Borderline HbA1c levels signal rising cardiovascular risk and need for early intervention

A HbA1c reading of 6.4%, just below the diabetes threshold, highlights the importance of early detection and intervention for prediabetes, as recent research links such levels with increased cardiovascular risk and challenges the perception of near-normal health.

An HbA1c reading of 6.4% sits only a tenth of a percentage point below the formal cut-off for diabetes, but leading clinical guidance does not treat it as a near-normal result that can be brushed aside. In the American Diabetes Association’s 2026 Standards of Care, 5.7% to 6.4% is classed as prediabetes, and that range identifies people at high risk of both future diabetes and cardiovascular problems. The association adds that risk is “continuous and curvilinear”, meaning it rises across the range rather than appearing abruptly at 6.5%. (diabetesjournals.org)

That matters because HbA1c is not a one-off snapshot. The CDC says the test reflects average blood sugar over roughly the previous three months and, unlike fasting glucose, does not require fasting beforehand. The National Institute of Diabetes and Digestive and Kidney Diseases says that if someone has no obvious symptoms, a diabetes or prediabetes result should usually be confirmed on a different day, and that point-of-care tests used in surgeries or clinics should not be used to make the diagnosis. (cdc.gov)

The closer the figure gets to 6.5%, the more serious the warning becomes. Evidence cited in the ADA’s 2026 standards says people with HbA1c levels from 6.0% to 6.5% face a five-year risk of developing diabetes of between 25% and 50%, with a relative risk around 20 times that of someone with an HbA1c of 5.0%. NIDDK makes the same point more plainly: within the prediabetes band, the higher the HbA1c, the greater the risk of type 2 diabetes. (diabetesjournals.org)

Recent research suggests the concern is not limited to whether diabetes eventually develops. Kaiser Permanente Northern California said in March 2026 that a study of more than 1.3 million adults aged 18 to 90, none with a prior diagnosis of diabetes or heart disease, recorded 28,615 atherosclerotic cardiovascular events over five years. Adults with prediabetes had a 21% higher cardiovascular risk than those with HbA1c below 5.7%. Lead author Jamal S. Rana said the findings indicate that “physiological changes are already underway” before diabetes is diagnosed. Kaiser also noted that more than 96 million adults in the US are estimated to have prediabetes. (divisionofresearch.kaiserpermanente.org)

The Kaiser data also suggested that the danger is not evenly distributed. About 31% of participants had obesity, and the combination of obesity and prediabetes was linked to a 32% higher cardiovascular risk, compared with 22% for prediabetes alone and 10% for obesity alone. Among adults aged 18 to 34, prediabetes was associated with a 54% higher cardiovascular risk; among those aged 35 to 44, the increase was 26%. Separate work published through UK Biobank in March 2026, analysing 278,697 adults over 13.5 years, also found higher cardiovascular risk under both ADA criteria and the stricter World Health Organization/International Expert Committee definition. An exploratory cardiac MRI analysis in that study suggested early concentric remodelling of the left ventricle. (divisionofresearch.kaiserpermanente.org)

Longer follow-up points in the same direction. A 2025 Diabetologia study using 10,310 participants in the Atherosclerosis Risk in Communities cohort followed adults for about 30 years. Roughly 60% had prediabetes at the outset, and about 30% later progressed to diabetes, with a median time to diagnosis of seven years. But that progression did not explain most of the harm. After the researchers accounted for people who later developed diabetes, 85% of the excess risk for any major complication and 81% of the excess atherosclerotic cardiovascular risk remained. The study concluded that progression to diabetes explained less than one-quarter of the clinical risk linked to prediabetes. (pubmed.ncbi.nlm.nih.gov)

None of that means HbA1c is infallible. The ADA, CDC and NIDDK all warn that the test can mislead in some circumstances, including pregnancy, haemoglobin variants, severe anaemia, kidney failure, liver disease, recent blood loss or transfusion, and other conditions that change red-cell survival. NIDDK also says some people of African, Mediterranean or Southeast Asian descent may carry haemoglobin variants that interfere with certain assays. When an HbA1c result does not fit the wider clinical picture, doctors may rely on fasting plasma glucose or an oral glucose tolerance test instead. (diabetesjournals.org)

So 6.4% is not a diabetes diagnosis, but neither is it a clean bill of health. The CDC says people whose result falls in the prediabetes range are often advised to repeat the test every one to two years, while the ADA says they should be told about their increased risk and counselled on ways to reduce it. The practical point is straightforward: a borderline HbA1c is a prompt for earlier action, not a reason to wait for a worse number. (cdc.gov)