough to use it on broader groups of patients than intended. Further in the future, the technology could also find additional uses, like as a monitoring tool in clinical trials to see whether drugs like cholesterol-lowering statins are working or not, he said.

“Physicians will want to see other physicians using it, and want to see it embedded in clinical guidelines,” Thomas says. “This doesn’t change medicine tomorrow, but it’s one step in potentially changing heart care.”