HbA1c in sickle cell disease is widely recognised as an unreliable marker for diagnosing and monitoring diabetes, yet many patients and clinicians remain unaware of the implications. In people with sickle cell disease (SCD), abnormal haemoglobin variants and significantly shortened red blood cell survival cause HbA1c to read falsely low — even when blood glucose is dangerously elevated. This can lead to missed diagnoses, under-treatment, and serious long-term complications. Understanding why HbA1c fails in SCD, and which alternative tests are recommended under NICE and NHS guidance, is essential for safe, accurate diabetes care in this population.
Summary: HbA1c is unreliable in sickle cell disease because shortened red blood cell survival causes falsely low readings, meaning alternative tests such as fasting plasma glucose, OGTT, fructosamine, or continuous glucose monitoring must be used instead.
- Sickle cell disease causes red blood cells to survive only 10–20 days (versus 90–120 days normally), reducing time for haemoglobin glycation and producing falsely low HbA1c results.
- NICE (NG28) and WHO guidance state that HbA1c should not be used to diagnose diabetes in people with haemoglobinopathies; fasting plasma glucose or OGTT should be used instead.
- Blood transfusions and hydroxycarbamide therapy, both common in SCD management, further distort HbA1c values and make interpretation even less reliable.
- Fructosamine and glycated albumin reflect glycaemic control over two to three weeks and are not affected by red cell lifespan, making them useful monitoring alternatives in SCD.
- SGLT2 inhibitors carry an MHRA-flagged risk of euglycaemic DKA and require particular caution in SCD due to dehydration risk; sick-day rules must be followed.
- Annual diabetes reviews in SCD should include renal function (eGFR), blood pressure, foot health, and eye screening, coordinated between haematology and diabetes teams.
Table of Contents
- Why HbA1c Is Unreliable in Sickle Cell Disease
- How Sickle Cell Disease Affects Red Blood Cell Turnover
- Alternative Tests for Monitoring Blood Glucose Control
- NICE and NHS Guidance on Diabetes Diagnosis in Sickle Cell Disease
- Managing Diabetes Safely Alongside Sickle Cell Disease
- When to Seek Specialist Advice from Your Clinical Team
- Frequently Asked Questions
Why HbA1c Is Unreliable in Sickle Cell Disease
HbA1c is unreliable in sickle cell disease because shortened red cell survival reduces glycation time, producing falsely low results; both NICE and WHO recommend alternative diagnostic tests in people with haemoglobinopathies.
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HbA1c (glycated haemoglobin) is the standard test used to diagnose and monitor diabetes in the general population. It measures the percentage of haemoglobin that has been glycated — that is, bound to glucose — over the preceding two to three months, reflecting average blood glucose levels during that period. However, in people with sickle cell disease (SCD), this test is widely recognised as unreliable and can produce misleading results.
The core problem lies in the nature of sickle cell disease itself. People with SCD have abnormal haemoglobin variants (most commonly HbS, but also HbC or HbS combined with beta-thalassaemia, depending on genotype — for example HbSS, HbSC, or HbSβ-thalassaemia) and their red blood cells have a significantly shortened lifespan. Because HbA1c reflects glycation over the life of a red blood cell, a shorter red cell survival time means there is less time for glycation to accumulate — leading to falsely low HbA1c readings even when blood glucose levels are actually elevated.
In addition to this biological effect, some HbA1c assay methods can produce spuriously high or low results in the presence of haemoglobin variants such as HbS or elevated foetal haemoglobin (HbF). Clinicians should liaise with their local laboratory to understand which assay method is in use and how it may be affected by haemoglobin variants. UK NEQAS and RCPath provide advisory guidance on method-dependent interference in haemoglobin variant states.
It is important to note that sickle cell trait (carrying one copy of the HbS gene, i.e. HbAS) is a different condition from sickle cell disease. People with sickle cell trait generally do not have significantly shortened red cell survival, and HbA1c reliability in this group depends largely on the assay method used; local laboratory advice should be sought.
