Can type 2 patients have a deceptively low HbA1c? The answer is yes — and understanding why matters for safe diabetes management. HbA1c is the standard NHS test for monitoring long-term blood glucose control in type 2 diabetes, but it is an indirect measure that depends on red blood cell lifespan. Certain medical conditions, medicines, and physiological states can cause HbA1c to appear reassuringly normal whilst actual glucose levels remain dangerously elevated. This article explains the key causes of a falsely low HbA1c, which additional tests may help, and when to speak to your GP.

Why HbA1c May Not Always Reflect True Blood Glucose Control

HbA1c can be deceptively low when red blood cells are destroyed or replaced more rapidly than usual, reducing the time for glycation to accumulate and producing a falsely reassuring result despite elevated blood glucose.

HbA1c — glycated haemoglobin — is the cornerstone test used to monitor long-term blood glucose control in people with type 2 diabetes. It measures the percentage of haemoglobin molecules that have glucose attached to them, reflecting average blood glucose levels over approximately 2 to 3 months, with the preceding 4 weeks contributing most to the result. In UK laboratories, HbA1c is reported in mmol/mol (with the older percentage figure sometimes shown in parentheses). However, HbA1c is an indirect measure, and in certain circumstances it can produce a result that appears reassuringly normal whilst true glucose control remains poor.

The test relies on the lifespan and turnover of red blood cells. Haemoglobin becomes glycated gradually over the life of a red blood cell, which typically survives around 120 days. If red blood cells are destroyed more rapidly than usual, or if new red blood cells are produced at an accelerated rate, there is less time for glycation to accumulate — and the HbA1c result will appear lower than expected. This is sometimes described as a falsely low or deceptively low HbA1c.

HbA1c is also unreliable during pregnancy and in the first 2 to 3 months after delivery, owing to changes in red cell turnover and plasma volume; alternative markers should be used in these situations.

For patients and clinicians alike, this distinction matters enormously. A person with type 2 diabetes may believe their glucose management is well-controlled based on a low HbA1c, when in reality their day-to-day blood glucose levels may be significantly elevated. This can lead to under-treatment, delayed medication adjustments, and an increased risk of long-term diabetic complications including retinopathy, nephropathy, and cardiovascular disease. Recognising the limitations of HbA1c is therefore a critical part of safe diabetes management, as acknowledged in NICE guideline NG28 (Type 2 diabetes in adults: management).

Medical Conditions That Can Falsely Lower HbA1c Results

Haemolytic anaemia, haemoglobinopathies, chronic kidney disease, liver cirrhosis, recent blood transfusion, and pregnancy are all recognised causes of a falsely low HbA1c in type 2 diabetes patients.

Several medical conditions are well-recognised as causes of a falsely low HbA1c, and clinicians should be alert to these when interpreting results in people with type 2 diabetes.

Haemolytic anaemia is one of the most significant. In this condition, red blood cells are destroyed prematurely, reducing the time available for glycation. Causes include autoimmune haemolytic anaemia, sickle cell disease, thalassaemia, and hereditary spherocytosis. Patients of African, Caribbean, South Asian, or Mediterranean heritage may be at higher risk of haemoglobinopathies that affect HbA1c accuracy.

Iron deficiency anaemia can sometimes raise HbA1c, but treatment with iron supplementation — which stimulates new red blood cell production — can transiently lower it. Similarly, vitamin B12 or folate deficiency, when treated, leads to a burst of new red cell production that may reduce HbA1c artificially.

Acute blood loss can also lower HbA1c by stimulating production of younger red cells with less accumulated glycation.

Other relevant conditions include:

• Chronic kidney disease (CKD): Uraemia can shorten red cell lifespan and interfere with some assay methods; the effect is most pronounced in advanced CKD and in patients on dialysis, where alternative markers or continuous glucose monitoring (CGM) are generally preferred

• Liver cirrhosis: Altered red cell turnover and haemodilution can affect results

• Splenomegaly (enlarged spleen): Accelerated red cell destruction can lower HbA1c

• Recent blood transfusion: Transfused red cells have not been exposed to the patient's glucose environment, diluting the HbA1c reading

• Pregnancy and early post-partum period: Increased red cell turnover and haemodilution render HbA1c unreliable; fasting plasma glucose or alternative markers should be used instead

• Haemoglobin variants: Certain haemoglobin variants (e.g., HbS, HbC, HbE) can interfere with some laboratory assay methods. The degree of interference is method-dependent; many UK NHS laboratories use high-performance liquid chromatography (HPLC), which is designed to detect and flag common variants on the report. Clinicians should check the laboratory comment when a variant is suspected

When a patient's HbA1c appears inconsistent with their symptoms or self-monitored glucose readings, clinical correlation is essential. NICE NG28 advises that HbA1c should not be used as the sole diagnostic or monitoring tool in these circumstances, and RCPath/ACB guidance provides further detail on assay interferences and appropriate alternative markers.

