Nursing care
Hemoglobin A1C: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 6 min read · Updated September 2026
Short answer
Hemoglobin A1C measures the percentage of haemoglobin coated with glucose, reflecting average blood glucose over the preceding two to three months. Because it captures a long window, it cannot be improved by a few days of tighter control before the test, unlike a fasting glucose reading. Normal is below 5.7%, with 5.7-6.4% indicating prediabetes and 6.5% or above indicating diabetes.
What the test measures
Glucose in the bloodstream binds to haemoglobin inside red blood cells in a slow, non-enzymatic process called glycation. The more glucose circulating over time, the more haemoglobin becomes glycated, and HbA1c reports that fraction as a percentage. Because red blood cells live for roughly 120 days, the result is weighted toward glucose exposure over the most recent 8 to 12 weeks, with the most recent month contributing more than the earliest.
This is what separates HbA1c from a fasting or random glucose check, both of which capture a single point in time. A patient can present a perfectly normal fasting glucose on the morning of an appointment after weeks of poor control, but HbA1c will not move in response to that one good week, because it reflects the cumulative average, not the most recent value. That is also why HbA1c is unreliable in conditions that shorten red blood cell lifespan, such as haemolytic anaemia or recent significant blood loss, where the result can read falsely low regardless of actual glucose control.
Normal ranges and what moves them
A normal HbA1c is below 5.7%. Prediabetes falls in the range of 5.7% to 6.4%, and a result of 6.5% or higher on two separate occasions, or on one occasion with classic hyperglycaemic symptoms, meets the diagnostic criterion for diabetes. In patients with an established diagnosis, most treatment targets aim for below 7%, though this is individualised and often relaxed to below 8% for older adults or those with significant comorbidities where tight control raises hypoglycaemia risk.
Sustained elevated blood glucose is what drives the number up, but conditions affecting red blood cell turnover move it independently of glucose. Anything that shortens red blood cell lifespan, including haemolytic anaemia, chronic kidney disease, or recent transfusion, tends to lower HbA1c artificially, since cells are not circulating long enough to accumulate glycation. Iron deficiency anaemia and conditions that extend red blood cell lifespan can push the result artificially high. Pregnancy also alters interpretation, and gestational diabetes is diagnosed with glucose tolerance testing rather than HbA1c.
What a high result means
An elevated HbA1c confirms that average glucose has been running high for weeks to months, not that a single bad day or missed medication dose caused it. In a newly diagnosed patient, a result of 6.5% or above establishes the diabetes diagnosis and prompts the start of lifestyle counselling and, typically, metformin as first-line therapy. In a patient with an existing diagnosis, a rising HbA1c signals that the current regimen is no longer adequate and usually triggers medication adjustment, addition of a second agent, or closer review of adherence and diet.
A very high result, above roughly 10%, correlates with an average glucose well above 240 mg/dL and often prompts a more urgent treatment escalation, sometimes including insulin initiation even in type 2 diabetes, because oral agents alone are unlikely to close that gap quickly. It is worth remembering that a single elevated HbA1c describes a trend, not a crisis; it does not by itself indicate diabetic ketoacidosis or hyperosmolar state, which are diagnosed from acute presentation and point-of-care glucose, not from HbA1c.
What a low result means
A low or falling HbA1c in a patient on treatment usually reflects improved glucose control and is the intended outcome of therapy, but a result that has dropped sharply and rapidly deserves a second look rather than automatic reassurance. Rapid drops can reflect genuine improvement, but they can also reflect recent significant blood loss, haemolysis, or a condition shortening red blood cell survival, any of which produces a falsely reassuring number that does not match the patient's actual glucose pattern.
In a patient not being treated for diabetes, a low HbA1c is not itself a clinical problem, but an unexpectedly low result in someone with known anaemia, chronic kidney disease, or a haemoglobinopathy should prompt correlation with fasting glucose or self-monitoring logs before the number is trusted on its own. Nurses should also be alert to overly aggressive treatment producing a low HbA1c in an older adult, where the corresponding hypoglycaemia risk may outweigh the benefit of tight control.
Nursing actions by result
For a newly elevated result meeting diagnostic criteria, the nursing role centres on initiating diabetes education: glucose monitoring technique, recognising hyperglycaemia and hypoglycaemia, dietary counselling, and medication teaching once therapy is prescribed. Document baseline knowledge and readiness to learn, since the education plan for a newly diagnosed patient differs substantially from a refresher for someone with years of established diabetes.
For a rising result in a treated patient, review adherence honestly with the patient before assuming the regimen has failed, since missed doses or dietary changes are common and reversible causes. For a result that seems inconsistent with the patient's self-monitoring log, or that is unexpectedly low in someone with anaemia or renal disease, escalate to the provider and anticipate a fructosamine test or direct correlation with glucose logs, since HbA1c may not be reliable in that patient. In all cases, HbA1c results inform the care plan; they do not replace point-of-care glucose checks for immediate treatment decisions.
Patient preparation and teaching
HbA1c requires no fasting and can be drawn at any time of day, which is worth telling patients directly since many arrive having fasted unnecessarily, sometimes to the point of feeling unwell before the blood draw. A single venous sample is usually sufficient, though some settings use a point-of-care fingerstick device for a faster result during the same visit.
The teaching point that matters most is also the one patients resist hearing: this test cannot be gamed by a good week before the appointment. Because it reflects roughly three months of average glucose, a patient who tightens their diet only in the days before testing will see little to no change in the result, and explaining that mechanism honestly, rather than letting the patient believe short-term effort will show up, builds more realistic expectations and better long-term engagement with the treatment plan. Testing frequency is typically every three to six months, timed to the red blood cell turnover the test relies on.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our endocrine practice questions are the closest set to what this page covers.
Common questions
How often should HbA1c be checked?
Every three to six months is standard, and testing more often than every three months adds little value because the result reflects a rolling three-month average that has not had time to shift meaningfully. Patients with newly adjusted therapy or unstable control are usually checked at the three-month end of that range.
Does HbA1c require fasting?
No. It can be drawn at any time of day regardless of when the patient last ate, since it measures cumulative glycation rather than a point-in-time glucose level. This is worth clarifying with patients who arrive having fasted unnecessarily.
Why would a patient with well-controlled daily glucose readings still have a high HbA1c?
Self-monitoring often captures only a few points a day, commonly fasting and pre-meal, and can miss sustained post-meal spikes or overnight highs. HbA1c integrates glucose exposure across the entire day and night over three months, so it can reveal patterns that a limited monitoring schedule misses.
Is HbA1c accurate in patients with anaemia?
Not reliably. Conditions that shorten red blood cell lifespan, such as haemolytic anaemia or chronic kidney disease, can produce a falsely low result, while iron deficiency anaemia can push it falsely high. In these patients, correlate the result with glucose logs or consider a fructosamine test, which reflects a shorter window and is not affected by red blood cell lifespan.
What HbA1c value confirms a diabetes diagnosis?
A result of 6.5% or higher on two separate tests, or on one test in a patient with classic symptoms of hyperglycaemia such as polyuria, polydipsia, and unexplained weight loss, meets the diagnostic threshold. A result between 5.7% and 6.4% indicates prediabetes rather than diabetes.