The clinical significance of HbA1c unreliability in SCD includes:
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A falsely low HbA1c may lead to missed or delayed diagnosis of diabetes or pre-diabetes
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It may give a false impression that blood glucose is well controlled in someone already diagnosed with diabetes
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Clinical decisions based on inaccurate HbA1c values could result in under-treatment and increased risk of long-term complications
Both NICE (NG28) and WHO guidance acknowledge that HbA1c is not appropriate for diagnosing diabetes in people with haemoglobinopathies, and that alternative testing methods must be used. Healthcare professionals should be aware of this important limitation, and patients should be informed that standard HbA1c-based screening may not apply to them.
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How Sickle Cell Disease Affects Red Blood Cell Turnover
In sickle cell disease, red blood cells survive only approximately 10–20 days rather than the normal 90–120 days, causing disproportionately low HbA1c values regardless of actual blood glucose levels.
To understand why HbA1c is unreliable in sickle cell disease, it is helpful to understand how the condition affects red blood cells at a physiological level. In healthy individuals, red blood cells (erythrocytes) survive for approximately 90 to 120 days. During this time, haemoglobin gradually becomes glycated in proportion to the ambient blood glucose concentration, and the resulting HbA1c level provides a reliable retrospective measure of glycaemic control.
In sickle cell disease, the abnormal HbS polymerises under low-oxygen conditions, causing red blood cells to adopt the characteristic rigid, sickle shape. These misshapen cells are fragile and are removed from circulation far more rapidly than normal red cells — typically surviving approximately 10 to 20 days, though this is an approximation and varies considerably between individuals, genotypes, and clinical state. This accelerated haemolysis (destruction of red blood cells) is a hallmark feature of SCD and contributes to the chronic anaemia seen in affected individuals.
Because the red cells are destroyed so quickly, there is simply less time for haemoglobin to become glycated, regardless of how high blood glucose levels may be. The result is a disproportionately low HbA1c relative to actual glycaemic exposure.
Several additional factors further complicate HbA1c interpretation in SCD:
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Blood transfusions, which are commonly used in SCD management, introduce donor red cells containing normal HbA, further distorting the HbA1c result. NICE advises that HbA1c should not be used for diagnosis within at least two months of a blood transfusion, and results may remain unreliable for longer depending on transfusion frequency.
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Hydroxycarbamide (hydroxyurea) therapy, used to increase foetal haemoglobin (HbF) production, alters haemoglobin composition. Elevated HbF can affect certain HbA1c assay methods (particularly immunoassay-based platforms), and hydroxycarbamide also influences red cell kinetics. Clinicians should consult local laboratory guidance and refer to British Society for Haematology (BSH) guidance on hydroxycarbamide in SCD for further detail.
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Reticulocytosis (an elevated proportion of young red cells in circulation) further reduces the average red cell age and skews HbA1c results downward.
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Assay method dependency: whilst some methods (such as HPLC or capillary electrophoresis) are better at separating haemoglobin variants, the underlying biological problem of shortened red cell survival means HbA1c remains unreliable in SCD regardless of the assay used.
These combined factors make HbA1c an inherently flawed marker in this population, necessitating the use of alternative glycaemic monitoring strategies.
Alternative Tests for Monitoring Blood Glucose Control
Fructosamine, glycated albumin, and continuous glucose monitoring (CGM) are the recommended alternatives to HbA1c in SCD; fasting plasma glucose or OGTT should be used for formal diabetes diagnosis.
Given the well-established limitations of HbA1c in sickle cell disease, clinicians and patients need to be aware of the alternative tests available for assessing glycaemic control. Several options exist, each with their own advantages and limitations.
