Time in range vs HbA1c is one of the most clinically important comparisons in modern diabetes management. HbA1c has guided treatment decisions for decades, offering a reliable retrospective average of blood glucose control. Time in range (TIR), derived from continuous glucose monitoring (CGM), reveals something fundamentally different: the moment-to-moment fluctuations, hypoglycaemic dips, and hyperglycaemic peaks that a single average figure cannot capture. Understanding what each metric measures, where they agree, and — crucially — where they diverge, helps patients and clinicians make better-informed decisions about diabetes care.
Summary: Time in range and HbA1c measure different aspects of blood glucose control — HbA1c reflects a two-to-three month average, while time in range shows the percentage of time glucose stays within a target range using continuous glucose monitoring data.
- HbA1c measures the proportion of glycated haemoglobin over approximately 8–12 weeks and is expressed in mmol/mol (IFCC) in the UK.
- Time in range (TIR) is derived from CGM data and targets glucose within 3.9–10.0 mmol/L; a minimum of 14 days with over 70% data capture is recommended for reliable interpretation.
- A normal HbA1c can mask significant hypoglycaemia; CGM metrics including time below range (TBR) reveal risks that HbA1c averages can obscure.
- NICE NG17 recommends CGM for all adults with type 1 diabetes; access for type 2 diabetes on insulin is criteria-based under NICE NG28.
- ATTD consensus targets TIR >70%, TBR <4% below 3.9 mmol/L, and TAR <25% above 10.0 mmol/L — these are not formal NICE targets but are widely used in UK practice.
- HbA1c and TIR are complementary tools; neither replaces the other, and both should be reviewed together at diabetes clinic appointments.
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What Time in Range and HbA1c Actually Measure
HbA1c reflects average glycaemic exposure over 8–12 weeks, while time in range measures the percentage of CGM readings within a target glucose range, capturing variability and hypoglycaemia that HbA1c cannot.
Understanding the difference between time in range (TIR) and HbA1c begins with recognising that they capture fundamentally different aspects of blood glucose control. HbA1c (glycated haemoglobin) reflects the average proportion of haemoglobin molecules that have become bound to glucose over approximately two to three months. Because red blood cells have a lifespan of roughly 120 days, HbA1c provides a retrospective snapshot of overall glycaemic exposure during that period — broadly the preceding eight to twelve weeks. It is expressed as a percentage or in mmol/mol under the IFCC standardisation used across the UK.
Time in range, by contrast, is a metric derived from continuous glucose monitoring (CGM) technology. It measures the percentage of time a person's glucose readings fall within a defined target range — typically 3.9–10.0 mmol/L for most adults with diabetes, though targets may be individualised. CGM devices record glucose levels every few minutes throughout the day and night, generating a rich dataset that HbA1c simply cannot replicate.
It is important to note that CGM sensors measure interstitial glucose (glucose in the fluid surrounding cells) rather than blood glucose directly. There is a physiological lag of a few minutes between changes in blood glucose and changes in interstitial glucose. For this reason, if your CGM reading does not match how you feel — particularly if you suspect hypoglycaemia — you should confirm the reading with a finger-prick capillary blood glucose test before acting on it. For reliable TIR interpretation, international consensus guidance recommends a minimum of approximately 14 days of CGM data with more than 70% data capture.
TIR also enables calculation of related metrics:
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Time below range (TBR): percentage of time glucose falls below 3.9 mmol/L (Level 1 hypoglycaemia) or below 3.0 mmol/L (Level 2 hypoglycaemia)
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Time above range (TAR): percentage of time glucose exceeds 10.0 mmol/L (Level 1) or 13.9 mmol/L (Level 2)
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Glucose management indicator (GMI): an estimated HbA1c derived from CGM data
These complementary figures give a far more granular picture of glycaemic variability — the peaks and troughs that an average figure like HbA1c inherently obscures. For patients experiencing frequent hypoglycaemic episodes, for example, TIR and TBR can reveal risks that a reassuringly normal HbA1c might mask entirely.
