Laboratory Testing for Type 1.5 Diabetes (LADA)
Test for glutamic acid decarboxylase antibodies (GADA) as the primary screening test, followed by additional islet autoantibodies (IA-2, insulin autoantibodies, and ZnT8) to confirm the diagnosis of latent autoimmune diabetes in adults (LADA). 1
Essential Autoantibody Panel
The core laboratory workup for suspected LADA requires measurement of islet autoantibodies:
GADA (Glutamic Acid Decarboxylase Antibodies): This is the most frequently positive autoantibody in LADA, present in 88% of cases 2. GADA is the single most important test and should be ordered first 3, 4.
IA-2 (Islet Antigen 2 Antibodies): Include this as part of the comprehensive autoantibody panel 1.
Insulin Autoantibodies (IAA): Present in only 8% of LADA cases but helps confirm diagnosis when positive 2.
ZnT8 (Zinc Transporter 8 Antibodies): Part of the complete screening panel recommended by current guidelines 1.
ICA (Islet Cell Antibodies): Found in 20% of LADA patients and provides additional diagnostic support 2.
Critical Supporting Laboratory Tests
Beyond autoantibodies, obtain these tests to assess disease severity and guide treatment:
C-peptide (fasting): Essential for evaluating residual beta-cell function 5. A low-normal C-peptide (even within the reference range) in the presence of hyperglycemia should prompt autoantibody testing 5. C-peptide levels decrease less rapidly in LADA compared to classic type 1 diabetes but will decline over time 6.
HbA1c: Assess glycemic control and disease severity 2.
Fasting plasma glucose and insulin levels: Help evaluate insulin resistance versus insulin deficiency 2.
Clinical Context for Testing
Order this autoantibody panel when adults present with apparent "type 2 diabetes" but have these features:
- Low or normal BMI (particularly BMI <25 kg/m²) 2
- Younger age at diagnosis (especially under 50 years) 1
- Unintentional weight loss 1
- Poor response to oral hypoglycemic agents 5
- Rapid progression to insulin requirement 4
- Absence of metabolic syndrome features (lower triglycerides, higher HDL cholesterol) 6
Important Diagnostic Considerations
Single autoantibody positivity is more common in LADA than in classic type 1 diabetes 6. Do not dismiss the diagnosis if only one autoantibody is positive—GADA alone may be sufficient, especially when clinical features align 2, 3.
The detection rate of LADA among newly diagnosed "type 2 diabetes" patients ranges from 6-50% depending on the population studied, with an overall prevalence around 7% 3, 7. Most LADA patients will require insulin within three years of diagnosis 3.
Common Pitfalls to Avoid
Do not rely solely on C-peptide being "normal": A C-peptide of 1.3 ng/mL (within the 0.8-5.2 ng/mL reference range) may seem reassuring but is actually low-normal in the context of significant hyperglycemia and should trigger autoantibody testing 5.
Do not assume type 2 diabetes based on age alone: The majority (73.7%) of adults presenting after age 30 who subsequently require insulin actually have autoimmune diabetes 4.
Standardized autoantibody assays are recommended for accurate classification 1.
Additional Supportive Testing
Consider these supplementary tests based on the clinical picture:
Thyroid function tests (TSH, TPOAb, TGAb): LADA patients show increased rates of thyroid autoimmunity 2.
Lipid profile: LADA patients typically have lower total cholesterol and triglycerides compared to type 2 diabetes 2, 6.
CD4+/CD8+ T cell ratio: May be decreased in LADA, though this is primarily a research tool rather than routine clinical practice 2.