Fructosamine is one of the most commonly used alternatives. It measures glycated serum proteins — primarily albumin — and reflects average blood glucose over the preceding two to three weeks. Because it is not dependent on red blood cell lifespan, it avoids the haemolysis-related distortion seen with HbA1c. However, fructosamine results can be affected by conditions that alter serum protein levels, such as hypoalbuminaemia, nephrotic syndrome, or liver disease, which may also be relevant in some patients with SCD. Fructosamine is a monitoring tool and is not validated for the diagnosis of diabetes; reference intervals and clinical interpretation should be discussed with the local laboratory.
Glycated albumin is a more specific measure of albumin glycation and offers a similar two- to three-week window of glycaemic information. It is increasingly used in specialist centres and may offer advantages over fructosamine in certain clinical contexts, though it is not yet universally available across NHS laboratories. Like fructosamine, it is affected by conditions altering albumin turnover and is not a diagnostic test.
Continuous glucose monitoring (CGM) and self-monitored blood glucose (SMBG) using fingerprick testing provide real-time glucose data and are particularly valuable for day-to-day management in patients with both SCD and diabetes. Intermittently scanned CGM (isCGM) and real-time CGM (rtCGM) devices are available on NHS prescription for eligible patients, subject to NICE criteria (set out in NICE NG28 for type 2 diabetes and NICE NG17 for type 1 diabetes) and NHS England access policies. These devices can provide detailed glucose trend data without relying on haemoglobin-based markers. Eligibility criteria should be discussed with the clinical team.
In practice, a combination of approaches is often recommended:
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Fructosamine or glycated albumin for medium-term monitoring
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CGM or SMBG for short-term and daily glucose assessment
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Fasting plasma glucose or oral glucose tolerance testing (OGTT) for diagnostic purposes (see below)
The choice of monitoring method should be individualised, agreed between the patient and their clinical team, and informed by local laboratory capability and specialist input.
NICE and NHS Guidance on Diabetes Diagnosis in Sickle Cell Disease
NICE NG28 and WHO guidance recommend fasting plasma glucose (≥7.0 mmol/L) or a two-hour OGTT (≥11.1 mmol/L) for diagnosing diabetes in people with haemoglobinopathies, as HbA1c is not appropriate.
NICE guidance on the management of type 2 diabetes in adults (NG28) acknowledges that HbA1c is not appropriate for diagnosing diabetes in people with haemoglobinopathies, including sickle cell disease. In these circumstances, NICE and WHO recommend that fasting plasma glucose or a two-hour oral glucose tolerance test (OGTT) should be used for diagnosis instead of HbA1c.
The NHS also recognises this limitation through its specialist haemoglobinopathy services and diabetes care pathways. NHS England's guidance on haemoglobinopathy care highlights the importance of multidisciplinary working between haematology, diabetes, and primary care teams to ensure that patients with SCD receive appropriate and accurate metabolic monitoring.
For diagnostic purposes, the following criteria apply when using plasma glucose-based tests, consistent with WHO (2011) and NICE standards:
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Fasting plasma glucose ≥7.0 mmol/L on two separate occasions (or once if symptomatic)
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Two-hour plasma glucose ≥11.1 mmol/L following a 75 g oral glucose load
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Random plasma glucose ≥11.1 mmol/L in the presence of classic symptoms of hyperglycaemia
In asymptomatic individuals, a single abnormal result should be confirmed with a repeat test on a separate day before a diagnosis is made.
The following thresholds indicate high-risk (pre-diabetic) states that warrant monitoring and lifestyle intervention:
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Impaired fasting glucose (IFG): fasting plasma glucose 6.1–6.9 mmol/L
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Impaired glucose tolerance (IGT): two-hour plasma glucose 7.8–11.0 mmol/L following a 75 g oral glucose load
It is important to note that there is no specific NICE guideline dedicated solely to diabetes in sickle cell disease, and recommendations are largely extrapolated from broader guidance on haemoglobinopathies and diabetes diagnosis. Clinicians are advised to exercise clinical judgement and refer to specialist haematology and diabetes teams when uncertainty exists. Patients should be informed that standard HbA1c-based screening does not apply to them, and that alternative testing is both appropriate and necessary for their care.
