Two patients walk in with an identical HbA1c of 7.0%. On paper, identical control.
One spends almost the whole day between 70 and 180 mg/dL, with gentle post-meal rises. The other swings between 50 and 300 several times daily, and the highs and lows average out to the same number.
Those are not the same patient, and they do not carry the same risk.
What HbA1c measures
HbA1c reflects the proportion of haemoglobin that has become glycated, giving an approximate three-month average of blood glucose. It is cheap, standardised and strongly linked to long-term complication risk. It is not going away.
But it has real limits:
- It is an average, so it conceals variability entirely.
- It says nothing about hypoglycaemia, which is an acute risk.
- It is distorted by anaemia, haemoglobin variants, kidney disease and pregnancy — all common in Indian patients.
- It is slow, reflecting a quarter that has already happened.
What Time in Range adds
Continuous glucose monitoring makes a different metric possible. Time in Range is simply the percentage of the day spent between 70 and 180 mg/dL, alongside time above and time below.
The international consensus targets for most adults with Type 1 or Type 2 diabetes:
- Time in range (70–180): above 70% of the day
- Time below 70: under 4%
- Time below 54: under 1%
- Time above 180: under 25%
Every additional 10% of time in range corresponds to roughly a 0.5% reduction in HbA1c — but with the variability made visible rather than averaged away.
Why variability matters
Glucose variability appears to drive oxidative stress and endothelial damage somewhat independently of average glucose. Two people at the same HbA1c with very different variability do not appear to face the same complication risk.
Variability is also what patients actually feel. The afternoon crash, the 3am hypo, the post-meal fog — none of those appear in an HbA1c, and all of them are visible in a fortnight of CGM data.
What it changes in practice
The clinical difference is speed and specificity. HbA1c tells you three months later that something was wrong. Time in Range tells you which meal, which dose, which time of day — this week.
That turns treatment adjustment from quarterly guesswork into a directed change. Instead of raising a dose because the number is high, you can see that the number is high because of one specific evening pattern, and change that instead.
Use both
This is not an argument for discarding HbA1c. It remains the metric with the longest outcome evidence behind it, and it is far more accessible.
The best picture uses both: HbA1c for the long arc and the evidence base, Time in Range for the texture and the week-to-week decisions. One tells you where you have been. The other tells you what to do on Tuesday.
Ask about CGM at your next consultation if your HbA1c looks acceptable but you do not feel well.