Mmol/mol is the internationally standardized unit for reporting HbA1c, a blood test that reflects your average blood sugar over the previous two to three months. If you have received blood test results showing a number like 42 or 53 mmol/mol, that figure tells you how many hemoglobin molecules in your blood have glucose attached to them, expressed per million molecules of total hemoglobin. The unit replaced the older percentage system in many countries to improve laboratory accuracy and reduce confusion between labs, though the transition has left plenty of people staring at their results and wondering what the numbers actually mean.
What HbA1c Actually Measures
Hemoglobin is the protein inside red blood cells that carries oxygen. As blood circulates, glucose in the bloodstream naturally sticks to hemoglobin in a process called glycation. The higher your blood sugar runs over time, the more glucose attaches. Because red blood cells live for roughly 120 days, the amount of glycated hemoglobin in a blood sample acts as a rolling average of your blood sugar over the past two to three months. The test does not capture what happened yesterday or last week in isolation; it integrates glucose exposure across the full lifespan of your red blood cells.
The rate at which glucose binds to hemoglobin depends on both how high your blood sugar is and how long each red blood cell has been circulating. Researchers have calculated HbA1c levels from plasma glucose averages by accounting for the in vivo rate of hemoglobin glycation combined with the different turnover rates for red blood cells of different ages.1PubMed Central. Glycated Hemoglobin, Plasma Glucose, and Erythrocyte Aging Younger red blood cells have had less time to accumulate glucose, while older ones carry more. The test captures a weighted average of all of them.
Why the Unit Changed from Percentage to mmol/mol
For decades, HbA1c was reported as a percentage. A result of 6.5% meant that 6.5% of your hemoglobin had glucose attached. This system worked well enough in clinical practice, but it had a serious problem behind the scenes: different laboratories using different methods did not always agree on what that percentage actually was. Standardizing the test between laboratories proved difficult, and most assays were calibrated to the values used in a landmark diabetes trial rather than to a universal chemical standard.2The British Journal of Diabetes & Vascular Disease. HbA1c – changing units
To fix this, an international body developed a reference measurement procedure of higher accuracy. This method works by breaking hemoglobin into smaller pieces and measuring the glycated fragments directly using precise laboratory techniques. Pure glycated and non-glycated hemoglobin are mixed to calibrate the system, and the resulting measurements are traceable to a single worldwide standard.3PubMed Central. HbA1c: A Review of Analytical and Clinical Aspects The numbers this method produces are reported in mmol/mol rather than percent, partly to avoid confusion with the old system and partly to conform with standard scientific units.4Current Opinion in Endocrinology, Diabetes and Obesity. HbA1c : how do we measure it and what does it mean?
The practical upshot: when your lab reports HbA1c in mmol/mol, the number behind it is anchored to a more precise and globally consistent measurement than the old percentage ever was. The biology being measured has not changed at all. Only the ruler is different.
How to Convert Between mmol/mol and Percentage
Because the percentage system is still widely used in some countries, you will encounter both units. A rough mental shortcut for converting: subtract 2 from the percentage, then multiply by about 11. Going the other direction, divide the mmol/mol number by 11, then add 2. These approximations get you close enough for everyday understanding. The key reference points most people need are:
- 42 mmol/mol: approximately 6.0%
- 48 mmol/mol: approximately 6.5%
- 53 mmol/mol: approximately 7.0%
- 64 mmol/mol: approximately 8.0%
- 75 mmol/mol: approximately 9.0%
Both units remain in official use internationally. Reporting of HbA1c has been agreed using mmol/mol for the international standard and percent for the older system.5PubMed Central. HbA1c standardisation: history, science and politics In practice, many lab reports now show both numbers side by side, which helps during the transition period.
What the Numbers Mean for Your Health
The clinical thresholds in mmol/mol follow the same medical criteria that applied under the percentage system, just re-expressed in the new unit. The widely recognized cutoffs used by most health organizations are:
- Below 42 mmol/mol: considered normal, no sign of diabetes or pre-diabetes
- 42 to 47 mmol/mol: the pre-diabetes range, sometimes called “at risk” or “borderline”
- 48 mmol/mol or above: meets the diagnostic threshold for diabetes
If you are already living with diabetes, your doctor will typically set an individual target for you. For many adults with type 2 diabetes, the target sits around 48 to 53 mmol/mol, though this varies depending on age, other health conditions, and how long you have had diabetes. Tighter targets carry a lower risk of long-term complications but a higher risk of low blood sugar episodes, so the right number is a conversation between you and your clinician rather than a universal rule.
A result in the pre-diabetes range deserves attention even though it is not a diabetes diagnosis. That 42 to 47 mmol/mol zone signals that your body is already struggling to manage glucose efficiently. Lifestyle changes at this stage, particularly around diet and physical activity, can slow or prevent the progression to full diabetes in many people.