How HbA1c Has Been Used to Guide Diabetes Care in the UK
HbA1c is the UK's standard long-term glycaemic metric, with NICE targets of 48 mmol/mol for most adults with type 1 or type 2 diabetes, though it can be unreliable in haemoglobinopathies, anaemia, and pregnancy.
HbA1c has been the cornerstone of diabetes monitoring in the UK for several decades, and its role is firmly embedded in clinical practice. NICE guidelines recommend HbA1c testing every three to six months for people with type 1 or type 2 diabetes until levels are stable, then every six months thereafter, with specific target thresholds guiding treatment decisions. For most adults with type 2 diabetes managed with lifestyle measures or metformin alone, NICE (NG28) suggests an HbA1c target of 48 mmol/mol (6.5%), rising to 53 mmol/mol (7.0%) for those on medications carrying a hypoglycaemia risk.
For type 1 diabetes, NICE guidance (NG17) recommends a target of 48 mmol/mol (6.5%) where achievable without problematic hypoglycaemia, acknowledging that individual circumstances must always be considered. HbA1c results directly inform prescribing decisions — whether to intensify therapy, add a second agent, or refer to specialist services.
Two important programmes use HbA1c in distinct ways. The NHS Diabetes Prevention Programme (NDPP) uses an HbA1c of 42–47 mmol/mol to identify people with non-diabetic hyperglycaemia who are at risk of developing type 2 diabetes. The Quality and Outcomes Framework (QOF) uses HbA1c thresholds as performance indicators for people already diagnosed with diabetes, measuring the quality of care delivered at practice level.
The widespread adoption of HbA1c is understandable: it is a standardised, reproducible, and relatively inexpensive blood test available in virtually every NHS laboratory. Large landmark trials — including the DCCT (type 1) and UKPDS (type 2) — demonstrated that lower HbA1c values correlate with reduced risk of microvascular complications such as retinopathy, nephropathy, and neuropathy. This evidence base has made HbA1c the default metric for assessing long-term glycaemic control.
However, HbA1c has well-recognised limitations. It can be falsely altered by conditions affecting red blood cell turnover, including haemolytic anaemia, haemoglobinopathies (such as sickle cell trait), iron deficiency, and chronic kidney disease — all of which are relevant considerations in a diverse UK population. Additionally, HbA1c is not recommended for diagnosing diabetes in pregnancy, in children and young people, in people with suspected type 1 or rapidly evolving diabetes, during acute illness, or in those with haemoglobinopathies. In these situations, alternative tests such as fasting plasma glucose or, where appropriate, CGM-derived metrics should be used.
When the Two Measures May Give Different Results
HbA1c and TIR can diverge significantly when hypoglycaemia offsets hyperglycaemia, producing an acceptable average; CGM data reveals both extremes, particularly in pregnancy, haemoglobinopathies, and frail older adults.
There are clinically important scenarios in which TIR and HbA1c can tell quite different stories about a person's glycaemic health. The most striking example is the discordance between HbA1c and CGM metrics caused by the averaging effect of HbA1c: a patient may achieve an HbA1c within the target range whilst simultaneously spending significant time in hypoglycaemia. If periods of low glucose are offset by periods of high glucose, the average — and therefore the HbA1c — can appear deceptively acceptable. CGM data, however, would clearly reveal both the hypoglycaemic burden and the hyperglycaemic excursions.
Conversely, a person with relatively stable glucose levels that sit consistently just above the target range might have a modestly elevated HbA1c, yet their TIR data could show minimal variability and very little time in hypoglycaemia — a profile that may carry a different risk to someone with the same HbA1c but extreme fluctuations.
Several specific clinical situations warrant particular attention:
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Pregnancy: Tighter targets apply. International consensus (ATTD) recommends TIR >70% within 3.5–7.8 mmol/L for pregnant women with diabetes; these are not formal NICE targets. NICE (NG3) provides UK capillary blood glucose targets for pregnancy, and HbA1c may be less reliable due to increased red cell turnover. Both sets of guidance should be considered alongside clinical judgement.
