An A1C test reports roughly three months of blood sugar in a single number. Sickle cell trait, carried by about one in five Nigerians, makes that number read lower than it should.
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A1C, written as HbA1c on most lab reports, measures the proportion of your haemoglobin that has glucose attached to it.
Haemoglobin is the protein inside red blood cells that carries oxygen and gives blood its colour. Glucose, the sugar your body runs on, circulates freely in the blood. But glucose is sticky. So when there is more glucose in the blood, more of it sticks to haemoglobin and stays there until the red blood cell dies.
Red blood cells live for around three to four months. So if you measured what fraction of haemoglobin carries glucose, you would have an accurate estimate of how much sugar has been in the blood for the past few weeks to months. That fraction is called glycated haemoglobin.
A fasting or random blood sugar test measures sugar directly, at one moment. Blood sugar moves constantly with food, illness, stress and sleep, so a single reading may not reflect your long-term health.
An A1C result reflects average blood sugar across roughly the previous three months, weighted toward the most recent weeks.
This has two practical consequences. A high A1C cannot be explained away by one heavy meal. And changes you make now begin to show in the number within about six weeks.
Different regions of the world use different units for A1C.
| Region | Unit |
|---|---|
| Nigeria, United States, most of the Caribbean | Percentage (%) |
| United Kingdom, much of Europe | mmol/mol |
The conversion is not intuitive. An A1C of 6.5 per cent is 48 mmol/mol.
| Category | Percentage | mmol/mol |
|---|---|---|
| Diabetes | 6.5% and above | 48 and above |
| Prediabetes, American and international criteria | 5.7% to 6.4% | 39 to 47 |
| Prediabetes, UK guidance | 6.0% to 6.4% | 42 to 47 |
The diabetes threshold of 6.5 per cent is consistent worldwide. The prediabetes floor is not. The American Diabetes Association sets it at 5.7 per cent, which is why most Nigerian, American and Caribbean lab reports print a reference range of "under 5.7%". UK guidance sets it at 42mmol/mol or 6.0 per cent.
A result of 5.9 per cent would be flagged as prediabetes in Lagos or Kingston and would attract no comment in London. A result of 6.4 per cent sits at the top of the prediabetes range under both standards, one step below a diabetes diagnosis.
An A1C in the prediabetes range means blood sugar has been running above normal for months. It is a warning with time attached to it, and the evidence on acting early is unusually strong.
The Diabetes Prevention Program enrolled 3,234 adults with raised blood sugar and gave one group a structured programme of food and activity changes. Over about three years, that group's rate of progressing to type 2 diabetes fell by 58 per cent. Metformin, the drug, achieved 31 per cent.
That trial also matters for who was in it. Forty-five per cent of participants came from minority ethnic groups, including African American participants, which is not true of much of the diabetes evidence base.
Haemoglobin is not identical in everyone, and several variants are far more common in African and Caribbean populations. The most important for A1C is sickle cell trait, the carrier state for sickle cell disease.
A systematic review of nearly 212,000 Nigerian children and adolescents found sickle cell trait in 21 per cent, roughly one in five. Among African Americans, the American Society of Hematology puts it at 8 to 10 per cent.
Carrying sickle cell trait shortens red blood cell survival slightly, which gives glucose less time to attach. The A1C comes back lower than the person's actual blood sugar warrants.
A 2017 study published in JAMA followed 4,620 African American adults, 367 of whom carried sickle cell trait. Applying standard A1C cut-offs to carriers identified 40 per cent fewer cases of prediabetes and 48 per cent fewer cases of diabetes than the same cut-offs identified in people without the trait.
One in five Nigerians carries this variant, and in carriers the standard test misses roughly half the diabetes it should be detecting.
Iron deficiency raises A1C, producing a result higher than the person's blood sugar justifies. Iron deficiency anaemia is common among women of reproductive age in Nigeria and across the Caribbean.
Two sources of error, running in opposite directions, both concentrated in the same populations.
A1C remains the most widely available screening test for type 2 diabetes, and these limitations are a reason to interpret it carefully rather than to distrust it.
If you are of African or Caribbean descent, raise the following at the appointment.
A borderline A1C in someone who carries sickle cell trait deserves a second test, not a reassurance.
Often yes, particularly in the prediabetes range. In the Diabetes Prevention Program, food and activity changes cut progression to diabetes by 58 per cent, outperforming metformin at 31 per cent.
No. It affects how accurately the A1C test measures your blood sugar. It does not change your blood sugar itself.
No. A fasting glucose test measures blood sugar at one moment. An A1C reflects roughly three months, because it measures sugar bound to red blood cells that live about that long.
No. A result of 6.4 per cent sits at the top of the prediabetes range. The diabetes threshold is 6.5 per cent, and a diagnosis normally requires a second abnormal test.
That depends on your result and your risk factors, and it is a decision for your doctor. People in the prediabetes range are commonly retested each year.