How HbA1c Relates to Daily Blood Sugar Readings
If you also monitor blood sugar with a finger-prick meter or a continuous glucose monitor, you might wonder how your HbA1c result connects to the numbers you see day to day. The relationship is strong but not perfect. Research using continuous glucose monitors shows that HbA1c correlates strongly with average glucose levels, with a correlation of about 0.80, and also correlates well with the amount of time spent in a healthy glucose range.6PubMed Central. Associations between HbA(1c) and continuous glucose monitoring-derived glycaemic variables However, HbA1c does a poor job of reflecting how often blood sugar dips too low. The correlation with time spent in hypoglycemia is weak, which makes sense: low blood sugar means less glucose sticking to hemoglobin, so frequent lows can actually drag your HbA1c down and make it look “better” than your day-to-day experience would suggest.
This is one reason continuous glucose monitors have become increasingly valued alongside HbA1c. They capture the swings, the lows, and the time in range that HbA1c smooths over. Think of HbA1c as the average grade on a semester report card, while daily glucose readings are the individual test scores. An average of 75% could mean consistently scoring 75, or it could mean alternating between 95 and 55. Both produce the same HbA1c, but the lived experience and health implications differ.
When HbA1c Results Can Be Misleading
HbA1c is a reliable test for most people most of the time, but certain situations can produce results that do not accurately reflect your real average blood sugar. Because the test depends on the normal behavior and lifespan of red blood cells, anything that changes how quickly red blood cells are made or destroyed will skew the result.7PubMed Central. Pitfalls in hemoglobin A1c measurement: when results may be misleading
Conditions that can cause falsely low HbA1c readings include:
- Blood loss or hemolysis: if you are losing red blood cells faster than normal (through bleeding, certain autoimmune conditions, or mechanical destruction), newer cells replace them. Younger cells have had less time to accumulate glucose, so HbA1c drops even if your blood sugar has not.
- Hemoglobin variants: some inherited hemoglobin types, including sickle cell trait and other variants more common in certain ethnic groups, can interfere with certain lab methods and produce inaccurate readings.
- Kidney disease: advanced kidney problems can shorten red blood cell lifespan and alter hemoglobin chemistry, making the test less reliable.
- Recent blood transfusion: donor blood introduces red blood cells with a different glycation history, temporarily distorting the average.
Conditions that can cause falsely high readings include iron deficiency anemia (red blood cells live longer when iron is scarce, giving glucose more time to attach) and certain less common hemoglobin variants. If your doctor suspects your HbA1c does not match your daily readings or clinical picture, alternative markers like fructosamine or glycated albumin can fill in the gaps. These tests measure glucose attachment to different proteins with shorter lifespans, giving a picture of blood sugar control over roughly two to three weeks rather than two to three months.
Special Considerations During Pregnancy
Pregnancy is one of the best-known situations where standard HbA1c thresholds should be interpreted cautiously. During pregnancy, the body ramps up red blood cell production significantly, especially in the first half. This flood of new, young red blood cells dilutes the proportion of glycated hemoglobin and pushes HbA1c readings down regardless of what is happening with blood sugar. A reduction in HbA1c before 20 weeks of gestation can be explained by increased new red blood cell production combined with lower fasting glucose levels that naturally occur early in pregnancy.8PubMed Central. Trimester-specific reference intervals for hemoglobin A1c in non-diabetic pregnancy in a Chinese population
Research into how well HbA1c tracks average glucose during pregnancy found that the relationship between the two remains strong but the numbers shift. A difference of about 11 mmol/mol in HbA1c corresponded to a smaller-than-expected difference in average glucose in pregnant women.9PubMed Central. Translating HbA(1c) measurements into estimated average glucose values in pregnant women with diabetes Clinicians managing diabetes during pregnancy therefore tend to rely more heavily on frequent blood sugar checks or continuous monitors and set trimester-specific HbA1c targets rather than applying the standard 48 mmol/mol threshold.
Point-of-Care Testing and Accuracy
You may have your HbA1c measured at your doctor’s office using a small portable device rather than having blood sent to a central laboratory. These point-of-care tests use a finger-prick blood sample and can return a result in minutes. The natural question is whether the quick version is as accurate as the full lab test.
Testing of point-of-care devices has shown strong agreement with reference laboratory methods. In one evaluation, results from finger-prick samples were very highly correlated with reference lab values, with a mean bias of only about negative 0.02 percentage points for finger-prick samples and essentially zero for venous samples.10PubMed Central. Accuracy and Precision of a Point-of-Care HbA1c Test In plain terms, the difference between a point-of-care result and a laboratory result is small enough to be clinically insignificant for most purposes. That said, when a result falls right on a diagnostic borderline, say 47 versus 48 mmol/mol, your doctor may want a confirmatory lab test before making a diagnosis. A small measurement wobble matters more when it crosses a threshold.
