What is the recommended initial dose of Lantus (insulin glargine) and the corresponding carbohydrate ratio and correction scale for a 33-week pregnant patient with a BMI of 27, who has passed the pregnancy glucose tolerance test, received betamethasone (corticosteroid) 12 hours ago, and is currently on intravenous insulin with drip rates of 0.5-2 units per hour?

Transitioning from IV Insulin to Subcutaneous Lantus After Betamethasone in Pregnancy

For this 33-week pregnant CF patient on IV insulin (0.5-2 units/hour) following betamethasone administration, start Lantus at 12-16 units once daily (calculated as 80% of her 24-hour IV insulin requirement), use an insulin-to-carbohydrate ratio of 1:10-1:12, and apply a correction factor of 1 unit per 30-40 mg/dL above 100 mg/dL. 1, 2

Calculating the Lantus Dose

The basal insulin dose should be derived from her current IV insulin infusion rate:

• If averaging 1 unit/hour IV: 24 units total daily → Start Lantus 18-20 units (75-80% of IV total) 1
• If averaging 0.75 units/hour IV: 18 units total daily → Start Lantus 14-16 units 1
• The conversion uses 75-80% of the 24-hour IV requirement because subcutaneous absorption differs from IV delivery 1, 2

Critical timing consideration: Administer the first Lantus dose and continue IV insulin for 2 hours before discontinuing the drip, as Lantus requires time to establish steady-state levels 1

Betamethasone Impact on Insulin Requirements

Betamethasone causes significant hyperglycemia lasting 1-3 days post-administration, with peak effect at 12-48 hours:

• 66 of 83 pregnant women (79%) without diabetes required insulin after betamethasone, averaging 12.25 units/day 3
• Since she received betamethasone 12 hours ago, she is currently at or approaching peak insulin resistance 3, 4
• Her insulin requirements will decrease substantially over the next 24-48 hours as betamethasone effects wane 3

This means her current IV rates likely overestimate her baseline needs—plan for dose reductions within 48 hours 3

Carbohydrate Ratio Determination

For a 74 kg pregnant woman at 33 weeks with demonstrated insulin resistance (requiring IV insulin), start with 1:10 to 1:12 ratio:

• The "500 rule" (500 ÷ total daily insulin dose) provides the starting point 2
• If her total daily insulin is estimated at 40-50 units (accounting for both basal and anticipated bolus needs): 500 ÷ 45 = 1:11 ratio 2
• Third-trimester insulin resistance typically requires more aggressive ratios than non-pregnant states 1

Pregnancy-specific consideration: In the third trimester, insulin requirements increase 5% per week through week 36, so this ratio will need weekly reassessment 1

Correction Scale (Insulin Sensitivity Factor)

Use the "1800 rule" for rapid-acting insulin correction doses:

• 1800 ÷ total daily insulin dose = correction factor 2
• If total daily insulin is 45 units: 1800 ÷ 45 = 1 unit per 40 mg/dL above target 2
• Target glucose for corrections should be 100 mg/dL (not 120 mg/dL) to achieve pregnancy targets 1

Correction scale example:

• Blood glucose 140 mg/dL → Give 1 unit (40 mg/dL above target)
• Blood glucose 180 mg/dL → Give 2 units (80 mg/dL above target)
• Blood glucose 220 mg/dL → Give 3 units (120 mg/dL above target)

Glycemic Targets for Monitoring

She must achieve strict pregnancy targets to prevent fetal complications:

• Fasting: <95 mg/dL (5.3 mmol/L) 1
• 1-hour postprandial: <140 mg/dL (7.8 mmol/L) 1
• 2-hour postprandial: <120 mg/dL (6.7 mmol/L) 1

Monitor fasting and postprandial glucose 4-7 times daily, with more frequent checks during the betamethasone effect period 1, 3

Critical Pitfalls to Avoid

Do not use her current IV insulin rates as a direct conversion without accounting for betamethasone:

• Her current requirements are artificially elevated by corticosteroid-induced insulin resistance 3, 4
• Failure to reduce insulin as betamethasone wears off will cause severe hypoglycemia 3

Do not delay transition from IV to subcutaneous insulin:

• The 2-hour overlap between starting Lantus and stopping IV insulin is essential to prevent hyperglycemic gaps 1
• Starting Lantus without this overlap risks 4-6 hours of inadequate basal coverage 1

Do not use pregnancy insulin doses postpartum:

• After delivery, insulin requirements drop by 50% immediately due to placental hormone removal 5, 2
• This patient will need completely different dosing postpartum 5

Adjustment Strategy Over Next 48-72 Hours

Plan for daily dose reductions as betamethasone effects resolve:

• Reduce Lantus by 10-20% daily if fasting glucose falls below 80 mg/dL 3
• Adjust carbohydrate ratio if postprandial values consistently fall below 100 mg/dL 1
• After betamethasone effects resolve (48-72 hours), reassess total daily insulin needs and recalculate ratios 3

Given her CF diagnosis, coordinate with pulmonary team regarding nutritional needs and potential for malabsorption affecting carbohydrate calculations 2