Guide to Hemoglobin A1c (HbA1c)

WRITTEN BY

The Levels Team

Updated: 05/22/2025|12 min read

Summary

The HbA1c test assesses your average blood sugar over the past two to three months by detecting glycated hemoglobin, which forms when glucose bonds to hemoglobin in red blood cells.

Why It Matters

When glucose enters your bloodstream, it can attach to proteins in a process called glycation. Hemoglobin, the oxygen-carrying protein in red blood cells, is particularly susceptible to this process, making it an excellent marker for tracking glucose exposure. Unlike a fasting glucose test, which captures just one moment, or even a glucose tolerance test, which shows a response to a specific challenge, HbA1c reveals the body's ongoing blood sugar patterns.

However, HbA1c has important limitations: it can remain normal for years in the early stages of insulin resistance. This means that by the time HbA1c becomes elevated, metabolic dysfunction may have been present for some time. Additionally, since HbA1c measures average glucose exposure, it might miss dangerous daily spikes and dips that could be causing damage despite a "normal" average.

Think of HbA1c as a report card for blood sugar control - it shows the overall pattern but might not catch early warning signs or daily glucose variability. This is why it's most valuable when viewed alongside other metabolic markers, particularly insulin and fasting glucose, to understand the full picture of metabolic health.

Associated Symptoms

HbA1c levels themselves are laboratory findings rather than medical conditions. However, abnormal levels may be associated with various health issues, each with its own symptoms.

Common symptoms that may indicate conditions associated with elevated HbA1c:

• Polydipsia: Increased thirst that can result from dehydration when excess glucose draws water from tissues
• Polyuria: Frequent urination can be due to kidneys working harder to filter excess glucose
• Fatigue: Persistent tiredness may result from cells' inability to efficiently use glucose for energy
• Vision changes: Blurry vision can be caused by glucose-induced swelling of the eye's lens
• Impaired healing: Slow wound healing may result from damaged blood vessels and reduced immune function
• Neuropathy: Numbness or tingling in feet can be due to nerve damage from sustained high blood sugar
• Recurrent infections: Increased susceptibility to infections can result from impaired immune function
• Weight changes: Unexplained weight loss despite normal or increased appetite may be due to energy inefficiency
• Mood disturbances: Irritability or mood changes can be related to glucose fluctuations affecting brain function
• Skin issues: Dry skin can result from dehydration and microvascular changes
• Polyphagia: Increased hunger may result from cells' inability to access glucose despite an abundant supply

If your HbA1c is low, you may not experience noticeable symptoms under normal circumstances. However, certain medications or medical conditions that cause low HbA1c (such as conditions with rapid red blood cell turnover) may be associated with symptoms of those underlying conditions. People taking medications that can cause hypoglycemia might experience symptoms of low blood sugar episodes, including shakiness, sweating, confusion, or dizziness.

It's important to understand that symptoms typically develop gradually as blood sugar levels rise over time, and many people with moderately elevated HbA1c may not notice any symptoms at all. This is why regular screening is valuable for early detection of prediabetes or diabetes.

Clinical Ranges

Lab Reference Range: <5.7 %

Lifestyle Choices That Can Impact It

Activities that tend to have a positive effect on HbA1c:

• Regular exercise activates GLUT4 transporters in muscles to improve insulin sensitivity and use glucose more efficiently
• Whole, natural food diet provides fiber, healthy fats, and protein that help maintain stable blood sugar levels
• Adequate sleep and feeling well-rested upon waking supports proper hormone regulation that controls blood sugar metabolism
• Stress management reduces levels of cortisol, a hormone that raises blood sugar
• Weight management helps maintain insulin sensitivity and reduces inflammation
• Regular meal timing, with periods of not eating or snacking between meals and avoiding snacking before bed, helps maintain consistent blood sugar patterns throughout the day
• Blood sugar monitoring allows for immediate feedback and adjustment of diet and activities
• Good hydration helps kidneys remove excess glucose and supports overall metabolism

Those that tend to have a negative effect:

• Eating sugar, highly refined carbohydrates, or processed foods causes frequent blood sugar spikes that lead to higher average glucose levels
• Smoking increases insulin resistance and inflammation, making blood sugar more challenging to control
• Excessive alcohol interferes with the liver's ability to regulate blood sugar and can cause dangerous fluctuations
• Dehydration concentrates blood glucose and reduces insulin's ability to bind effectively to cell receptors

Other Factors That Can Impact It

Medications

• Metformin: lowers HbA1c by improving insulin sensitivity
• Corticosteroids: increase HbA1c by raising blood sugar
• Iron supplements: can affect results if iron deficient
• Erythropoietin: lowers HbA1c by increasing new red blood cells
• Vitamin C: can interfere with some test methods at high doses and can interfere with continuous glucose monitor readings
• HIV medications: can affect red blood cell lifespan
• Statins: can increase HbA1c
• Berberine: can lower Hba1c
• Mood stabilizers, some antidepressants, and other medications that can cause weight gain can also impact

Medical Conditions

• Kidney disease: can affect results through anemia
• Iron deficiency anemia: can falsely elevate HbA1c levels since this is commonly seen
• Liver disease: may affect red blood cell lifespan
• Thyroid disorders: impact metabolism and blood sugar
• Conditions that cause anemia or increased blood cell turnover can underestimate the HbA1c
• Sickle cell anemia can falsely lower HbA1c levels

Testing Accuracy and Stability

This test is very stable overall. Recent meals won't affect it, and you don't need to fast beforehand.

That said, blood conditions, such as recent transfusions, blood loss, and anemia can lead to inaccurate results on your HbA1c test. And as mentioned earlier, pregnancy and certain medications can skew readings.

HbA1c also has limitations, such as falsely low readings in conditions that shorten red blood cell lifespan (e.g., sickle cell trait) and the inability to capture daily glucose fluctuations. Because it reflects the average blood glucose, it can miss glucose spikes. It can lag as an indicator of insulin resistance.

How it Relates to Other Markers

• High HbA1c with normal fasting glucose: May indicate post-meal glucose spikes or dawn phenomenon; worth investigating insulin response.
• High HbA1c with high fasting glucose: Suggests consistently high blood sugar; consider testing insulin and C-peptide levels.
• Normal HbA1c with high fasting glucose: Could indicate recent lifestyle changes or that glucose control has recently worsened.
• High HbA1c with normal insulin: May suggest beta cell dysfunction or early Type 1 diabetes.
• High HbA1c with high insulin: Could indicate insulin resistance and prediabetes or Type 2 diabetes.
• Low HbA1c with variable glucose: Could indicate increased red blood cell turnover or certain hemoglobin variants.

Follow-up Considerations

You should always talk to your doctor if you have medical concerns or questions.

When Re-Testing May Be Appropriate

• Borderline: Retest in 3-6 months after implementing lifestyle changes
• High: Immediate repeat testing to confirm diagnosis, then every 3 months
• Very high: Urgent repeat testing, then frequent monitoring as directed by a medical team

Additional Testing Your Doctor May Consider

• Continuous glucose monitoring
• Glucose tolerance test
• GAD antibodies
• C-peptide

When Additional Care May be Warranted

• Borderline: If levels don't improve after 3-6 months of lifestyle changes
• High: Immediate follow-up for diabetes evaluation and treatment planning

Further Reading

What is HbA1c, and why does it matter?

Bibliography