The blood sugar test that misreads sickle cell carriers
A1C is how diabetes gets caught early. But it reads lower than it should in people with sickle cell trait, which is one in five Nigerians.
A1C is how diabetes gets caught early. But it reads lower than it should in people with sickle cell trait, which is one in five Nigerians.
Yemi is in the kitchen with her mother when the notification comes through. Test results ready to view. She takes her phone up to her room, away from mum, who might see her face and worry.
HbA1c: 6.4%.
Beside it, in a column marked Reference, it says <5.7%. So she knows her number is high. What she does not know is what high means, except that her mother takes a daily pill for her blood sugar, and her great uncle in Nigeria lost a toe to diabetes.
From Port Harcourt to Port of Spain, someone is sitting with the same screen and the same question. Type 2 diabetes arrives earlier in people of African heritage and it arrives more often. In the UK, African and Caribbean communities develop it at about three times the rate of white Europeans, roughly ten years younger. A recent pooling of 60 Nigerian studies put adult prevalence at 7 per cent, close to double what the International Diabetes Federation had estimated.
The test on Yemi's screen is the main tool for catching it early. Here is what it is doing.
Your blood gets its colour from a protein called haemoglobin. Haemoglobin sits inside your red blood cells and carries oxygen around your body.
Your blood also carries glucose, the sugar your body runs on. Glucose is sticky. When there is not much of it, it moves around freely. When there is a lot, it starts sticking to whatever it touches, and haemoglobin is right there.
So the more sugar in your blood, the more of your haemoglobin ends up coated in it. Take a sample, count what fraction of the haemoglobin is coated, and you have a measure of how sweet the blood has been. That coated fraction is glycated haemoglobin. HbA1c.
That test exists. A fasting blood glucose test measures it directly. The problem is that it only tells you about that morning, and mornings vary.
Red blood cells live about three months before your body retires them and makes new ones. So the coated fraction is a record that builds up over that whole period. A high A1C means the sugar has been high for a while, and no single bad breakfast can fake it.
In Nigeria, the US and most of the Caribbean, A1C is reported as a percentage. In the UK and much of Europe, it comes as mmol/mol. Same test, different scale, and the conversion is not intuitive: 6.5 per cent is 48 mmol/mol.
The thresholds differ too, and this matters more than the units.
The gap is real and it has consequences. Someone at 5.9 per cent is flagged as prediabetic under the American and international criteria and is told nothing under the UK ones. Yemi at 6.4 sits at the top of the range under both, one notch below a diabetes diagnosis.
Prediabetes is the part of this story where something can still be done, and the evidence for that is unusually good.
The Diabetes Prevention Program followed 3,234 adults with raised blood sugar and gave one group a structured programme of food and movement changes. Over about three years, that group's rate of going on to develop diabetes fell by 58 per cent. Metformin, the drug, managed 31 per cent. The programme beat the pill.
And unlike many of the trials that shape diabetes care, this one recruited beyond white participants. Forty-five per cent came from minority ethnic groups, African American among them.
So Yemi's number is not a sentence. It is early, and early is the whole point of the test.
Here is what a lab report will not tell you.
Haemoglobin is not identical in everyone, and some variants are far more common in African and Caribbean people. 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 it runs at 8 to 10 per cent.
Carrying the trait changes what your A1C reads. A 2017 study in JAMA followed 4,620 African American adults, 367 of them with sickle cell trait, and found their A1C sat lower than their actual blood sugar warranted. Applying the standard cut-offs to them turned up 40 per cent fewer cases of prediabetes and 48 per cent fewer cases of diabetes than the same cut-offs found in people without the trait.
Read that again. One in five Nigerians carries this, and in carriers the standard test misses roughly half the diabetes it should be catching.
The problem runs the other way too. Iron deficiency pushes A1C up, so an anaemic woman can get a number that overstates her blood sugar. Between the two, a great many African and Caribbean patients are being handed a figure that is quietly off.
A1C is still the best widely available screening test there is, and this is not a reason to distrust your result. It is a reason to tell your doctor what you carry. If you have sickle cell trait, or you are anaemic, or you do not know, say so, and ask whether a fasting glucose or an OGTT should sit alongside the A1C. A borderline number in a carrier deserves a second look.
Yemi puts her phone away. Downstairs, her mother is waiting, and her mother has been taking that daily pill for years without anyone ever explaining what the number on her own results meant.
Yemi has something her mother did not have at forty: a number, early, and time to use it.