Why Some Countries Still Use Percentages
If mmol/mol is the internationally agreed standard, you might expect it to be universal by now. It is not. Adoption of the mmol/mol system has been slow outside of Europe. In much of Asia, the Americas, and other regions, HbA1c continues to be reported either only in percent or with both units listed side by side.11PubMed Central. Methods, units and quality requirements for the analysis of haemoglobin A1c in diabetes mellitus The UK, the Netherlands, and several other European countries switched fully to mmol/mol years ago, while the United States continues to report primarily in percent.
The slow transition has created a headache for anyone who moves between countries, reads international research, or looks up diabetes information online. A target of “below 7%” and a target of “below 53 mmol/mol” mean the same thing, but encountering both without context can make people worry their results have changed when only the unit has. If you relocate or see results from a different reporting system, the conversion reference points listed earlier in this article are worth bookmarking. The underlying measurement is the same; the packaging is what differs.
Why Your HbA1c and Your Meter Might Disagree
One of the most common frustrations people with diabetes encounter is a disconnect between their daily glucose readings and their HbA1c result. You might check your blood sugar four times a day, see numbers that look reasonable, and then get an HbA1c that seems too high. Or the opposite: you feel your control has been poor, but your HbA1c comes back looking fine.
Several factors explain this. Finger-prick tests capture snapshots, and unless you happen to test at your highest or lowest points, you can miss the peaks and valleys that glucose quietly climbs to between meals or overnight. A continuous glucose monitor removes this blind spot, which is one reason the correlation between continuous monitor data and HbA1c is stronger than the correlation between spot checks and HbA1c.6PubMed Central. Associations between HbA(1c) and continuous glucose monitoring-derived glycaemic variables
There is also genuine biological variation between individuals. Two people with identical average blood sugar can have measurably different HbA1c results because their red blood cells have slightly different lifespans or because glucose attaches to their hemoglobin at slightly different rates. Researchers have documented this “glycation gap” for years. It is not an error in the test; it reflects real person-to-person variation in how hemoglobin behaves. If your HbA1c consistently runs higher or lower than your daily readings would predict, your doctor can use alternative glycemic markers to check whether the discrepancy is clinically meaningful.
Conditions That Share the mmol/mol Unit
If you see mmol/mol on a lab report, it almost certainly refers to HbA1c. But mmol/mol is not an HbA1c-specific unit; it is a general-purpose way of expressing a ratio of one molecule to another. Other laboratory tests occasionally use the same unit for different purposes, most commonly when reporting a substance’s concentration relative to creatinine in urine samples. You are unlikely to confuse the two in practice because the context on a lab report makes the test identity clear, but it is worth knowing that mmol/mol simply means “millimoles of substance X per mole of substance Y.” In the case of HbA1c, substance X is glycated hemoglobin and substance Y is total hemoglobin.
This is also why the mmol/mol numbers for HbA1c are so much larger than the old percentages. A “normal” HbA1c of about 5.5% translates to roughly 37 mmol/mol. The jump from a single-digit percentage to a double-digit mmol/mol figure alarms some people when they first see it, but the two numbers describe the same degree of glycation. The scale is just different, the way that a temperature of 72°F sounds a lot higher than 22°C even though they describe identical warmth.
Practical Tips for Reading Your Results
If you have just received a lab report in mmol/mol and are not sure what to do with it, a few practical points can help. First, look for whether the report also shows the percentage equivalent. Many labs now include both, which saves you the conversion math. If only mmol/mol is given, the reference points in the conversion section above cover the most clinically important thresholds.
Second, track trends rather than fixating on a single number. A one-off HbA1c reading is a useful piece of information, but two or three readings over six to twelve months are far more telling. A slow upward drift from 40 to 44 to 47 mmol/mol over a year paints a clearer picture of where your glucose management is heading than any single test. Third, remember that the test reflects an average. If you have recently made significant dietary or medication changes, it takes about six to eight weeks for the HbA1c to begin reflecting those changes fully, and a full three months for the old glucose exposure to cycle out of your red blood cells entirely. Checking too soon after a lifestyle change will undercount your progress.
Finally, if your results seem inconsistent with your experience or with other measurements, bring that up with your doctor rather than dismissing either data source. The situations described earlier where red blood cell behavior skews HbA1c are well-characterized, and your clinician can order additional tests to determine whether your HbA1c is giving an accurate picture or whether an alternative marker would serve you better.