What are the blood glucose requirements for a Positron Emission Tomography (PET) scan?

Blood Glucose Requirements for PET Scan

Blood glucose must be below 200 mg/dL (11 mmol/L) for clinical FDG PET scans, and the scan should be rescheduled if glucose exceeds this threshold. 1, 2

Acceptable Blood Glucose Ranges

The upper limit for proceeding with a clinical PET scan is clearly defined:

• Clinical scans: Blood glucose must be <200 mg/dL (<11 mmol/L) 1, 2
• Optimal target range: 70-126 mg/dL (4-7 mmol/L), representing normal fasting glucose levels 2
• Research studies: More stringent upper limit of 126-150 mg/dL (7-8.3 mmol/L) 2

The European Association of Nuclear Medicine establishes 200 mg/dL as the clinical cutoff, above which competitive inhibition between glucose and FDG significantly degrades image quality. 1, 3

Pre-Scan Glucose Monitoring Protocol

Check blood glucose twice: upon patient arrival and immediately before FDG injection. 2, 4

• Use a calibrated glucometer for screening purposes 4
• Early arrival is recommended for diabetic patients to allow time for glucose correction if needed 2
• Document all glucose values in the patient record 2

This two-point checking system identifies problems early and avoids wasting the patient's time if glucose is unacceptable. 4

Patient Preparation Requirements

Patients must fast for a minimum of 4 hours before FDG injection. 1, 2

The physiologic rationale is critical to understand:

• Fasting ensures low blood glucose and low insulin levels 2
• Insulin directly increases glucose uptake by non-tumor cells, degrading image quality 2
• Glucose-containing IV fluids must not be used within 4 hours of FDG injection 2

Management When Glucose is Too High (≥200 mg/dL)

Reschedule the scan when glucose is ≥200 mg/dL, except in urgent clinical situations. 1, 2

Practical management approach:

• Most patients can be successfully rescheduled and scanned within one week 5
• In urgent clinical situations, hyperglycemia is not an absolute contraindication, though image quality will be compromised 2
• For glucose levels between 201-300 mg/dL, empagliflozin (20 mg oral) can safely reduce glucose to acceptable levels within 2-4 hours without causing hypoglycemia 3

Critical pitfall: Do not administer insulin immediately before FDG injection, as this causes excessive muscle uptake and severely degrades image quality. 2, 4 If insulin is necessary, it must be given at least 4-6 hours before the study. 4, 6

Management When Glucose is Too Low (<70 mg/dL)

Do not proceed with FDG injection when glucose is <70 mg/dL (<4 mmol/L). 2, 4

Immediate treatment protocol:

• Administer 15-20 grams of oral glucose (dextrose tablets preferred) 4
• Recheck glucose every 15 minutes until ≥70 mg/dL 4
• Delay FDG injection until glucose normalizes 4
• Alternatives if dextrose tablets unavailable: 6-8 oz juice, 1 tablespoon sugar or honey, or 15-25 jellybeans 4

Hypoglycemia compromises both patient safety and scan quality, making this an absolute contraindication to proceeding. 2, 4

Special Considerations for Diabetic Patients

Type 1 diabetes: Continue prescribed daily insulin with meals, but avoid short-acting insulin 4-6 hours before the study. 4

Type 2 diabetes: Hold oral hypoglycemic medications during the fasting period and avoid insulin 4-6 hours before the study. 4

In practice, only 1.5% of patients cannot proceed with scanning at first attendance due to unacceptable glucose levels, and nearly all are successfully scanned within one week. 5 Interestingly, non-diabetic patients virtually never present with glucose >150 mg/dL, suggesting routine glucose checking may be limited to known diabetic patients. 7

Documentation Requirements

Always record in the patient record: 2

• Blood glucose level at time of scan
• Whether SUV values are glucose-corrected
• Use validated laboratory methods for serial scans to ensure consistency