I've been quite bullish on Exact Sciences (NASDAQ:EXAS) stock for a while now. In January, I pointed out some reasons the molecular-diagnostics stock should go much higher. Since I wrote that article, Exact Sciences' share price indeed has gone much higher -- almost doubling. 

After this huge run-up, can Exact Sciences stock still go a lot higher? Here's the bear case on the stock from someone who has been nothing but a bull on Exact Sciences so far.

Toy bear and bull

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The price of Cologuard could fall

Exact Sciences' only current product on the market is the Cologuard DNA screening test for colorectal cancer. Sales for Cologuard have been soaring, but Exact Sciences could be in trouble if the price for the DNA test falls significantly -- even if volume sold continues to grow. And it's possible that could happen.

The current list price of Cologuard is $649. However, few, if any, payers actually shell out that much for the DNA test. Medicare currently pays $512 per patient. Exact Sciences stated in its first-quarter results that the average selling price for Cologuard was $485.

A challenge the company faces is that Medicare will start reimbursing based on the weighted median of private payer rates in January 2018. Medicare covers roughly 47% of patients in the target population for Cologuard. If the rate that Medicare pays declines significantly, Exact Sciences' overall revenue could also drop.

Physicians might not prescribe as much as hoped

While the numbers of first-time physicians prescribing Cologuard are growing, there's a possibility that physicians might not prescribe the test as much as hoped. There are actually some signs of this in Exact Sciences' reported numbers. The prescriptions-per-physician ratio has only grown modestly, suggesting that doctors aren't prescribing a lot more of the DNA tests after the first order.

Why might Exact Sciences run into problems on this front? One challenge is education. Not every doctor is yet familiar with Cologuard.

The bigger issue, though, could be that many physicians aren't comfortable with the accuracy of the test. In clinical studies, Cologuard was found to have a 94% sensitivity in detecting early-stage colorectal cancer, and 92% sensitivity in detecting cancer in stages 1 through 4. Although those are actually pretty good levels, the numbers also mean some patients might have cancer that goes undetected.

DNA strands

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Blood-based DNA-testing could be just around the corner

Perhaps the greatest long-term threat to Cologuard (and, therefore, to Exact Sciences) is that stool-based DNA testing could become obsolete. One company working hard to make a blood-based DNA test to detect multiple types of cancer at an early stage is Grail.

Grail was formed in early 2016 by genomic-sequencing pioneer Illumina (NASDAQ:ILMN) and other investors. The company's goal from the outset was to achieve the "holy grail in oncology" by identifying biomarkers that could reliably detect the presence of cancer at an early stage.

Although Illumina sold some of its stake in Grail earlier this year and no longer has majority ownership, Grail still has several big-name investors funding its operations. It's too soon to know if Grail will succeed in its quest, but if it does, Cologuard could go by the wayside.

Barely bearish

Are these arguments enough to flip this Exact Sciences bull to a bear? Not really.

While each of the bear cases against the company have some merit, each argument also contained reasons to remain invested in Exact Sciences stock. Yes, Cologuard's price could fall. But if you look closely at the numbers, that price decline shouldn't be catastrophic for the company, and it could easily be offset by increased volume.

Yes, physicians might not prescribe Cologuard as much as expected. However, with the nation's largest health insurer, UnitedHealth Group, announcing that it will reimburse for the DNA test, I suspect physicians will gain more confidence in prescribing the DNA test -- especially for the many patients who simply don't want to undergo a colonoscopy.

And, yes, Cologuard could be rendered obsolete by new and improved technology. But it's not like Exact Sciences is sitting still. The company has its own research underway on liquid biopsies for detecting various types of cancer at early stages. It's even working with the prestigious Mayo Clinic on these efforts. 

At some point, Exact Sciences stock could run out of steam. This bull doesn't see that happening just yet.

Keith Speights has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Illumina. The Motley Fool recommends UnitedHealth Group. The Motley Fool has a disclosure policy.