| Issue / Factor | Effect on HbA1c in SCD | Clinical Risk | Recommended Alternative |
|---|---|---|---|
| Shortened red cell survival (10–20 days vs 90–120 days) | Falsely low HbA1c due to reduced glycation time | Missed or delayed diabetes diagnosis; under-treatment | Fasting plasma glucose or OGTT for diagnosis |
| Haemoglobin variants (HbS, HbC, HbSβ-thalassaemia) | Assay-dependent spuriously high or low results | Inaccurate glycaemic assessment regardless of assay | Liaise with local laboratory; use HPLC or capillary electrophoresis where possible |
| Blood transfusions (common in SCD management) | Donor HbA distorts HbA1c result | Unreliable for ≥2 months post-transfusion; longer if frequent | Fructosamine or glycated albumin for monitoring |
| Hydroxycarbamide (hydroxyurea) therapy | Elevated HbF interferes with immunoassay-based platforms; alters red cell kinetics | Further distortion of HbA1c accuracy | Consult local laboratory; refer to BSH guidance on hydroxycarbamide in SCD |
| Reticulocytosis | Higher proportion of young red cells lowers average red cell age, skewing HbA1c downward | Underestimation of glycaemic exposure | Fructosamine or continuous glucose monitoring (CGM) |
| Diabetes diagnosis (NICE NG28 / WHO 2011) | HbA1c explicitly not recommended for diagnosis in haemoglobinopathies | Diagnostic error if HbA1c used as per standard pathway | Fasting plasma glucose ≥7.0 mmol/L or 2-hr OGTT ≥11.1 mmol/L |
| Ongoing glycaemic monitoring | HbA1c unreliable for medium- or long-term monitoring in SCD | False reassurance of good control; risk of complications | Fructosamine or glycated albumin (2–3 weeks); CGM or SMBG for daily assessment |
Managing Diabetes Safely Alongside Sickle Cell Disease
Diabetes management in SCD requires individualised pharmacological choices, with caution around metformin in renal impairment and SGLT2 inhibitors due to DKA and dehydration risk; glycaemic monitoring should use fructosamine, glycated albumin, or CGM rather than HbA1c.
Managing diabetes in a person with sickle cell disease requires careful consideration of both conditions simultaneously, as each can influence the other in clinically meaningful ways. Hyperglycaemia is associated with oxidative stress and endothelial dysfunction, which may compound the vascular complications already associated with SCD, including stroke, acute chest syndrome, and renal impairment. Conversely, the physiological stresses of sickle cell crises — including dehydration, infection, and hypoxia — can destabilise blood glucose control.
Lifestyle measures remain the foundation of diabetes management and are appropriate for all patients, including those with SCD:
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A balanced, low-glycaemic diet to support stable blood glucose levels
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Regular, appropriate physical activity, taking care to avoid known triggers for sickle cell crises (such as extreme cold, dehydration, or overexertion)
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Adequate hydration, which is important for both glycaemic control and sickle cell management
When pharmacological treatment is required, the choice of agent should be made carefully in discussion with the clinical team:
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Metformin remains a first-line option for type 2 diabetes in many patients, but should be used with caution in those with renal impairment — a common complication of SCD. The Summary of Product Characteristics (SmPC) and NICE NG28 provide guidance on eGFR thresholds for dose adjustment and discontinuation. Metformin should generally be withheld during acute severe illness, significant dehydration, hypoxia, or before procedures involving iodinated contrast media; patients should be given clear sick-day guidance by their clinical team.
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SGLT2 inhibitors (such as dapagliflozin and empagliflozin) carry a risk of dehydration and urinary tract infections, which may be particularly problematic in SCD. Importantly, the MHRA has issued Drug Safety Updates warning of a risk of diabetic ketoacidosis (DKA) with SGLT2 inhibitors, which can occur even when blood glucose is not markedly elevated (euglycaemic DKA). SGLT2 inhibitors should be withheld during acute illness, dehydration, surgical procedures, or prolonged fasting (sick-day rules); patients should be counselled on this by their prescriber. Renal function thresholds for use are specified in the relevant SmPCs.
