Understanding HbA1c, eGFR, and their role in the risk calculation

SCORE2-Diabetes asks for three values that a standard blood panel doesn't always provide right away. Here's what they mean, why they matter particularly for assessing the cardiovascular risk of someone with diabetes, and how to interpret them.

HbA1c, blood sugar's memory over several months

Unlike a simple blood sugar reading, which reflects blood sugar at one specific moment, glycated hemoglobin (HbA1c) reflects the average blood sugar level over the past 2 to 3 months. It measures the proportion of hemoglobin — the protein in red blood cells that carries oxygen — that has become irreversibly bound to glucose in the blood. The higher average blood sugar has been over that period, the higher this proportion rises.

It has become the reference marker for assessing someone with diabetes' actual blood sugar control over time, less sensitive to one-off variations (a meal, temporary stress, an infection) than a single blood sugar reading. A prolonged high HbA1c reflects greater cumulative exposure of the cardiovascular system to excess sugar — which explains why it carries so much weight in the risk calculation. Target HbA1c goals vary depending on each person's profile, generally discussed and adjusted with the primary care doctor or endocrinologist based on age, how long diabetes has been present, and other risk factors present.

eGFR, an indicator of kidney health — and heart health

The estimated glomerular filtration rate (eGFR) measures the kidneys' ability to filter blood, expressed in milliliters per minute. It may seem unrelated to cardiovascular health at first glance, but the link is actually direct and well documented in the medical literature: reduced kidney function is associated with markedly higher cardiovascular risk, particularly in people with diabetes, in whom the small kidney blood vessels are often among the first affected by prolonged excess sugar.

This early kidney damage, sometimes silent for years, is a broader warning sign about the overall state of a person's vascular network — if the small kidney blood vessels are already affected, it's statistically likely that other blood vessels, particularly the coronary arteries, are affected too, to varying degrees. This is why severe kidney impairment (very low eGFR) even excludes the use of the SCORE2-Diabetes calculation: at that stage, risk is already considered high without needing a further calculation, and the situation calls for direct, close medical care.

A few numerical reference points for reading your results

For HbA1c. The result is expressed as a percentage, and sometimes in mmol/mol. Diabetes is defined by an HbA1c of 6.5% (48 mmol/mol) or higher under international guidelines. In someone with diabetes, the most common target is an HbA1c of 7% (53 mmol/mol) or below. The French National Authority for Health (HAS) suggests a stricter target, 6.5%, for recently diagnosed diabetes in someone with no cardiovascular disease, and a more flexible one, 8% or even 9%, in frail older adults, to avoid low blood sugar episodes. Be sure to check which unit appears on your lab report before entering the value.

The value of this control is well established. The large British UKPDS study, published in 2000 in the BMJ (Stratton et al.), showed that each one-point reduction in HbA1c was associated with about 14% fewer heart attacks and 37% fewer small-vessel complications.

For eGFR. It is estimated by the lab from blood creatinine levels, age, and sex, and is expressed in mL/min/1.73 m². An eGFR of 90 or above is normal. Between 60 and 89, the decline is mild and often of no consequence if urine tests are normal. Below 60 for more than three months, this is called chronic kidney disease: moderate between 30 and 59, severe between 15 and 29. eGFR naturally declines with age.

The kidney workup includes a second test, just as important in someone with diabetes: checking for albumin in the urine, expressed as the albumin-to-creatinine ratio. Its presence is one of the earliest signs of kidney damage, sometimes appearing while eGFR is still normal, and it too is associated with higher cardiovascular risk. The HAS recommends doing both tests at least once a year.

Age at diagnosis, a marker of exposure duration

Diabetes diagnosed at 35 doesn't carry the same weight in the calculation as diabetes diagnosed at 65, even at the same current age — how long the cardiovascular system has been exposed to the disease matters directly, following a logic similar to that seen with other cumulative risk factors like smoking. This is why this seemingly simple piece of data is one of the variables SCORE2-Diabetes takes into account, weighted more heavily the longer ago diagnosis occurred.

Where to find this information

These three values usually appear on a diabetes follow-up blood panel already done as part of standard medical care — HbA1c and eGFR are routine tests for someone with diabetes followed regularly, generally measured at least once or twice a year. If these values aren't recent or available, a doctor can easily order them at a follow-up appointment, with no complex or invasive testing — a standard blood draw is enough for both.

What to do if a value looks abnormal

If the tool's result flags an HbA1c or eGFR value outside the usual range, this isn't a diagnosis in itself, but an invitation to discuss it with the primary care doctor or endocrinologist managing the diabetes. These professionals can put the value in context within the full medical record, and adjust the treatment strategy if needed — something well beyond what a simple online calculator can offer.

Why these three variables rather than others

The choice of these three specific variables — age at diagnosis, HbA1c, eGFR — rather than other potentially relevant markers (such as the type of diabetes medication used, or the presence of certain minor complications) comes from rigorous statistical work aimed at identifying the elements with the most robust predictive power across large cohorts of people with diabetes followed over time. Other variables, while clinically relevant, haven't shown enough statistical contribution to justify including them in a model meant to be both precise and simple to use — two goals that are sometimes in tension when designing this kind of tool.

An assessment to repeat over time

As with the standard cardiovascular risk calculation, these three variables change over time — HbA1c in particular can vary significantly from one year to the next depending on changes in treatment and lifestyle. It's therefore worth repeating this calculation periodically, especially after a notable change in diabetes management (a change in treatment, improved or worsened blood sugar control), to have an up-to-date risk estimate rather than one based on old data.

A note of caution on the accuracy of reported values

The reliability of the result you get depends directly on the accuracy of the values entered — an approximate HbA1c or eGFR, or one from several years ago, will produce a less reliable estimate than recent, exact values. It's therefore recommended to refer to your latest blood panel rather than estimating these values from memory, and to repeat the assessment after a new panel if the previous one is more than a year old, a point beyond which blood sugar control or kidney function may have changed significantly without the person necessarily realizing it.

To get your own estimate, take the “What's my cardiovascular risk with my diabetes?” test.