What Three Tests Are Included in a Standard Diabetes Screening?

When evaluating for diabetes or prediabetes, clinicians routinely rely on three cornerstone laboratory tests—each with distinct physiological relevance, diagnostic thresholds, and clinical applications. These are fasting plasma glucose (FPG), hemoglobin A1c (HbA1c), and the 2-hour plasma glucose value following a 75-gram oral glucose tolerance test (OGTT).

Fasting plasma glucose measures circulating blood glucose after an overnight fast of at least eight hours. A level of ≥7.0 mmol/L (126 mg/dL) on two separate occasions confirms a diagnosis of diabetes; values between 6.1–6.9 mmol/L (110–125 mg/dL) indicate impaired fasting glucose—a prediabetic state.

Hemoglobin A1c reflects average glycemia over the preceding two to three months by quantifying the percentage of hemoglobin molecules glycated at the N-terminal valine. An HbA1c ≥6.5% is diagnostic for diabetes when performed in a certified laboratory using NGSP- and IFCC-aligned methodology; values from 5.7% to 6.4% signify increased risk for progression to diabetes.

The oral glucose tolerance test remains the most sensitive method for detecting early dysglycemia, particularly in high-risk populations such as women with prior gestational diabetes or individuals with polycystic ovary syndrome. After fasting, the patient ingests 75 g of anhydrous glucose dissolved in water; venous plasma glucose is measured precisely two hours later. A 2-hour value ≥11.1 mmol/L (200 mg/dL) meets diagnostic criteria for diabetes; values between 7.8–11.0 mmol/L (140–199 mg/dL) define impaired glucose tolerance.

Importantly, discordance among these tests occurs in up to 20% of cases—underscoring the need for repeat testing, clinical correlation, and consideration of confounding factors such as anemia, chronic kidney disease, or recent acute illness before final diagnosis.