How Type 2 Diabetes Management Can Affect HbA1c Accuracy

Erythropoiesis-stimulating agents, dapsone, and ribavirin can artificially lower HbA1c, while SGLT2 inhibitors carry a risk of euglycaemic DKA that may not be reflected in the HbA1c value.

Beyond underlying medical conditions, certain aspects of diabetes treatment can influence HbA1c readings in ways that may not accurately reflect a patient's true glycaemic status.

Erythropoiesis-stimulating agents (ESAs), such as epoetin alfa and epoetin beta, are sometimes used in patients with CKD-related anaemia. These drugs stimulate the production of new red blood cells, which — being younger — carry less glycated haemoglobin. This can produce a deceptively low HbA1c even when blood glucose levels remain elevated. This effect is documented in the relevant Summary of Product Characteristics (SmPC) for ESAs, available via the electronic Medicines Compendium (eMC).

Dapsone, used in the treatment of dermatitis herpetiformis and certain infections, causes haemolysis and can lower HbA1c artificially. Ribavirin, used in hepatitis C treatment, has a similar haemolytic effect. These effects are noted in the respective SmPCs. Clinicians managing patients on these medicines should exercise particular caution when interpreting HbA1c results. Patients who suspect a medicine is causing an adverse effect — such as signs of haemolysis (unusual fatigue, pallor, or jaundice) — should report this through the MHRA Yellow Card scheme (available at yellowcard.mhra.gov.uk).

Some newer diabetes medicines may also warrant careful interpretation:

• SGLT2 inhibitors (e.g., dapagliflozin, empagliflozin) are associated with a modest, documented rise in haematocrit (as noted in their European Medicines Agency EPARs and SmPCs). Any consequent effect on HbA1c measurement is likely to be small and of uncertain clinical relevance; the primary concern with these medicines and glycaemic monitoring relates to the risk of euglycaemic diabetic ketoacidosis (DKA) — see the safety note below

• GLP-1 receptor agonists and insulin regimens that cause significant glucose variability — with frequent hypoglycaemic episodes followed by rebound hyperglycaemia — may produce an HbA1c in the target range that masks dangerous swings in blood glucose. This is an issue of interpretation rather than analytical inaccuracy: the HbA1c result may be technically correct, yet the average it represents conceals clinically important instability

Important safety note — SGLT2 inhibitors and DKA: If you are taking an SGLT2 inhibitor (such as dapagliflozin, empagliflozin, or canagliflozin), seek same-day urgent medical advice (contact your GP, call NHS 111, or go to A&E) if you develop nausea, vomiting, abdominal pain, excessive thirst, difficulty breathing, confusion, or unusual fatigue. DKA can occur with these medicines even when blood glucose is not markedly elevated. This risk is highlighted in MHRA Drug Safety Updates on SGLT2 inhibitors.

In cases of significant glucose variability, continuous glucose monitoring (CGM) or structured self-monitoring of blood glucose (SMBG) provides a far more complete clinical picture than HbA1c alone.

When to Request Additional Tests Alongside HbA1c

Fructosamine, fasting plasma glucose, full blood count, reticulocyte count, and continuous glucose monitoring should be considered when HbA1c appears inconsistent with symptoms or self-monitored glucose readings.

Given the limitations of HbA1c, there are specific clinical scenarios in which additional or alternative tests should be considered to ensure accurate assessment of glycaemic control in people with type 2 diabetes.

Fructosamine is the most widely available alternative marker in NHS laboratories. It reflects average blood glucose over a shorter period — approximately 2 to 3 weeks — by measuring glycated serum proteins rather than haemoglobin, making it unaffected by red cell lifespan. It is particularly useful in patients with haemolytic anaemia, haemoglobinopathies, or those who have recently received a blood transfusion.

Glycated albumin offers a similar 2 to 3-week window and is used in some specialist settings, particularly in patients with CKD or liver disease. However, it is not widely available across all NHS laboratories; clinicians should check local provision before requesting it.

Fasting plasma glucose (FPG) or oral glucose tolerance test (OGTT) may also be used when HbA1c is deemed unreliable or discordant, in line with NICE NG28.

