Managing diabetes effectively requires a well-rounded strategy that includes regular monitoring of blood glucose levels. For individuals taking diabetes medications, the HbA1c test (also called glycated hemoglobin or A1C) is one of the most critical tools in assessing long-term glycemic control. Unlike daily fingerstick checks or continuous glucose monitor (CGM) readings, which capture instantaneous values, the HbA1c test provides a two- to three-month average of blood sugar levels. This article explains why routine HbA1c testing is essential for those on pharmacological therapy, how often it should be performed, and what the results mean for adjusting your treatment plan. We’ll also cover practical tips for getting the most accurate results and integrating HbA1c data with other monitoring approaches.

What Is the HbA1c Test?

The HbA1c test measures the percentage of hemoglobin molecules in red blood cells that have glucose attached to them via a nonenzymatic process called glycation. Because red blood cells have a lifespan of roughly 120 days, the HbA1c value reflects the average blood glucose concentration over the preceding 8–12 weeks. In individuals without diabetes, a normal HbA1c is typically below 5.7%. Values between 5.7% and 6.4% indicate prediabetes, while an HbA1c of 6.5% or higher on two separate tests confirms a diagnosis of diabetes.

For most people with diabetes who are on medications, the general target is an HbA1c below 7.0% (essentially an average blood glucose around 154 mg/dL or 8.6 mmol/L). However, targets can be individualized — for example, stricter goals (below 6.5%) might be appropriate for younger patients with new-onset diabetes and no cardiovascular disease, while less stringent goals (below 8.0%) may be suitable for older adults with a history of severe hypoglycemia or limited life expectancy. Your healthcare provider will set a personalized target based on your age, duration of diabetes, medication regimen, and comorbid conditions.

Why Regular HbA1c Testing Matters When on Medications

When you are taking diabetes medications — whether they are oral agents like metformin, sulfonylureas, or SGLT-2 inhibitors, or injectable therapies like insulin or GLP-1 receptor agonists — the HbA1c test serves several critical purposes.

Assessing Medication Efficacy

Medications are prescribed to lower blood glucose levels. An HbA1c test reveals whether your current regimen is achieving the desired reduction in average glucose. A high or rising HbA1c indicates that the medication combination may not be sufficient or that adherence needs improvement. Conversely, a low HbA1c (especially below 6.0% in someone on insulin or sulfonylureas) could signal an increased risk of hypoglycemia, which may require dose adjustments.

Guiding Treatment Adjustments

Diabetes management is dynamic. Over time, beta-cell function may decline, insulin resistance may change, or weight fluctuations can alter medication effectiveness. Regular HbA1c testing enables your healthcare team to make data-driven decisions — for example, adding a new class of drug, adjusting insulin doses, or changing to a more potent formulation. The American Diabetes Association (ADA) recommends that patients whose HbA1c is above target should have their therapy intensified at least every three months until the goal is achieved.

Monitoring for Diabetes Complications

Consistently high HbA1c levels are strongly associated with the development and progression of microvascular complications, including diabetic retinopathy, nephropathy, and neuropathy. Data from landmark studies like the Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS) show that each 1% reduction in HbA1c lowers the risk of these complications by roughly 35–40%. Regular testing helps you and your provider stay ahead of these risks.

While HbA1c reflects average glucose, it does not capture daily fluctuations (glycemic variability). However, in combination with self-monitored blood glucose (SMBG) or CGM data, the HbA1c trend can highlight periods of sustained hyperglycemia or frequent hypoglycemia that might otherwise go unnoticed. For example, a patient with an HbA1c of 7.5% but frequent low glucose readings may need a different medication or dosing schedule that reduces variability.

According to the American Diabetes Association Standards of Medical Care in Diabetes 2024, the frequency of testing should be individualized:

  • For patients meeting treatment goals (HbA1c at or below target) and stable glycemic control: At least twice per year (every 6 months).
  • For patients whose therapy has changed or who are not meeting glycemic goals: Quarterly (every 3 months) until the HbA1c stabilizes within the target range.
  • For patients with type 1 diabetes, pregnancy, or complex insulin regimens: Quarterly testing is generally recommended even if goals are met, given the higher risk of rapid glucose fluctuations.

