HbA1c has been the gold standard for measuring long-term glucose control for decades. But as continuous glucose monitors have become more common, a different metric has been gaining ground: Time in Range (TIR). Research now shows that TIR isn't just a useful day-to-day number — it directly correlates with the risk of serious diabetes complications.
This article is based on the lecture Time in Range and Diabetes Complications by Roy W. Beck, MD, PhD, Jaeb Center for Health Research, Tampa, Florida.
What Is Time in Range?
Time in Range is the percentage of time your blood glucose spends between 70 and 180 mg/dL (3.9–10 mmol/L). A CGM can measure this precisely, tracking your glucose around the clock and reporting what fraction of the day fell inside the target zone.
At an HbA1c of 8%, TIR is roughly 50% on average — meaning about 47% of the day is spent above 180 mg/dL, and 3% below 70. In other words, for someone at that HbA1c, the time spent out of range is almost entirely time spent in hyperglycemia. This is why TIR and "time above 180" are so closely linked: for most people, time out of range is hyperglycemia.
How TIR and HbA1c Relate — and Where They Diverge
TIR and HbA1c correlate at around r = 0.73–0.75, which sounds strong but still leaves a lot of room for disagreement. For any given HbA1c, there is a surprisingly wide spread of possible TIR values — and vice versa.
Much of that discordance appears to come from differences in red blood cell lifespan between individuals. HbA1c measures glycated hemoglobin, which depends on how long red blood cells live. If your RBC lifespan is shorter than average, HbA1c may underestimate your true glucose exposure. CGM-based TIR doesn't have that biological bias — it measures actual glucose concentrations directly.
When researchers corrected for this effect, the correlation between TIR and HbA1c jumped from ~0.75 to above 0.93. The takeaway: both metrics are largely measuring the same glycemic reality, just through different lenses.
TIR and Retinopathy Risk
The landmark data on TIR and complications comes from the DCCT (Diabetes Control and Complications Trial), the trial that established the importance of tight glucose control. DCCT participants performed 7-point blood glucose tests every three months — a far cry from CGM, but enough data to estimate TIR retrospectively.
When researchers used that data to compute TIR and then compared it to rates of retinopathy progression, the results were striking:
- Each 10 percentage points lower TIR → 64% increase in the risk of retinopathy progression
- Each 5 percentage points lower TIR → 28% increase in risk
To put that in context: a 6.2% drop in TIR carried roughly the same retinopathy risk as a 0.5% rise in HbA1c. The curves for TIR and HbA1c looked remarkably similar when plotted against complication rates.
A large cross-sectional study from China — over 32,000 patients with type 2 diabetes — confirmed the pattern. Even with just three days of CGM data, lower TIR was consistently associated with greater severity of diabetic retinopathy. The association held when looking at mild, moderate, and vision-threatening retinopathy.
TIR and Kidney Disease
The DCCT data showed a similar link for kidney disease. Each 10 percentage points lower TIR was associated with a 40% increase in the risk of developing microalbuminuria — an early marker of diabetic nephropathy.
The pattern again mirrored what was observed with HbA1c: higher glucose burden, whether measured by HbA1c or TIR, tracked with greater risk of renal complications.
A separate study in over 2,000 patients with type 2 diabetes measured carotid intima-media thickness (CIMT), a marker of macrovascular disease. Higher TIR was associated with lower prevalence of abnormal CIMT — suggesting that the relationship between TIR and complications extends beyond microvascular disease.
TIR in Pregnancy: Even Small Differences Matter
In pregnancy, the stakes are even higher — and the numbers smaller. Research combining CGM data from pregnant women with diabetes found that a 5% difference in TIR (roughly one hour per day) was significantly associated with:
- Higher rates of large-for-gestational-age newborns
- Neonatal hypoglycemia
- Neonatal intensive care admissions
Five percent sounds trivial. One hour a day does not. This finding underscores how meaningful even modest improvements in TIR can be.
Why TIR May Eventually Replace HbA1c
HbA1c became the standard glycemic metric because, when the DCCT was run in the 1980s and 1990s, there was no way to measure day-to-day glucose levels realistically. HbA1c was a practical surrogate for what we actually care about: glucose exposure over time.
Now that CGM can measure actual glucose concentrations continuously, the argument for using a surrogate grows weaker. TIR provides the direct measurement — without the red blood cell lifespan confound — and as research accumulates, it is increasingly being considered as an endpoint for clinical trials in its own right.
What This Means in Practice
The research message is clear: more time in range is directly associated with lower rates of serious complications. Every percentage point matters. Every hour counts.
That makes tracking TIR one of the most actionable things you can do for your long-term health. Not just knowing today's readings, but watching the trend — week over week, across different meal types, different activity patterns, different times of day.
Dia-Log helps you do exactly that. It logs your meals, insulin doses, and corrections, giving you a clear picture of where your glucose tends to fall and when. Patterns that are invisible in isolated readings become obvious over time — and that's the kind of information worth bringing to every clinic visit.
This article is for educational purposes only and is based on the lecture Time in Range and Diabetes Complications by Roy W. Beck, MD, PhD (Jaeb Center for Health Research). All diabetes management decisions should be made in consultation with your care team.