A high Signatera result means the test detected elevated levels of tumor DNA circulating in your blood, reported as a number called MTM/mL (mean tumor molecules per milliliter of plasma). There is no single universal cutoff that defines “high” versus “low” positive, but higher MTM/mL values generally correlate with greater tumor burden in the body. Understanding what your specific number means requires context: whether it’s your first test or part of a series, what cancer type you have, and whether the number is rising or falling over time.
How Signatera Works
Signatera is a personalized blood test designed to detect tiny fragments of tumor DNA floating in your bloodstream. Unlike standard tumor markers, it’s built specifically for you. A lab analyzes your tumor tissue to identify 16 unique genetic mutations, then designs a custom test panel that searches your blood for those exact mutations. This personalized approach allows the test to pick up incredibly small amounts of cancer DNA, validated to detect levels as low as 0.01% tumor fraction in the blood.
A blood sample is considered positive when at least two of those 16 tracked mutations are found above a confidence threshold. The result is then quantified as MTM/mL, giving your oncologist a number that reflects how much tumor DNA is circulating. A negative result means the test did not detect those mutations at a meaningful level. A positive result, whether the number is modest or high, means tumor DNA is present.
What MTM/mL Numbers Mean
The MTM/mL value represents the concentration of tumor-derived DNA molecules in each milliliter of your plasma. A higher number means more tumor DNA is circulating, which generally reflects a larger volume of cancer activity somewhere in the body. Research in head and neck cancer, breast cancer, and colorectal cancer has consistently shown that MTM/mL levels correlate with overall tumor burden.
There is no published threshold that separates a “mildly high” result from a “very high” one in the way that, say, cholesterol has defined ranges. The number matters most in relation to your own baseline and trend over time. A result of 50 MTM/mL in someone who previously tested at 5 MTM/mL tells a very different story than a stable reading of 50 MTM/mL across several draws. Your oncologist interprets the number in the context of your specific cancer, treatment history, and imaging results.
Why the Trend Matters More Than One Number
Signatera is designed for serial monitoring, meaning repeated blood draws over weeks or months. A single high result provides a snapshot, but the trajectory of your results over time is what drives clinical decision-making.
A rising MTM/mL level across consecutive tests suggests the cancer may be growing or spreading, even if imaging hasn’t caught up yet. A declining level after surgery or during treatment suggests the therapy is working and tumor burden is shrinking. A level that drops to undetectable (negative) is the most reassuring pattern, as it suggests no measurable disease remains in the body.
If your result went from negative to positive, or from a low positive to a significantly higher number, your oncologist will likely want to investigate further with imaging or additional testing. The change between draws is often more informative than the absolute number on any single test.
How Early Signatera Detects Recurrence
One of the test’s key strengths is lead time. In a study of breast cancer patients, rising Signatera levels predicted metastatic relapse a median of 10.5 months before the cancer became visible on scans, with some cases detected up to 38 months ahead of imaging. This window can give oncologists the opportunity to adjust treatment plans earlier than traditional surveillance methods would allow.
In colorectal cancer, serial testing after surgery identified relapse with 88% sensitivity and 98% specificity in a prospective study of 125 patients with stage I through III disease. That means the test correctly flagged recurrence in the vast majority of cases while rarely producing false alarms. A high or newly positive result in this setting is taken seriously, even when CT scans still look clean.
What Happens After a High Result
A high or rising Signatera result does not automatically trigger a change in treatment. The test provides molecular information, but your oncologist combines it with imaging, symptoms, physical exams, and other blood markers like CEA to form a complete picture. Standard surveillance for cancers like colon cancer still relies on CT scans of the chest, abdomen, and pelvis, colonoscopy, and regular physical exams.
In practice, a newly positive or rising result typically leads to more frequent imaging to look for a source. If scans reveal a recurrence, your oncologist can discuss treatment options at that point. If scans remain clear despite a positive Signatera result, you may be monitored more closely with repeat blood draws and imaging at shorter intervals. The goal is to catch any recurrence as early as possible while it may still be treatable with curative intent.
It’s worth noting that major guideline organizations have not yet fully incorporated Signatera into standard recommendations for all cancer types. Some insurers still classify it as investigational. This doesn’t mean the test lacks value, but it does mean clinical protocols around positive results are still evolving and vary between oncologists and institutions.
Factors That Affect Your Result
Several things can influence what your MTM/mL number looks like on any given draw. The timing of the blood draw relative to surgery or treatment matters. Levels are expected to be higher before surgery and should drop rapidly afterward if the tumor was fully removed. A sample drawn too soon after a procedure may still reflect residual DNA from the removed tumor rather than active disease.
The type and location of your cancer also play a role. Some tumors shed more DNA into the bloodstream than others. A cancer in a highly vascular organ may produce higher MTM/mL readings than a small, isolated tumor in a less blood-rich area, even if both represent a similar stage of disease. This is one reason why comparing your numbers to someone else’s results is not particularly useful. Your own trend line is the most reliable guide.