Some clinical groups recommend more frequent testing for individuals with conditions that affect red blood cell turnover (e.g., chronic kidney disease, anemia, recent blood transfusion) because those conditions can artificially alter the HbA1c value. In such cases, alternative measures like glycated albumin or fructosamine may be used.

Preparing for the HbA1c Test: What You Need to Know

One of the advantages of the HbA1c test is that it does not require fasting or special preparation. Unlike a fasting plasma glucose test, you can eat and drink normally before the blood draw. However, there are important factors to consider that can affect test accuracy.

Factors That May Interfere With HbA1c Results

Not all HbA1c results are reliable. The test measures hemoglobin A1 — a specific subtype of hemoglobin. Conditions that affect red blood cell lifespan or hemoglobin structure can produce misleading results:

  • Anemia: Iron deficiency anemia can falsely elevate HbA1c, while hemolytic anemia or recent blood loss can lower it.
  • Hemoglobin variants: Individuals with sickle cell trait, hemoglobin C trait, or other hemoglobinopathies may have inaccurate HbA1c values when using certain methods. Laboratories typically note if the test is affected by common variants.
  • Pregnancy: During the second and third trimesters, red blood cell turnover increases, potentially lowering HbA1c. The ADA recommends using the oral glucose tolerance test for gestational diabetes diagnosis, but HbA1c can still be used for monitoring with caution.
  • Chronic kidney disease: Uremia and reduced red blood cell lifespan can alter HbA1c. In advanced kidney disease, glycated albumin or fructosamine may be more accurate.
  • Blood transfusions: A recent transfusion can significantly affect HbA1c because donor cells have a different glycation history.

Always inform your healthcare provider about any known blood disorders, recent transfusions, or pregnancy before interpreting your HbA1c results. If discrepancies arise between your HbA1c and self-monitored glucose levels, further investigation may be needed. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) provides additional guidance on interpreting HbA1c in special populations. Read more on the NIDDK website.

Understanding Your HbA1c Results in Context

An HbA1c percentage can be converted to an estimated average glucose (eAG) using the formula: eAG (mg/dL) = 28.7 × HbA1c − 46.7, derived from the ADAG study. For example, an HbA1c of 7% corresponds to an average glucose of 154 mg/dL. Many laboratories now report eAG alongside the percentage to make the number easier to relate to daily meter readings.

It is important to remember that an HbA1c is an average — it does not tell you about extreme highs or lows. Two patients with identical HbA1c values can have vastly different glycemic profiles: one might spend most of the day near target with occasional mild highs, while another might oscillate between dangerous hypoglycemia and severe hyperglycemia. Therefore, HbA1c should always be interpreted in collaboration with daily blood glucose logs or CGM data.

The Limitations of HbA1c Alone

While the HbA1c test is a powerful tool, it has well-known limitations:

  • It may underestimate glycemic control in individuals with increased red blood cell turnover (e.g., hemolytic anemia, treatment with erythropoietin).
  • It provides no insight into hypoglycemia frequency or severity.
  • It cannot detect postprandial glucose spikes unless they are sustained enough to raise the average.
  • Certain racial and ethnic groups can have slightly higher HbA1c levels for the same average glucose levels, although the absolute difference is small. More research is ongoing to understand these disparities.

Integrating HbA1c Testing With Other Monitoring Approaches

For patients on medications, a comprehensive monitoring plan combines HbA1c testing with one or more of the following methods:

Self-Monitoring of Blood Glucose (SMBG)

Fingerstick glucose checks provide immediate feedback about the effect of meals, exercise, and medication timing. Patients on insulin may need to test multiple times daily, while those on oral agents often test less frequently. HbA1c results help validate the patterns seen in SMBG logs.

Continuous Glucose Monitoring (CGM)

CGM devices provide real-time glucose readings and retrospective data on time-in-range, time-above-range, and time-below-range. Several studies have shown that using CGM alongside HbA1c leads to better glycemic outcomes. The ADA now recommends that individuals with diabetes who are on intensive insulin therapy use CGM, but even those on non-insulin therapies can benefit from intermittent use.

Time-in-Range (TIR)

TIR is the percentage of time your glucose stays within the target range (typically 70–180 mg/dL). It’s becoming an alternative metric to HbA1c, especially when HbA1c results are unreliable. Research demonstrates a strong correlation between TIR and HbA1c — for most people, each 10% improvement in TIR corresponds to approximately a 0.8% reduction in HbA1c.

