Most Likely Diagnosis: Type 2 Diabetes Mellitus with Diabetic Ketoacidosis
Based on the negative autoantibody panel (IA-2, insulin autoantibody, islet cell antibody, and GAD65 all negative) combined with normal C-peptide, this patient most likely has Type 2 diabetes mellitus that presented with diabetic ketoacidosis, not latent autoimmune diabetes (LADA) or classic Type 1 diabetes.
Diagnostic Reasoning
The antibody profile is critical here. While a positive IA-2 antibody would suggest autoimmune diabetes 1, this patient has all negative antibodies with preserved beta-cell function (normal C-peptide). This pattern strongly indicates ketosis-prone Type 2 diabetes rather than autoimmune diabetes 2.
Key Distinguishing Features:
- Negative autoantibodies: Rules out LADA and Type 1 diabetes 3
- Normal C-peptide: Indicates preserved insulin secretion capacity 2
- Age 45 years: Compatible with either diagnosis, but combined with other features points to Type 2
- DKA presentation: Can occur in Type 2 diabetes, particularly in certain populations 2
Important caveat: In one South African cohort, 47% of adults presenting with first-episode DKA were antibody-negative with preserved C-peptide, representing ketosis-prone Type 2 diabetes rather than Type 1 2. This phenotype is increasingly recognized globally.
Initiating Basal-Bolus Subcutaneous Insulin
Transition from DKA Protocol
Once the patient is stable (anion gap normalized, acidosis resolved, hemodynamically stable, tolerating oral intake), transition to subcutaneous insulin 4.
Calculate total daily dose (TDD) from IV insulin infusion:
- Average the insulin infusion rate over the last 12 hours before transition
- Multiply by 24 to get estimated TDD
- Example: 1.5 units/hour × 24 = 36 units/day 4
Basal-Bolus Regimen Structure
For insulin-naive patients or those with low prior insulin exposure 4, 5:
Total Daily Dose: 0.3-0.5 units/kg body weight 4
- Use 0.3 units/kg for patients at higher hypoglycemia risk (older age, renal impairment, poor oral intake)
- Use 0.5 units/kg for younger, otherwise healthy patients
Distribution 4:
- 50% as basal insulin: Long-acting analog (glargine U100/U300, degludec, or detemir) given once or twice daily
- 50% as bolus insulin: Rapid-acting analog (lispro, aspart, or glulisine) divided equally before three meals (approximately 17% of TDD per meal)
Example for 80 kg patient:
- TDD = 0.4 units/kg × 80 kg = 32 units/day
- Basal: 16 units once daily (glargine or degludec)
- Bolus: 5-6 units before each meal
- Add correctional insulin scale for glucose >180 mg/dL
Insulin Selection
Preferred basal insulins 5:
- Glargine U300 or degludec (lower hypoglycemia risk than glargine U100)
- Detemir (alternative)
- Avoid NPH insulin due to higher hypoglycemia risk 5
Preferred bolus insulins 5:
- Rapid-acting analogs (lispro, aspart, glulisine) over regular human insulin
- More predictable onset/offset, lower hypoglycemia risk
Critical Management Points
Do NOT use sliding scale insulin alone 4. This approach is condemned in guidelines and associated with poor glycemic control. Always use basal-bolus regimen with correctional insulin added to scheduled doses.
Monitor for hypoglycemia: The basal-bolus approach carries 12-30% risk of mild hypoglycemia in controlled settings 4. For this patient with preserved C-peptide and Type 2 diabetes, insulin requirements may decrease substantially after acute illness resolves 2.
Reassess insulin needs: Given the normal C-peptide and negative antibodies, this patient may achieve insulin independence or require minimal insulin after metabolic stabilization 2. In similar phenotypes, 68% achieved insulin independence at 12 months 2. Consider systematic insulin weaning once HbA1c normalizes and oral intake is stable.
Avoid premixed insulins in the hospital setting due to unacceptably high hypoglycemia rates 4.
Monitoring Requirements
- Glucose checks before meals and bedtime (minimum 4 times daily) 6
- Adjust doses by 10-20% every 2-3 days based on patterns
- Target pre-meal glucose <140 mg/dL and random glucose <180 mg/dL 4
- Consider continuous glucose monitoring if available 4
Future consideration: Once stable, evaluate for transition to oral agents ± GLP-1 receptor agonist given the Type 2 diabetes phenotype and preserved beta-cell function 5, 7.