Management of Hyperglycemia During Fasting for Laboratory Tests
For hospitalized patients with diabetes who must fast for laboratory tests, use a basal-plus insulin approach consisting of basal insulin (0.1-0.25 U/kg per day) with correctional insulin every 4-6 hours, while holding all prandial insulin doses during the fasting period 1.
Algorithmic Approach Based on Patient Status
For Patients Already on Insulin Therapy
- Continue basal insulin at the usual dose or slightly reduced (80% of home dose if on high doses ≥0.6 U/kg/day to prevent hypoglycemia) 1
- Hold all prandial/mealtime insulin during the fasting period
- Administer correctional insulin every 4-6 hours based on point-of-care blood glucose monitoring 1, 2
- Target glucose levels below 10.0 mmol/L (180 mg/dL) 1
For Patients on Oral Antidiabetic Medications Only
- Use sliding scale insulin alone (correctional insulin only) if the patient has good baseline metabolic control 1
- Add basal insulin if unable to maintain glucose below 10.0 mmol/L (180 mg/dL) with correctional insulin alone 1
- Hold metformin if the fasting period involves procedures with contrast or if there are concerns about reduced oral intake that could increase lactic acidosis risk 1
For Patients Without Known Diabetes
- Use correctional insulin alone for glucose elevations during the fasting period 1
- Avoid starting basal insulin unless hyperglycemia persists beyond the fasting period
Key Monitoring Parameters
- Check blood glucose every 4-6 hours during the fasting period 1, 2
- In high-risk patients (age ≥65, BMI ≤27 kg/m², insulin dose ≥0.6 units/kg, chronic kidney disease stage ≥3), consider real-time continuous glucose monitoring with confirmatory point-of-care testing if resources available 2
Critical Pitfalls to Avoid
Do NOT use premixed insulin (70/30) during fasting periods—this carries an unacceptably high hypoglycemia risk 1. The basal-plus approach significantly reduces hypoglycemia risk compared to traditional sliding scale-only regimens while maintaining adequate glycemic control in fasting patients.
Do NOT continue sulfonylureas or other insulin secretagogues during prolonged fasting—these medications stimulate insulin release regardless of food intake and dramatically increase hypoglycemia risk.
Do NOT use basal-bolus insulin (scheduled prandial insulin) in fasting patients—this is the regimen for patients with normal oral intake and carries 4-6 times higher hypoglycemia risk than appropriate fasting regimens 1.
Rationale for This Approach
The basal-plus strategy addresses the physiological reality that hospitalized fasting patients still have hepatic glucose output requiring basal insulin coverage 3, while avoiding the hypoglycemia risk of prandial insulin when no carbohydrate intake occurs. The 2021 Lancet guideline explicitly recommends this approach for "patients with diabetes that are fasting or are expected to undergo procedures" 1, and the 2022 Endocrine Society guideline supports correctional insulin administration at 4-6 hour intervals in nil per os patients 2.