Your healthcare team can use these complementary tools to make more nuanced medication adjustments. For example, if your HbA1c is 7.5% but CGM shows frequent hypoglycemia overnight, the goal might be to slightly reduce long-acting insulin and add a short-acting option for meals, rather than aiming for a lower HbA1c at all costs.

Lifestyle Factors and Their Impact on HbA1c

While medications are vital, lifestyle modifications significantly influence HbA1c levels. Diet, physical activity, sleep, and stress management all play a role. A diet rich in fiber and low in refined carbohydrates can lower HbA1c by 0.5–1.0% on its own. Regular exercise improves insulin sensitivity and helps maintain a healthy weight. Conversely, chronic sleep deprivation and stress can raise blood glucose through hormonal pathways (e.g., increased cortisol).

When reviewing your HbA1c results, consider whether recent lifestyle changes could explain a trend. For instance, if your HbA1c rises despite consistent medication adherence, it might be time to re-evaluate your carbohydrate intake or exercise routine. The Centers for Disease Control and Prevention (CDC) offers practical tools for integrating healthy habits into your diabetes management plan. Visit the CDC’s diabetes eating well page.

Special Considerations for Specific Populations

HbA1c in Type 1 Diabetes

People with type 1 diabetes require insulin around the clock and are at high risk for both hypoglycemia and diabetic ketoacidosis. Quarterly HbA1c testing is standard, but great care must be taken not to over-treat the number. A low HbA1c (e.g., <6.5%) in a person with type 1 diabetes may indicate frequent lows rather than ideal control. The use of CGM and insulin pumps has made it possible to achieve lower HbA1c with fewer hypoglycemic events.

HbA1c in Gestational Diabetes

During pregnancy, HbA1c is not typically used for screening or diagnosis of gestational diabetes mellitus (GDM) because of physiological changes. However, once diagnosed, some clinicians use HbA1c to monitor glycemic control in women with preexisting diabetes. Targets are stricter (usually <6.0% or <6.5%) to minimize risks to the fetus. Women with GDM should follow their obstetrician’s monitoring recommendations closely.

HbA1c in Older Adults

The risk-benefit balance shifts with age. For older adults who are frail, have a long duration of diabetes, or have multiple comorbidities, less stringent HbA1c targets (e.g., <8.0%) are appropriate to avoid hypoglycemia and medication burden. Regular testing remains important to ensure that treatment does not become overly aggressive.

Communicating With Your Healthcare Team About HbA1c Results

Your HbA1c result is a conversation starter, not a verdict. Bring your meter or CGM data to appointments and ask how the HbA1c aligns with your daily numbers. If there is a discrepancy, explore potential causes together — it might be a lab error, a medication timing issue, or an interfering condition. Do not hesitate to ask for a repeat test if you suspect an inaccurate result. Also, clarify your personal target and what steps to take if the number is above or below that range.

The American Diabetes Association provides patient-friendly resources for understanding HbA1c. Check their A1C overview page.

Conclusion: Making HbA1c Testing a Pillar of Your Diabetes Care

For anyone on diabetes medications, regular HbA1c testing is not optional — it is a cornerstone of safe and effective management. It tells you and your provider whether your current treatment plan is working, guides necessary adjustments, and helps prevent the devastating long-term complications of uncontrolled diabetes. To get the most value from this test:

  • Follow the recommended testing frequency based on your stability and treatment intensity.
  • Understand what influences your HbA1c and discuss any interfering conditions with your provider.
  • Use HbA1c in combination with daily glucose monitoring to get a complete picture of your control.
  • Pair your testing with healthy lifestyle choices that support your medication therapy.

By staying proactive with HbA1c testing and collaborating with your healthcare team, you can achieve better glycemic outcomes, reduce the risk of complications, and maintain a high quality of life. Remember that the goal is not a perfect number — it is the healthiest way of living with diabetes that works for you. For further reading on personalized diabetes targets, consult the American Diabetes Association’s Professional Practice Committee guidelines. View the Standards of Care here.

Additionally, the National Center for Biotechnology Information (NCBI) hosts numerous studies on HbA1c accuracy and clinical outcomes. Learn more about HbA1c from the NCBI Bookshelf.