Why HbA1c doesn’t tell the full story about blood sugar

HbA1c is one of the most commonly requested blood tests in primary care. It often appears quietly on a results list, sometimes highlighted, sometimes waved through with brief reassurance. “It’s normal.” Or “It’s only slightly raised.” Then life moves on.

For a test that plays such an important role in diagnosing diabetes and assessing long-term risk, HbA1c is surprisingly misunderstood.

HbA1c reflects your average blood sugar over the previous two to three months. It works by measuring how much glucose has attached to red blood cells during their lifespan, which is around 120 days. This makes it a useful long-term marker rather than a snapshot of a single day.

It is a well-validated, evidence-based test and remains an important part of health care. The issue is not what HbA1c shows, but what it cannot show on its own.

To understand this, it helps to look at how blood sugar regulation works in the body.

Every time you eat, particularly foods containing carbohydrates, glucose enters your bloodstream. Your body releases insulin, a hormone that helps move glucose into cells so it can be used for energy or stored. When this system works well, blood sugar rises gently after meals and returns to baseline without difficulty.

Over time, however, cells can become less responsive to insulin. When this happens, the body compensates by producing more insulin to keep blood sugar within a normal range. This state is known as insulin resistance.

Here is where HbA1c can be misleading.

In the early stages of insulin resistance, blood sugar levels may still appear completely normal. HbA1c can sit comfortably within range while insulin levels are rising quietly in the background. The body is working harder to maintain balance, but the test result does not reflect that effort.

This helps explain why people are sometimes told their results are “fine” despite symptoms such as fatigue, difficulty losing weight, cravings, or energy dips after meals. It also explains why type 2 diabetes can develop in people who are slim or physically active, while others never develop it at all. Weight alone is not the deciding factor. Hormones, genetics, inflammation, sleep, stress, muscle mass, and life stage all influence how blood sugar is regulated.

HbA1c also hides another important detail: variability.

Two people can have the same HbA1c but very different daily blood sugar patterns. One may have stable levels throughout the day. Another may experience repeated spikes and crashes after meals. The average looks the same, but the physiological stress on the body is not.

Those fluctuations matter. Repeated spikes are associated with inflammation and metabolic strain, even when average blood sugar appears acceptable. HbA1c smooths over these highs and lows and cannot show how hard the system is working.

This does not make HbA1c a bad test. It makes it an incomplete one when used in isolation.

The more helpful question is often not just “Is this number normal?” but “What is driving this number, and how sustainable is the balance behind it?”

What we do at Bespoke Health Clinic

At Bespoke Health Clinic, HbA1c is usually the starting point of a conversation, not the end. We help interpret results in context, alongside symptoms, lifestyle, and goals. Where appropriate, this may include looking at insulin, inflammation, hormones, or nutritional markers, or using short-term continuous glucose monitoring to understand real-world responses to food, stress, sleep, and activity. The aim is not perfect numbers or unnecessary testing. It is clarity, so decisions are informed, proportionate, and personalised.

If this raises questions about your own health, a free Clarity Call can help you decide whether further support or investigation would be useful.

This article is for general information and education only and is not intended to replace personalised medical advice. HbA1c is a well-established and important test within health care, particularly for diagnosing and monitoring diabetes.

If you have concerns about blood sugar, have been advised on treatment, or are managing a long-term condition, decisions should always be made with a clinician who understands your personal medical history. This article is intended to support understanding, not to override clinical advice.