If you or someone you know has been evaluated for Alzheimer’s disease, you may have heard of an amyloid PET scan. These scans show the amount of amyloid plaque buildup in the brain. But different medical centers use different machines and methods to read these scans. The Centiloid Scale is a standard measurement tool that allows doctors and researchers to compare amyloid PET scan results from any machine, at any hospital, anywhere in the world. It takes the raw data from a scan and turns it into a single number on a 0 to 100 scale. A score of zero means no amyloid buildup is detected. A score of 100 represents the level typically seen in a person with Alzheimer’s disease.
Why Was the Centiloid Scale Created?
Before the Centiloid Scale existed, comparing amyloid PET scans was difficult. Each type of PET scanner and each brand of radioactive tracer produced slightly different images. A “positive” result at one clinic might look different at another. This made it hard for researchers to combine data from multiple studies or track a patient’s disease progression over time if they moved to a different city.
The Centiloid Scale was developed by a global team of researchers known as the Centiloid Project. Their goal was to create a universal ruler for amyloid. The scale was first published in 2015 in the Journal of Nuclear Medicine. Since then, it has become the standard way to report amyloid PET results in clinical trials. The scale works by calibrating all scans to a common reference point. This reference point comes from a group of young healthy controls and a group of people with confirmed Alzheimer’s disease. Once a scan is calibrated, the result is a Centiloid value that means the same thing regardless of where the scan was performed.
How Does the Centiloid Scale Work?
The process starts with a standard amyloid PET scan. The patient receives an injection of a radioactive tracer that binds to amyloid plaques in the brain. After about an hour, a scanner takes images of the brain. The raw data from this scan is a set of numbers representing tracer activity in different brain regions.
To convert this into a Centiloid value, the raw data goes through a specific calculation. First, the tracer activity in a brain region known to accumulate amyloid is measured. This is typically the cortex. Then, the activity in a reference region that does not accumulate amyloid is measured. The cerebellum is often used for this. The ratio between these two measurements is calculated. This ratio is then transformed using a standard formula that references the global dataset of healthy controls and Alzheimer’s patients. The final result is a single number on the Centiloid scale.
The table below shows how Centiloid values are generally interpreted in clinical research settings.
| Centiloid Value | Interpretation |
|---|---|
| 0 – 10 | Negative scan. No significant amyloid buildup. Typical of a young healthy adult. |
| 10 – 20 | Borderline. Some amyloid may be present but below the threshold typically associated with Alzheimer’s pathology. |
| 20 – 40 | Low positive. Amyloid accumulation is detected. This range is often seen in early stages of amyloid buildup. |
| 40 – 60 | Moderate positive. Significant amyloid plaque burden is present. This level is commonly associated with mild cognitive impairment due to Alzheimer’s. |
| 60 – 100 | High positive. The level of amyloid is similar to that seen in a typical Alzheimer’s disease dementia patient. |
What Are the Benefits of Using the Centiloid Scale?
The biggest benefit is standardization. Because the Centiloid Scale is universal, a result from a study in Japan can be directly compared to a result from a study in the United States. This allows researchers to pool data from many smaller studies into larger analyses. Larger datasets give more reliable answers about how amyloid buildup relates to memory loss and other symptoms.
Another benefit is tracking changes over time. A patient can have a scan today and another scan in two years. If both scans are converted to Centiloid values, the doctor can see exactly how much the amyloid burden has increased. This is far more precise than simply saying the scan is “positive” or “negative.” The scale also helps in clinical trials for new Alzheimer’s drugs. Researchers can use the Centiloid value to select patients who have enough amyloid to be likely to benefit from treatment. They can also use the scale to measure whether a drug is actually reducing amyloid plaques in the brain.
Research published in the journal Alzheimer’s & Dementia has shown that Centiloid values correlate well with the amount of amyloid found in the brain at autopsy. This means the scale is not just a number. It reflects real biological changes in the brain.
What Are the Limitations of the Centiloid Scale?
The Centiloid Scale is a powerful tool, but it has limits. First, it only measures amyloid plaques. Alzheimer’s disease is complex. It also involves tau tangles, inflammation, and brain cell death. A person can have a high Centiloid score but no memory problems. This is called being “asymptomatic.” The scale tells you about amyloid burden, but it does not tell you about symptoms or how fast the disease will progress.
