From the Guidelines
Diagnosing type 1 diabetes requires a combination of clinical evaluation and blood tests, including A1C, fasting plasma glucose (FPG), 2-h plasma glucose (2-h PG) during a 75-g oral glucose tolerance test (OGTT), or random glucose value accompanied by classic hyperglycemic symptoms/crises criteria, as recommended by the American Diabetes Association in 2024 1. To diagnose type 1 diabetes, the following steps can be taken:
- Measure A1C levels, with results of 6.5% or higher indicating diabetes 1
- Measure fasting plasma glucose (FPG) levels, with values above 126 mg/dL (7.0 mmol/L) on two separate occasions suggesting diabetes 1
- Perform a 75-g oral glucose tolerance test (OGTT), with 2-h plasma glucose (2-h PG) values above 200 mg/dL (11.1 mmol/L) indicating diabetes 1
- Measure random glucose levels, with values above 200 mg/dL (11.1 mmol/L) accompanied by classic hyperglycemic symptoms such as polyuria, polydipsia, and unexplained weight loss, indicating diabetes 1 In the absence of unequivocal hyperglycemia, confirmatory testing is required, as stated in the 2024 standards of care in diabetes 1. It is essential to note that the detection rates of different screening tests vary in both populations and individuals, and FPG, 2-h PG, and A1C reflect different aspects of glucose metabolism, identifying different groups 1. The diagnosis of type 1 diabetes should be based on the most recent and highest-quality evidence, which is the 2024 standards of care in diabetes from the American Diabetes Association 1.
From the Research
Diagnosing Type 1 Diabetes
To diagnose type 1 diabetes, several methods can be employed, including:
- Oral Glucose Tolerance Tests (OGTTs) to assess the body's ability to regulate blood sugar levels after consuming a sugary drink
- Analysis of alternative OGTT indexes, such as the 60- and 90-min glucose levels, which have been shown to be highly predictive of type 1 diabetes 2
- Use of a metabolic risk score, such as the Diabetes Prevention Trial-Type 1 risk score (DPTRS), which can accurately predict type 1 diabetes 3
- Measurement of first-phase insulin response (FPIR) during OGTTs, which can serve as an alternative to intravenous glucose tolerance testing (IVGTT) in predicting type 1 diabetes 4
Oral Glucose Tolerance Tests
OGTTs can be used to predict type 1 diabetes in individuals with normal glucose tolerance, with certain alternative OGTT indexes appearing to be more predictive than standard indexes 2. A 1-hour OGTT can predict type 1 diabetes as accurately as a 2-hour OGTT, with minimal risk of missing a diagnosis before the next visit 3. OGTT measures, such as Index60 and C-peptide index, have been shown to correlate with FPIR and have high predictive accuracy for type 1 diabetes 4.
Predictive Ability
The predictive ability of OGTTs for type 1 diabetes has been demonstrated in several studies, with areas under receiver operating curves (ROCAUCs) ranging from 0.73 to 0.847 2, 3, 4. The use of OGTTs in diagnosing type 1 diabetes is supported by the evidence, which suggests that subtle metabolic abnormalities are present several years before diagnosis 2.