In an adult patient with elevated C‑peptide, a positive zinc transporter 8 (ZnT8) antibody and a negative glutamic acid decarboxylase 65 (GAD‑65) antibody, does this pattern indicate latent autoimmune diabetes in adults (LADA)?

This Pattern is Suggestive but Not Diagnostic of LADA

Your patient's presentation—elevated C-peptide with positive ZnT8 antibody but negative GAD-65—suggests possible LADA, but the diagnosis is not definitive and requires additional evaluation. The presence of any islet autoantibody (including ZnT8) indicates autoimmune diabetes, but the high C-peptide complicates the picture and demands further investigation 1.

Understanding the Antibody Pattern

The current ADA guidelines emphasize that GAD antibodies should be the primary antibody measured first, followed by IA-2 and/or ZnT8 when GAD is negative 2. Your patient has the reverse pattern—ZnT8 positive with GAD negative—which occurs in a meaningful subset of autoimmune diabetes patients:

• ZnT8 antibodies alone are found in approximately 2-11% of adult-onset autoimmune diabetes patients 3, 4, 5
• Research shows that ZnT8 testing increases LADA diagnostic sensitivity by about 1% when added to GAD and IA-2 testing 5
• ZnT8-positive patients tend to be younger and have higher HbA1c levels 6

The C-Peptide Conundrum

The elevated C-peptide is the critical issue here. According to the 2024 ADA diagnostic algorithm 2:

• C-peptide >600 pmol/L (>1.8 ng/mL) suggests type 2 diabetes
• C-peptide <200 pmol/L (<0.6 ng/mL) confirms type 1 diabetes/LADA
• C-peptide 200-600 pmol/L (0.6-1.8 ng/mL) is indeterminate

Critical caveats about C-peptide interpretation:

• The test must be done with concurrent glucose measurement 2
• If glucose is <70 mg/dL or the patient was fasting, the result may be falsely low
• C-peptide should not be tested within 2 weeks of a hyperglycemic emergency 2
• A "low-normal" C-peptide (like 1.3 ng/mL in one case report) should raise suspicion when autoantibodies are present 7

Diagnostic Algorithm for Your Patient

Step 1: Verify the C-peptide measurement

• Was it done with concurrent glucose? If glucose was >100 mg/dL and C-peptide is truly elevated (>1.8 ng/mL), this argues against classic LADA
• Consider repeating if testing conditions were suboptimal

Step 2: Complete the autoantibody panel

• Test IA-2 antibodies—ZnT8 positivity is more strongly associated with IA-2 positivity than GAD positivity (38.3% vs 10.7%) 5
• Consider insulin autoantibodies (IAA) if not yet on insulin 2

Step 3: Apply the AABBCC clinical framework 1:

• Age: Is patient <35 years? (favors type 1/LADA)
• Autoimmunity: Personal/family history of autoimmune disease?
• Body habitus: BMI <25 kg/m²? (favors type 1/LADA)
• Background: Family history of type 1 diabetes?
• Control: Inability to achieve glycemic goals on non-insulin therapies?
• Comorbidities: Any relevant exposures (e.g., checkpoint inhibitors)?

Most Likely Scenarios

If C-peptide is truly elevated (>1.8 ng/mL) with only ZnT8 positive:

• This represents early-stage LADA where significant beta-cell function remains
• The autoimmune process is present but not yet advanced
• These patients typically progress more slowly than those with multiple antibodies 6

If C-peptide is in the indeterminate range (0.6-1.8 ng/mL):

• Strongly consider LADA diagnosis given the positive autoantibody 7
• The presence of autoimmunity should take precedence over borderline C-peptide
• Plan for repeat C-peptide testing after >3 years if initially indeterminate 2

Treatment Implications

The presence of any islet autoantibody fundamentally changes management:

• Patients with autoantibodies have worse glycemic control regardless of which specific antibody is positive 4
• Those with triple antibody positivity show the most severe phenotype (lowest C-peptide, highest glucose/HbA1c) 4, 6
• Earlier insulin initiation should be strongly considered even with preserved C-peptide, as these patients will progress to insulin dependence 7

Common Pitfalls to Avoid

1. Don't dismiss autoimmunity based on "normal" C-peptide alone—the combination of any positive autoantibody with clinical features warrants LADA consideration 7

2. Don't assume GAD-negative means no autoimmune diabetes—5-10% of type 1 diabetes patients are antibody-negative, and ZnT8 can be the sole positive marker 2, 3

3. Don't delay appropriate treatment—recognizing LADA early prevents complications and prompts appropriate pharmacotherapy 7

Your patient likely has LADA with preserved beta-cell function, representing an earlier stage of the autoimmune process. Complete the antibody panel, verify C-peptide with proper glucose measurement, and prepare for eventual insulin therapy even if current C-peptide is elevated.