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There is currently no specific MHRA or NICE contraindication to particular antidiabetic agents solely on the basis of SCD, but individual risk assessment and specialist input are essential.
Monitoring should rely on the alternative glycaemic markers discussed above (fructosamine, glycated albumin, or CGM/SMBG) rather than HbA1c. Regular review of renal function (eGFR), blood pressure, and cardiovascular risk is also recommended, given the overlapping complications of both conditions.
If you experience any suspected side effects from your diabetes medicines, you can report these to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk.
When to Seek Specialist Advice from Your Clinical Team
People with SCD and diabetes should seek urgent assessment for symptoms of DKA or HHS, and should ask their GP or haematologist to review whether fructosamine, glycated albumin, or CGM is more appropriate than HbA1c for their monitoring.
People living with sickle cell disease who also have — or are at risk of — diabetes should have access to a coordinated, multidisciplinary clinical team. Given the complexity of managing both conditions together, there are several situations in which seeking prompt specialist advice is particularly important.
Contact your GP or specialist team if you notice:
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Symptoms suggestive of high blood glucose, such as increased thirst, frequent urination, unexplained fatigue, or blurred vision
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Difficulty maintaining stable blood glucose readings on your home monitoring device
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Signs of hypoglycaemia (low blood sugar), including shakiness, sweating, confusion, or palpitations — particularly if you are taking insulin or sulphonylureas
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Any new or worsening symptoms that could indicate a sickle cell crisis, as these may also affect blood glucose stability
Seek urgent same-day medical assessment or call 999 / attend your nearest A&E if you experience symptoms that may suggest diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), including:
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Vomiting, severe abdominal pain, or inability to keep fluids down
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Drowsiness, confusion, or difficulty staying awake
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Rapid or deep breathing
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Very high blood glucose readings on your monitor
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Ketones detected in your urine or blood (if you have a testing kit)
Do not wait for a routine appointment if you have these symptoms. DKA and HHS are medical emergencies. If you take an SGLT2 inhibitor, be aware that DKA can occur even when blood glucose is not very high; check for ketones if you feel unwell.
For those already diagnosed with diabetes, annual reviews should include assessment of renal function (eGFR and urine albumin-to-creatinine ratio), blood pressure, foot health, and eye screening — all of which are relevant to both diabetes and SCD-related complications. These reviews should ideally be coordinated between your haematology team and your diabetes care provider.
If you are unsure whether your diabetes monitoring is appropriate for your condition, ask your GP or haematologist to review your testing method. You have the right to ask whether fructosamine, glycated albumin, or continuous glucose monitoring might be more suitable for you than standard HbA1c testing.
If you are pregnant and have both SCD and diabetes or gestational diabetes, urgent specialist referral is essential. Both conditions carry significant obstetric risks that require expert joint management from haematology, obstetrics, and diabetes teams throughout pregnancy, in line with NICE guidance on diabetes in pregnancy (NG3).
Frequently Asked Questions
Can HbA1c be used to diagnose diabetes in sickle cell disease?
No. NICE (NG28) and WHO guidance state that HbA1c should not be used to diagnose diabetes in people with haemoglobinopathies such as sickle cell disease. Fasting plasma glucose or an oral glucose tolerance test (OGTT) should be used instead.
What is the best alternative to HbA1c for monitoring blood glucose in sickle cell disease?
Fructosamine and glycated albumin are the most commonly used alternatives, as they reflect glycaemic control over two to three weeks without relying on red blood cell lifespan. Continuous glucose monitoring (CGM) or fingerprick blood glucose testing can also be used for day-to-day assessment.
Does sickle cell trait also make HbA1c unreliable?
Sickle cell trait (HbAS) is different from sickle cell disease; people with sickle cell trait generally do not have significantly shortened red cell survival. However, HbA1c reliability in this group depends on the assay method used, and local laboratory advice should be sought.
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