What Causes a False Negative — on a PET Scan?

A negative PET result means no area produced enough metabolic signal to register as abnormal on that scan, on that day, with that tracer. Whether that result can be trusted depends on your cancer type, the size of any suspicious areas, and how the result fits your symptoms and other test findings. Your oncologist will interpret it in that full context. A normal PET does not remove the need for follow-up, and for some cancer types it does not rule out disease that is present but metabolically quiet.

Yes. PET scans use a glucose-based tracer, and elevated blood sugar at the time of the scan means there is more competing glucose in circulation. This can dilute the tracer signal from tumours and increase the chance of a false negative. Before every PET scan you will be asked to fast and your blood sugar will be checked. If your reading is elevated on the day, the scan is typically rescheduled rather than performed with a compromised result. Let your team know if you have diabetes or if your blood sugar is poorly controlled.

A clear PET after treatment is a positive finding and is used in most protocols to assess treatment response. But for some cancers — particularly mucinous or low-grade disease — residual tumour may still be present but metabolically too quiet to show up. Your team will assess the post-treatment PET alongside your clinical examination, tumour markers, and CT findings before drawing a conclusion about response. A clear PET is an encouraging signal; it is not always a confirmation that all disease is gone.

A rising tumour marker with a negative PET is a recognised clinical pattern and should not be dismissed. It often means disease is present but not metabolically active enough to register on standard FDG PET. Your team may recommend a specialist tracer PET, cross-sectional imaging with CT or MRI, or a biopsy of an area that appears suspicious on other imaging. Tell your oncologist about a rising marker result alongside a negative PET — the combination changes what needs to happen next and should be discussed, not assumed to be reassuring.

Yes, for several reasons. Hepatocellular carcinoma can vary significantly in its FDG uptake — well-differentiated tumours often show little or no signal. Liver metastases from mucinous primaries can also return falsely negative results because the tumour cells are dispersed within mucin rather than forming a dense, metabolically active mass. MRI with a liver-specific contrast agent is generally more sensitive for detecting liver lesions and is often used alongside or instead of PET for liver assessment. If the liver is a clinical concern and the PET result does not address it, ask whether MRI liver is indicated.

Yes, that is a reasonable step. If your symptoms, examination findings, or other test results do not fit the PET result, asking for a formal second read of the imaging — or a second clinical opinion — is appropriate. Nuclear medicine reporting can vary between centres, and a radiologist with specific expertise in your cancer type may reach a different conclusion on a borderline case. Your oncologist can arrange a second read, or you can seek an opinion at a centre with a dedicated multidisciplinary team. A scan that does not fit the clinical picture deserves further investigation.