Will an infection show up on a PET scan?

Yes, an infection can light up on a PET scan because the scan detects areas of high metabolic activity, and infections often cause increased glucose uptake by inflammatory cells. However, PET scans cannot always distinguish between infection and other causes of inflammation, such as cancer or autoimmune conditions.

Can a PET scan tell the difference between infection and cancer?

A PET scan alone cannot reliably differentiate between infection and cancer, as both can appear as areas of increased metabolic activity. Additional tests, such as blood work, biopsies, or specialized imaging, are often needed to confirm the cause.

What types of infections show up on a PET scan?

Infections that cause significant inflammation, such as abscesses, osteomyelitis (bone infection), infected prostheses, or soft tissue infections, are most likely to be detected on a PET scan. Chronic or low-grade infections may not always produce enough metabolic activity to be visible.

How accurate is a PET scan for detecting infection?

PET scans are highly sensitive for detecting areas of infection-related inflammation, but their specificity is moderate, meaning they can produce false positives from other inflammatory conditions. The accuracy improves when combined with CT imaging or other clinical findings.

Will a PET scan detect a hidden infection?

A PET scan can help locate hidden infections, especially in cases of fever of unknown origin, by identifying metabolically active areas that may correspond to an abscess or other infection. However, it is not a standalone diagnostic tool and results must be interpreted in context with other medical evaluations.

Will an Infection Light Up on a PET Scan? Key Facts

Yes, an infection can light up on a PET scan. A PET (positron emission tomography) scan detects areas of high metabolic activity. Immune cells fighting an infection consume large amounts of glucose, so they show up as bright “hot spots.” But the scan cannot tell you whether that hot spot is caused by infection, inflammation, or cancer. It only shows that something is metabolically active. That is why PET scans are often combined with CT scans (PET/CT) to give doctors a clearer picture.

How PET Scans Detect Metabolic Activity

A PET scan works by injecting a small amount of radioactive sugar (called FDG) into your bloodstream. Cells that are working hard (cancer cells, immune cells fighting an infection, or cells repairing an injury) take up more of this sugar than resting cells. The radioactive tracer emits signals that a scanner picks up, creating a 3D map of metabolic activity.

The key point is that any process that increases cellular metabolism can produce a hot spot. This includes:

Bacterial or viral infections
Inflammatory conditions (like rheumatoid arthritis or sarcoidosis)
Healing wounds or surgical sites
Cancerous tumors

Because of this overlap, a PET scan alone cannot diagnose an infection. It can only tell your doctor that something is going on in a particular location.

Why FDG Uptake Is Not Specific to Infection

The FDG tracer is a glucose analog. White blood cells (neutrophils, macrophages, and lymphocytes) rely heavily on glucose when they are activated to fight an infection. So a pocket of pus or an abscess will often show intense FDG uptake. But a growing tumor also consumes large amounts of glucose.

This lack of specificity is the main limitation. A hot spot on a PET scan requires follow-up (often with blood tests, cultures, or a biopsy) to determine whether it is an infection, inflammation, or malignancy.

When a PET Scan Is Used for Infection

Despite the limitation, PET/CT scans are useful in specific infection-related scenarios.

Fever of Unknown Origin (FUO)

When a patient has a persistent fever and standard tests (blood work, X-rays, ultrasound) cannot find the cause, a whole-body PET/CT scan can help locate hidden infections. It can reveal infected lymph nodes, abscesses in the abdomen, or infections in bones or joints that other imaging missed.

Suspected Osteomyelitis (Bone Infection)

Bone infections can be hard to detect on regular X-rays or CT scans, especially in the early stages. A PET/CT scan can show increased metabolic activity in the infected bone, often before structural changes appear. It is also useful for distinguishing active infection from healed or chronic bone changes.

Infected Implants or Prosthetics

Joint replacements, metal plates, and other surgical hardware can become infected. PET/CT can help identify infection around the implant, though inflammation from the surgery itself can also cause hot spots. Doctors often combine PET results with blood tests (like C-reactive protein and white blood cell count) to make a diagnosis.

Limitations and False Positives

A PET scan is sensitive (it picks up small areas of activity) but it is not very specific. This means it can produce false positives, where a hot spot looks like an infection but turns out to be something else.

Common Causes of False Positives

Post-surgical inflammation: Healing tissue can remain metabolically active for weeks or months after surgery.
Radiation therapy: Tissues that have been irradiated can show increased FDG uptake for months.
Inflammatory diseases: Conditions like sarcoidosis, tuberculosis, or fungal infections can mimic infection on PET.
Benign tumors: Some non-cancerous growths also take up FDG.

A negative PET scan (no hot spots) is more useful. If a patient has a fever and a whole-body PET/CT shows no abnormal activity, it makes a serious hidden infection much less likely.

How Doctors Reduce False Positives

To improve accuracy, doctors sometimes use dual-time-point imaging. They take images at two different times after the FDG injection. Cancer cells tend to keep accumulating FDG over time, while inflammatory cells may peak and then wash out. This pattern can help distinguish infection from malignancy, though it is not foolproof.

What to Expect During a PET Scan for Infection

If your doctor orders a PET/CT scan to look for a possible infection, here is what typically happens:

Preparation: You fast for 4-6 hours before the scan (only water). This ensures low blood sugar so the FDG tracer is taken up by active cells rather than competing with glucose.
Injection: A small amount of FDG is injected into a vein in your arm.
Wait time: You rest quietly for about 45-60 minutes while the tracer circulates. Talking, reading, or moving can cause muscle uptake that may confuse the results.
Scan: You lie on a table that slides through the PET/CT scanner. The scan itself takes about 20-30 minutes.
Results: A radiologist interprets the images and sends a report to your doctor. Results are usually available within a day or two.

The radiation exposure from a PET/CT scan is roughly equivalent to a few years of natural background radiation. For most people, the benefit of finding a hidden infection outweighs this small risk.

Key Takeaways

A PET scan can show an infection as a hot spot of metabolic activity, but it cannot distinguish infection from cancer or inflammation on its own.
The scan is most useful for finding hidden infections (fever of unknown origin), bone infections, and infected surgical hardware.
False positives are common (any area of healing, inflammation, or benign growth can light up). Follow-up tests are almost always needed.
A negative PET scan is more reliable: no hot spots makes a serious hidden infection unlikely.
PET/CT combines metabolic and anatomical imaging, giving doctors more information than PET alone.
Always consult your doctor to interpret PET scan results in the context of your symptoms, blood work, and other tests.

Will an Infection Light Up on a PET Scan? What to Know