If the sight of blood makes you cringe, consider this: Blood plays a vital role in your body — it carries oxygen and nutrients, regulates temperature, and helps protect against infection. And in recent years, scientists have discovered something extraordinary about blood: It may hold the secret to detecting certain types of cancer.
Most malignant tumors start in a single organ (think: breast cancer or prostate cancer). As the cancer grows, minuscule fragments of cancer cells, known as circulating tumor DNA (ctDNA), begin to shed into the bloodstream. Although that idea might make you shiver, the use of ctDNA in blood tests is drastically changing how cancer is detected, treated, and monitored. But in the words of Paul Limburg, MD, chief medical officer for screening at Exact Sciences, “While these tests certainly offer a lot of hope, there’s also a lot of hype.” That’s why we’re sharing both the upsides and the downsides of cancer-detecting blood tests, including their power, promise, and potential pitfalls.
Downside: They aren’t very good at detecting precancers or many early-stage cancers
Let’s start with the bad news: If you’re thinking one simple blood test can detect whether you’re cancer-free, things are unfortunately a lot more complicated than that, especially when it comes to cancers that already have recommended screening methods available. Take, for example, colorectal cancer: Who wouldn’t want to forego their once-a-decade colonoscopy for a quick blood test? While opting for a single-organ blood test might sound tempting, it’s important to understand the limitations of this screening method, such as its limited ability to detect stage 1 colorectal cancer and precancers, crucial indicators of colorectal cancer. “Precancers haven’t broken out of their tissue boundary, so they’re not yet invasive,” explains Dr. Limburg. So, if you’ve got a polyp, it’s not likely to release enough DNA fragments into your bloodstream to be detectable, and your blood test result may be clear. If you use a stool test, it’s more likely to identify a precancerous polyp, which can be removed during a colonoscopy. Choosing a colorectal cancer screening test that helps prevent cancer through effective detection of precancer — such as a stool test, like the Cologuard® test, or colonoscopy — is essential. “A single organ blood test is better than nothing,” says Dr. Limburg, “But for colon cancer screening, blood tests should not replace the current standard of care.”
They’re less effective at detecting cancer in its early stages. This is because the smaller the tumor, the less ctDNA it will release into the bloodstream. “Single-organ blood tests miss about half of stage-one colon cancers,” explains Dr. Limburg, “Which is unfortunate because that’s the point at which cancer is most curable.”
Downside: The cross-reactivity problem
If you get a blood test for colon cancer and it comes back positive, your doctor will then order a colonoscopy to identify and biopsy any tumors that are identified visually. But what happens if the colonoscopy says you’re cancer-free?
Cross-reactivity occurs when a test detects something other than its intended target because the two substances share a similar structure. As a result, even though all cancers are different, they can sometimes produce similar signals in test results. “Carcinogenesis, the process in which a normal cell turns into invasive cancer, isn’t exactly unique to each organ,” explains Dr. Limburg. “Many of the markers these tests look for could come from cancer anywhere in the body. So now you know there’s a suspicion of cancer, but it could be in your kidneys, ovaries, or somewhere else in the body, which can make the test result difficult to evaluate. This could lead to a diagnostic odyssey, and there’s very little guidance for physicians on what to do in these scenarios.”
Upside: Certain tests can help detect some deadly cancers before symptoms appear
So if a blood test doesn’t always detect precancer, what’s the point? Well, there are plenty of cancers that don’t have screenings to detect them at any stage, let alone when they’re still precancerous.
Except for colorectal, breast, cervical, lung cancer, and some prostate cancers, most cancers are only diagnosed when symptoms prompt a patient to see a doctor. Take pancreatic cancer — a fast-growing and aggressive disease — for example. By the time symptoms like jaundice or unexplained weight loss appear, the tumor has usually already grown and spread. This late detection is a major reason why pancreatic cancer has such a high mortality rate.
This is where the multi-cancer early detection test (MCED test) could be an absolute game-changer. Like a single-organ blood test, an MCED test looks for ctDNA or other markers in the blood as a signal that cancer may be present. MCED tests can help screen for aggressive cancers that currently lack a screening method. That means potentially detecting deadly and aggressive cancers like pancreatic, liver, ovarian, and stomach cancer at earlier stages when treatment is generally more effective. Current MCED tests, like other blood tests, aren’t likely to find precancer. However, for many cancers without screening options, finding them earlier could be a significant benefit.
Upside: If you’ve already had cancer, some tests can pretty accurately predict if you’ll have a recurrence
ctDNA isn’t just helpful in diagnosing new cancer cases; it can also tell you if your cancer might return. Molecular residual disease (MRD) tests look for DNA fragments in blood as evidence that there are still tumor cells somewhere in your body. According to Rick Baehner, MD, chief medical officer of precision oncology at Exact Sciences, “MRD tests, which are relatively new in cancer care, can determine if there is ctDNA from the patient’s tumor circulating in the blood, which can help providers to determine if cancer cells are present after surgery or chemotherapy.”
So if your MRD test is positive, how do you know if it’s a cancer recurrence, or a totally different cancer that’s sprung up independently? Get ready to have your mind blown: Instead of just checking the blood sample against a general panel of mutations or a single protein marker, they can individualize your test from your specific tumor. In a tumor-informed blood test, scientists sequence the DNA from a piece of your tumor, and then test your blood for genetic mutations specific to your cancer tissue to see if tumor cells are still present in your blood. Dr. Baehner says, “There is a very strong association between detecting ctDNA in an MRD test and developing a clinically detectable cancer recurrence in the future.”
What does that mean for the patient? It can help determine if a surgery or therapy was successful. “Patients often ask if the surgeon was able to entirely remove the cancer from their body,” explains Dr. Baehner. “If this test shows ctDNA is detected, there’s a much higher likelihood their tumor will return.”
Upside: They can help some patients avoid chemotherapy
Even if a tumor appears to be completely removed during surgery, microscopic traces of cancer can sometimes remain in the body. And if left untreated, these remnants can grow. That’s why chemotherapy is sometimes recommended after surgery — to eliminate any remaining cancer cells and reduce the risk of recurrence. “One fundamental change we’re going to see from MRD tests is which patients are at a higher risk of a relapse, which may help providers more accurately identify who needs chemotherapy after surgery, and who doesn’t,” says. Dr. Baehner. “If the test result is negative after surgery or chemotherapy, a provider can continue to monitor the patient at regular checkups using that same tumor-informed blood test, to ensure their result stays negative.”
Blood tests that can help detect cancer are a reason to remain hopeful: They can spot signals in the blood from some of the deadliest cancers before symptoms even show up, and give survivors peace of mind by monitoring for signs of recurrence. But new doesn’t always mean better, and these blood tests may still miss important precancers and early-stage cancers. The science is constantly progressing, and some of these tests are likely to take a greater role in the fight against cancer in the future; but for now, many of these tests are more like sidekicks (not superheroes) in the ongoing fight against cancer.