Second, the scale works best for the most common tracers used in PET imaging. Some newer tracers may need additional calibration to fit the scale properly. The calculation also assumes the reference regions in the brain are free of amyloid. In some older adults, the cerebellum can accumulate small amounts of amyloid, which could slightly skew the results.
Third, Centiloid values are not the same as a diagnosis. A high score does not mean a person definitely has Alzheimer’s disease. It means they have amyloid buildup. Some people with high amyloid never develop dementia. The scale is one piece of information that a doctor uses alongside cognitive tests, medical history, and other imaging results.
How Is the Centiloid Scale Used in Clinical Practice?
As of 2026, the Centiloid Scale is used more in research than in routine clinical care. Many memory clinics still report amyloid PET results as simply “positive” or “negative.” However, this is changing. The Alzheimer’s Association and other groups have published guidelines encouraging the use of Centiloid values in clinical practice. Some major medical centers now include the Centiloid number in their standard PET scan reports.
For a patient, this means the doctor can be more precise. Instead of saying “your scan shows amyloid,” the doctor can say “your Centiloid value is 75, which is high and consistent with Alzheimer’s disease pathology.” This precision can help with treatment decisions. For example, the FDA-approved drug lecanemab (Leqembi) is indicated for patients with mild cognitive impairment or mild dementia who have confirmed amyloid pathology. A Centiloid value above a certain threshold can help confirm eligibility for such treatments.
The scale is also used to monitor patients in clinical practice. If a patient starts a treatment aimed at removing amyloid, follow-up scans can show whether the Centiloid value is decreasing. Some studies have shown that effective treatments can reduce Centiloid scores by 20 to 30 points over 18 months. This gives doctors and patients a concrete way to see if a treatment is working.
Common Misconceptions About the Centiloid Scale
A common myth is that a Centiloid score of zero means a person will never get Alzheimer’s disease. This is not true. A score of zero simply means no amyloid was detected at the time of the scan. Amyloid buildup can begin years or even decades before symptoms appear. A person with a zero score today could have a positive score five years from now.
Another misconception is that the scale measures disease severity. It does not. A person with a Centiloid score of 80 may have no memory problems, while a person with a score of 40 may have significant dementia. The relationship between amyloid burden and symptoms is not linear. Other factors like brain reserve, tau protein tangles, and overall brain health play a major role in whether a person develops symptoms.
Some people also believe that the Centiloid Scale is a direct measure of Alzheimer’s disease itself. It is not. It is a measure of one specific pathology associated with Alzheimer’s. Many older adults have mixed pathologies in their brains. They may have amyloid plaques alongside vascular damage or other protein deposits. The Centiloid Scale does not capture these other problems.
What to Avoid When Interpreting Centiloid Results
Avoid treating the Centiloid number as a definitive diagnosis. It is a biomarker, not a standalone test. Doctors should never make a diagnosis based on the Centiloid value alone. The value must be considered alongside the patient’s cognitive testing, medical history, and other clinical findings.
Avoid using the scale to compare different types of dementia. The Centiloid Scale is specifically calibrated for Alzheimer’s disease pathology. Other dementias, like frontotemporal dementia or Lewy body dementia, involve different proteins. A negative Centiloid scan does not rule out these other conditions.
Avoid assuming that a decreasing Centiloid value means a cure. Some treatments can remove amyloid plaques, and this will show as a drop in the Centiloid score. However, clinical trials have shown that removing amyloid does not always stop cognitive decline. The brain damage that has already occurred may not be reversible. A falling Centiloid number is a good sign, but it is not the same as getting better.
Frequently Asked Questions
What does a Centiloid score of 30 mean?
A score of 30 is considered a low positive result. It indicates that amyloid buildup is present but at a level lower than what is typically seen in Alzheimer’s dementia.
Is the Centiloid scale used for diagnosis?
It is used to support a diagnosis but is not a standalone diagnostic test. Doctors combine it with cognitive tests and medical history to make a clinical judgment.
Can the Centiloid scale track disease progression?
Yes. Serial scans over time can show whether the Centiloid value is increasing, which reflects the accumulation of amyloid plaques in the brain.
Does a negative Centiloid scan rule out Alzheimer’s?
It makes Alzheimer’s disease unlikely as the cause of symptoms, but it does not rule out other types of dementia or mixed pathology.
