Is a high fasting blood glucose level (>200 mg/dL) a contraindication for an 18F‑FDG PET scan?

High Fasting Blood Glucose and FDG PET/CT Scanning

A high fasting blood glucose level above 200 mg/dL (11 mmol/L) is NOT an absolute contraindication for FDG PET/CT scanning in clinical practice, though the scan should ideally be rescheduled when possible. 1

Blood Glucose Thresholds for Clinical Decision-Making

For Clinical (Non-Research) Scans

• If blood glucose is <200 mg/dL (11 mmol/L): Proceed with the scan 1, 2
• If blood glucose is ≥200 mg/dL (11 mmol/L): Reschedule the scan or proceed based on clinical urgency 1, 2
• Hyperglycemia should not represent an absolute contraindication, particularly in patients with unstable diabetes, infection, or urgent clinical indications, as fasting hyperglycemia does not completely eliminate the clinical value of FDG PET 1

For Research Studies

• The threshold is much stricter at 126-150 mg/dL (7-8.3 mmol/L), and patients exceeding this are typically excluded from protocols 1, 2

Management Algorithm for Elevated Blood Glucose on Scan Day

Step 1: Check Glucose Upon Arrival

• Measure blood glucose when the patient arrives at the imaging center to identify problems early 1, 2, 3
• Diabetic patients should arrive earlier than usual to allow time for correction if needed 1

Step 2: If Glucose is 200-250 mg/dL

• Have the patient hydrate and ambulate, then recheck glucose periodically until an acceptable level is achieved 1
• This non-pharmacologic approach can lower glucose in patients just above the threshold 1

Step 3: If Glucose Remains ≥200 mg/dL After Conservative Measures

• Consider rescheduling unless there is clinical urgency 1, 2
• If the scan must proceed due to urgent clinical circumstances (e.g., infection, unstable patient), document the glucose level and proceed with imaging 1

Step 4: Insulin Administration (Use With Extreme Caution)

• Insulin should NOT be given to reduce glucose levels unless there is at least a 4-hour interval between insulin administration and FDG injection, as insulin causes excessive muscle uptake of FDG and degrades image quality 1, 4
• If insulin must be used, administer rapid-acting insulin subcutaneously (peaks at 60 minutes, effective for 2-4 hours), then wait at least 4 hours before FDG injection 1, 4
• Avoid regular/short-acting, intermediate-acting, or long-acting insulin on the day of scanning 1
• Recent evidence suggests intravenous insulin protocols may be feasible, with one study showing successful imaging 90 minutes post-insulin administration in patients with glucose >160 mg/dL 5, though this contradicts established guidelines and should be used only in specialized centers with experience

Evidence Supporting Scanning Despite Hyperglycemia

Image Quality Considerations

• A retrospective study of 116 hyperglycemic patients (>200 mg/dL) found no significant difference in muscle-to-liver ratio, muscle SUV, or frequency of positive PET findings compared to normoglycemic patients 6
• Hyperglycemic patients showed only ~10% higher liver SUV, which is clinically negligible 6
• The study concluded that hyperglycemia in a fasting state does not negatively influence diagnostic quality 6

Differential Effects on Malignant vs. Inflammatory Lesions

• FDG uptake by malignant lesions is slightly negatively affected by elevated glucose (r=-0.21) 7
• Interestingly, FDG uptake by inflammatory lesions is positively associated with serum glucose (r=0.43), suggesting hyperglycemia may not impair detection of inflammatory processes 7

Critical Pitfalls to Avoid

Never Proceed If Glucose is Too Low

• Do not inject FDG if glucose is <70 mg/dL (4 mmol/L), as this compromises both patient safety and scan quality 1, 2, 3
• Treat hypoglycemia with 15-20 grams of oral glucose and recheck every 15 minutes until ≥70 mg/dL 3

Insulin Timing Errors

• The most common error is giving insulin too close to FDG injection, which causes marked muscle uptake and renders the scan non-diagnostic 1, 4
• If insulin was given within 4 hours, reschedule the scan 1

Documentation Requirements

• Always record the blood glucose level in the patient record 1, 2
• Document whether SUV values are glucose-corrected, though this is not standard practice in most clinical centers 1

Practical Implementation Success

A large retrospective study of 13,063 patients showed that implementing proper preparation protocols reduced rescheduling rates from 9% to 2.2%, with only 0.1% of patients ultimately unable to undergo scanning due to persistently elevated glucose 8. This demonstrates that with appropriate patient preparation and screening, very few patients need to be denied PET/CT imaging due to hyperglycemia 8.

Another study of 200 patients found only 1.5% could not proceed with scanning at first attendance due to glucose ≥200 mg/dL, and all were successfully scanned within one week 9.