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Haemoglobinopathies: HbA1c results may be unreliable; CGM-derived GMI or fructosamine may be preferable.
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Older or frail adults: Hypoglycaemia risk is high; TBR data from CGM provides safety information that HbA1c cannot.
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Insulin pump users: Rapid glucose fluctuations are common; TIR better reflects the effectiveness of pump therapy.
Research published in Diabetes Care (Beck et al.) has demonstrated a broadly consistent mathematical relationship between TIR and HbA1c — approximately a 10% increase in TIR corresponds to a 5 mmol/mol reduction in HbA1c — but this relationship varies between individuals, reinforcing the value of using both metrics together rather than treating them as interchangeable.
| Feature | Time in Range (TIR) | HbA1c |
|---|---|---|
| What it measures | Percentage of time glucose stays within 3.9–10.0 mmol/L | Average proportion of glycated haemoglobin over ~8–12 weeks |
| Technology required | Continuous glucose monitor (CGM); measures interstitial glucose | Standard NHS laboratory blood test; no specialist device needed |
| Time window reflected | Real-time and day-by-day; minimum 14 days data (>70% capture) recommended | Retrospective average of preceding 8–12 weeks |
| Glycaemic variability | Reveals peaks, troughs, TBR, and TAR; captures hypo and hyper episodes | Averages out fluctuations; hypos offset by hypers can give falsely reassuring result |
| UK clinical targets | TIR >70%; TBR <4% (<1% below 3.0 mmol/L); TAR <25% (ATTD consensus, used in UK practice) | 48 mmol/mol (6.5%) for most adults; 53 mmol/mol (7.0%) if hypoglycaemia risk (NICE NG17/NG28) |
| Key limitations | Requires CGM access; interstitial lag; confirm hypoglycaemia with finger-prick test | Unreliable in haemoglobinopathies, anaemia, CKD, pregnancy; not for diagnosing type 1 or gestational diabetes |
| Role in UK guidance | Complementary metric; NICE NG17 recommends CGM for all adults with type 1 diabetes | Primary metric for diagnosis, complication risk, QOF benchmarking, and NDPP screening |
Current NHS and NICE Guidance on Using Both Metrics
NICE NG17 recommends CGM for all adults with type 1 diabetes; TIR, TBR, and TAR targets from ATTD consensus are widely used alongside HbA1c in UK practice, though TIR does not replace HbA1c.
Recognition of CGM's clinical value has grown substantially in recent years, and UK guidance has evolved accordingly.
NICE guideline NG17 (Type 1 Diabetes in Adults, updated 2022) recommends that CGM should be offered to all adults with type 1 diabetes — a significant policy shift that has expanded access to TIR data across NHS services.
For type 2 diabetes, NICE guideline NG28 (updated 2022 onwards) sets out criteria under which real-time CGM (rtCGM) or intermittently scanned CGM (isCGM) should be offered or considered for adults on insulin therapy. Access is criteria-based rather than universal, and patients should discuss eligibility with their GP or diabetes team. Separately, NICE technology appraisal TA943 covers hybrid closed-loop systems for type 1 diabetes and should not be confused with CGM access criteria for type 2 diabetes.
NHS England commissioning frameworks have supported CGM rollout, with flash glucose monitoring (such as the FreeStyle Libre system) now widely available on NHS prescription for eligible patients. Professional bodies including Diabetes UK and the Association of British Clinical Diabetologists (ABCD), alongside the Diabetes Technology Network UK (DTN-UK), have published consensus guidance recommending that TIR, TBR, and TAR be reported and reviewed alongside HbA1c at clinic appointments.
For CGM-derived metrics, the following targets are drawn from ATTD international consensus (2019; updated 2023) and are widely used in UK clinical practice, though they are not formal NICE targets:
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TIR >70% (time within 3.9–10.0 mmol/L) for most adults with type 1 or type 2 diabetes
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TBR <4% below 3.9 mmol/L (Level 1), and <1% below 3.0 mmol/L (Level 2)
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TAR <25% above 10.0 mmol/L
Importantly, NICE and NHS guidance does not position TIR as a replacement for HbA1c but rather as a complementary tool. HbA1c remains the standard metric for diagnosing diabetes (where applicable), assessing long-term complication risk, and benchmarking care at a population level. The two measures are intended to work in tandem, providing both the broad overview and the granular detail needed for truly personalised diabetes management.
