Should You Check a Fasting Plasma Glucose as the Next Step?
No, you do not need to check a fasting plasma glucose (FPG) as the next step—an HbA1c of 6.3% already confirms the diagnosis of prediabetes and requires no additional testing for diagnostic purposes. 1
Why Additional Testing Is Not Required
A single HbA1c value between 5.7% and 6.4% definitively establishes prediabetes according to the American Diabetes Association's 2023 diagnostic criteria, and your patient's HbA1c of 6.3% falls squarely within this range. 1
The ADA guidelines specify that confirmatory testing with a second test is only required when diagnosing diabetes (not prediabetes), and only when there is diagnostic uncertainty, discordant results between different test types, or values near diagnostic margins. 1, 2
For prediabetes specifically, one abnormal test result is sufficient to establish the diagnosis and initiate appropriate interventions. 1
When You Would Consider FPG Testing
While not needed for diagnosis, there are specific clinical scenarios where obtaining an FPG might still be valuable:
Situations Where FPG Adds Clinical Value:
If you suspect HbA1c unreliability: Check FPG when conditions affecting red blood cell turnover are present, including hemoglobin variants, hemolytic anemia, iron deficiency anemia, recent blood loss or transfusion, chronic kidney disease, or pregnancy—these conditions can falsely lower or raise HbA1c independent of glycemic control. 1, 3, 2
For risk stratification within prediabetes: Patients with both elevated HbA1c (5.7-6.4%) and elevated FPG (100-125 mg/dL) have substantially higher diabetes progression risk compared to those with only one abnormal test, with 5-year progression rates approaching 25-50% when both are elevated. 1, 4
To guide intensity of intervention: Research demonstrates that HbA1c ≥6.0% predicts 5-year diabetes risk of 25-50%, while HbA1c 5.7-6.0% predicts 9-25% risk—adding FPG data can help determine whether aggressive lifestyle intervention versus less intensive monitoring is most appropriate. 1
Critical Pitfall to Avoid
Never assume you need confirmatory testing for prediabetes diagnosis. The ADA explicitly states that the requirement for two abnormal tests applies to diabetes diagnosis (HbA1c ≥6.5%, FPG ≥126 mg/dL, or 2-hour glucose ≥200 mg/dL), not to prediabetes. 1, 2 This is a common source of confusion that leads to unnecessary testing and delays in initiating preventive interventions.
What You Should Do Instead
Proceed directly to diabetes prevention interventions: With an HbA1c of 6.3%, your patient is at the higher end of the prediabetes range and has a 25-50% risk of developing diabetes within 5 years. 1
Initiate lifestyle modification counseling focusing on weight loss (7% body weight reduction target) and physical activity (150 minutes weekly of moderate-intensity exercise), which reduces diabetes incidence by 58% in this population. 1
Consider metformin for diabetes prevention, particularly if the patient has additional risk factors such as BMI ≥35 kg/m², age <60 years, or history of gestational diabetes. 1
Schedule follow-up HbA1c testing in 3-6 months to monitor response to interventions and assess for progression toward diabetes. 1
Evidence Quality Note
The 2023 ADA Standards of Care 1 represent the most current and authoritative guidance, superseding earlier recommendations from 2019 1 and 2012 1. Research studies consistently demonstrate that HbA1c alone is sufficient for prediabetes diagnosis, with HbA1c showing lower short-term variability (coefficient of variation 1.6%) compared to FPG and stronger prediction of diabetes progression. 5, 4