Guidelines for diabetes screening can vary slightly between organizations (like the American Diabetes Association (ADA) and the U.S. Preventive Services Task Force (USPSTF)), but generally include:
•    Age-Based Screening: 

• All adults generally starting at age 35, and then every 3 years if results are normal. Some guidelines previously recommended age 45, but this has been lowered.

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•    Individuals with Risk Factors (regardless of age): 

• Overweight or obesity (BMI ≥ 25 kg/m$^2$, or ≥ 23 kg/m$^2$ for Asian Americans).

• First-degree relative with diabetes (parent, sibling).

• High-risk race/ethnicity: African American, Latino, Native American, Asian American, Pacific Islander.

• History of cardiovascular disease.

• High blood pressure (≥ 130/80 mmHg or on medication for hypertension).

• Abnormal cholesterol levels: HDL cholesterol < 35 mg/dL (0.9 mmol/L) and/or triglyceride level > 250 mg/dL (2.8 mmol/L).

• Women who had gestational diabetes (diabetes during pregnancy). These individuals should be screened lifelong at least every 3 years.

• Women with Polycystic Ovary Syndrome (PCOS).

• Physical inactivity.

• Other clinical conditions associated with insulin resistance (e.g., severe obesity, acanthosis nigricans - dark, velvety patches of skin).

• History of prediabetes (A1C 5.7%-6.4%, impaired fasting glucose, or impaired glucose tolerance) - these individuals should be tested yearly.

• HIV infection.

Common Tests Used for Diabetes Screening and Diagnosis:
These tests measure blood glucose (sugar) levels and can diagnose prediabetes or diabetes. For a diagnosis of diabetes, an abnormal result usually needs to be confirmed with a repeat test on a different day, unless there are clear symptoms of high blood sugar.
1.    A1C Test (Glycated Hemoglobin Test):

• What it measures: Your average blood glucose level over the past 2 to 3 months. It measures the percentage of hemoglobin (a protein in red blood cells) that is coated with sugar.

• Preparation: No fasting required.

• Results: 

          Normal: Less than 5.7%
         Prediabetes: 5.7% to 6.4%
         Diabetes: 6.5% or higher
         Advantages: Convenient, no fasting, less day-to-day variability.

2.    Fasting Plasma Glucose (FPG) Test:

• What it measures: Your blood glucose level at a single point in time after a period of fasting.

• Preparation: You must fast for at least 8 hours (no food or drink except water) before the test. It's usually done in the morning.

• Results: 

        Normal: Less than 100 mg/dL (5.6 mmol/L)
        Prediabetes (Impaired Fasting Glucose - IFG): 100 mg/dL to 125 mg/dL (5.6 to 6.9 mmol/L)
        Diabetes: 126 mg/dL (7.0 mmol/L) or higher

3.    Oral Glucose Tolerance Test (OGTT):

• What it measures: How your body processes sugar after a glucose load.

• Preparation: You fast overnight. Your fasting blood glucose is measured, then you drink a sugary liquid (usually 75 grams of glucose). Blood glucose levels are then tested at specific intervals, typically 2 hours later.

• Results (2 hours after drinking the glucose solution): 

       Normal: Less than 140 mg/dL (7.8 mmol/L)
       Prediabetes (Impaired Glucose Tolerance - IGT): 140 mg/dL to 199 mg/dL (7.8 to 11.0 mmol/L)
       Diabetes: 200 mg/dL (11.1 mmol/L) or higher
       When used: Often used to diagnose gestational diabetes in pregnant women, or when other tests are inconclusive.

4.    Random (or Casual) Plasma Glucose Test:

• What it measures: Your blood glucose level at any time of day, without regard to when you last ate.

• When used: Primarily used if you have classic symptoms of diabetes (increased thirst, frequent urination, unexplained weight loss, blurred vision) and a quick diagnosis is needed.

• Results: 

       Diabetes: 200 mg/dL (11.1 mmol/L) or higher, along with symptoms of diabetes.

Gestational Diabetes Screening:
•    All pregnant individuals without a previous diagnosis of diabetes should be screened for gestational diabetes between 24 and 28 weeks of pregnancy.
•    Those with high risk factors (e.g., marked obesity, history of GDM in a previous pregnancy, strong family history) may be screened earlier in pregnancy.
•    Tests typically involve a glucose challenge test (drinking a sugary solution and then a blood test 1 hour later) followed by a 3-hour oral glucose tolerance test if the 1-hour test is positive.