Which Measure Is Most Useful for Managing Your Diabetes
Neither metric is universally superior; HbA1c suits those on oral therapy, while TIR is most valuable for insulin users, type 1 diabetes, pregnant women, and anyone with hypoglycaemia unawareness.
The honest answer is that neither measure is universally superior — the most useful metric depends on your individual circumstances, treatment regimen, and clinical goals. For many people, HbA1c remains a practical and meaningful indicator of overall glycaemic control, particularly for those managing type 2 diabetes with oral medications or lifestyle measures alone, where glucose variability tends to be less pronounced and CGM may not yet be routinely available.
However, for people living with type 1 diabetes, those using insulin (particularly multiple daily injections or pump therapy), pregnant women with diabetes, and anyone with a history of hypoglycaemia unawareness, TIR and its associated metrics offer insights that HbA1c simply cannot provide. Knowing that you are spending 15% of your time below 3.9 mmol/L — even if your HbA1c looks acceptable — is clinically important information that should prompt a review of your treatment plan.
Practical guidance for patients:
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If you use a CGM device, ask your diabetes team to review your TIR, TBR, and TAR data at each appointment, not just your HbA1c.
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Remember that CGM measures interstitial glucose, which can lag slightly behind blood glucose. If your CGM reading does not match your symptoms — particularly if you think you may be hypoglycaemic — confirm with a finger-prick capillary blood glucose test before treating.
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If your HbA1c appears well-controlled but you are experiencing symptoms of hypoglycaemia (shakiness, sweating, confusion), discuss CGM access with your GP or diabetes nurse. For type 2 diabetes, access to CGM depends on criteria set out in NICE NG28; your team can advise on eligibility.
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Do not adjust insulin doses or medications based on CGM data alone without guidance from your healthcare team.
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Contact your GP or diabetes team promptly if you are experiencing frequent hypoglycaemic episodes, unexplained HbA1c changes, or if your CGM data shows persistent time below range.
When to seek urgent or emergency help:
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Call 999 or go to A&E immediately if you or someone else has a severe hypoglycaemic episode requiring assistance from another person, or if you cannot be roused.
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Seek urgent same-day medical advice (via your GP, NHS 111, or diabetes team) if you have persistently high glucose readings alongside ketones, abdominal pain, vomiting, or drowsiness, as these may be signs of diabetic ketoacidosis (DKA).
Ultimately, the goal of diabetes management is not to optimise a number on a laboratory report, but to support long-term health, prevent complications, and maintain quality of life. Used together thoughtfully, time in range and HbA1c provide a more complete picture than either measure can offer alone.
Frequently Asked Questions
Can time in range replace HbA1c for monitoring diabetes in the UK?
No — UK NHS and NICE guidance positions time in range as a complementary tool rather than a replacement for HbA1c. HbA1c remains the standard metric for diagnosing diabetes, assessing long-term complication risk, and benchmarking care at a population level, while TIR provides additional detail on glucose variability and hypoglycaemia.
What is a good time in range target for adults with diabetes?
International ATTD consensus guidance, widely used in UK clinical practice, recommends a TIR of greater than 70% within 3.9–10.0 mmol/L for most adults with type 1 or type 2 diabetes, with time below range kept under 4% below 3.9 mmol/L. These are not formal NICE targets, so individual goals should be agreed with your diabetes team.
Why might my HbA1c look normal even if I am having hypoglycaemic episodes?
HbA1c reflects an average blood glucose level over approximately 8–12 weeks, so periods of low glucose can be offset by periods of high glucose, producing a deceptively normal result. Continuous glucose monitoring and time below range data can reveal hypoglycaemic episodes that HbA1c entirely masks.
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