For day-to-day clinical management, the following additional assessments may be warranted:

• Continuous glucose monitoring (CGM): Provides real-time glucose data, time-in-range metrics, and identifies glucose variability that HbA1c cannot capture

• Self-monitoring of blood glucose (SMBG): Structured finger-prick testing at key time points (fasting, post-meal) can reveal patterns inconsistent with a low HbA1c

• Full blood count (FBC): Helps identify anaemia, haemolysis, or abnormal red cell indices that may explain a discordant HbA1c

• Reticulocyte count: Elevated reticulocytes suggest increased red cell turnover, which may be lowering HbA1c artificially

Clinicians should consider requesting these tests when a patient's HbA1c appears inconsistent with their symptoms, self-reported glucose readings, or clinical presentation. Requests should be aligned with local laboratory guidance; when a haemoglobin variant is suspected, the laboratory report comment should be reviewed carefully, as many UK labs using HPLC will flag variants and advise on result reliability. A multidisciplinary approach — involving the GP, diabetes specialist nurse, and where appropriate a haematologist — is often the most effective strategy.

NICE and NHS Guidance on Interpreting HbA1c in Type 2 Diabetes

NICE NG28 recommends individualised HbA1c targets and explicitly advises use of alternative markers such as fructosamine or fasting plasma glucose when HbA1c is unreliable.

NICE guideline NG28 (Type 2 diabetes in adults: management) recommends HbA1c as the primary monitoring tool for long-term glycaemic control, with targets individualised to the patient. As a general guide:

• 48 mmol/mol (6.5%) is the target for adults managed by lifestyle measures or medicines that do not carry a risk of hypoglycaemia

• 53 mmol/mol (7.0%) is a commonly used target for adults on medicines associated with hypoglycaemia risk (e.g., sulphonylureas or insulin)

• Targets should be relaxed individually for older adults, those with frailty, significant comorbidities, or a history of severe hypoglycaemia — in some cases up to 58–64 mmol/mol (7.5–8.0%) or higher, based on clinical judgement and patient preference

NICE NG28 also explicitly acknowledges that HbA1c may be unreliable in certain populations and clinical circumstances, and advises use of alternative markers in these situations.

UK NHS laboratories — many of which use HPLC methods — are designed to detect and flag common haemoglobin variants on reports, in line with RCPath and IFCC guidance on HbA1c assay interferences. No single method is entirely free from interference, and clinicians should treat laboratory comments as an integral part of result interpretation rather than relying on the numerical value alone.

Key NICE-aligned recommendations include:

• Do not rely solely on HbA1c in patients with known haemoglobinopathies, haemolytic anaemia, pregnancy, or recent transfusion

• Use alternative markers such as fructosamine (or FPG/OGTT) when HbA1c is deemed unreliable

• Consider CGM for patients with significant glucose variability or recurrent hypoglycaemia. NICE NG28 supports CGM access for adults with type 2 diabetes on insulin who experience recurrent hypoglycaemia, impaired hypoglycaemia awareness, inability to self-monitor, or where the monitoring burden is high. NHS England implementation policies set out local access criteria

• Ensure HbA1c targets are individualised, taking into account age, comorbidities, frailty, hypoglycaemia risk, and patient preference

Clinicians are encouraged to use clinical judgement alongside laboratory results rather than treating HbA1c as an infallible measure.

Talking to Your GP If You Suspect a Misleading HbA1c Result

Patients should speak to their GP if home glucose readings consistently exceed what their HbA1c suggests, or if they have anaemia, kidney disease, a haemoglobin variant, or have recently started a new medicine.

If you have type 2 diabetes and feel that your HbA1c result does not reflect how your blood glucose has actually been behaving, it is important to raise this concern with your GP or diabetes care team. Many patients and clinicians encounter situations where the HbA1c result and the clinical picture do not align.

Consider speaking to your GP if:

• Your home blood glucose readings are consistently higher than your HbA1c would suggest

• You have been diagnosed with or treated for anaemia, a blood disorder, or kidney disease

• You have recently had a blood transfusion, started a new medicine, or are pregnant or recently post-partum

• You are experiencing symptoms of high blood glucose (increased thirst, frequent urination, fatigue, blurred vision) despite a normal HbA1c

• You have a family background associated with haemoglobin variants (e.g., sickle cell trait, thalassaemia trait)

Seek same-day urgent medical help — contact your GP urgently, call NHS 111, or go to A&E — if you develop any of the following: severe thirst, vomiting, abdominal pain, rapid or laboured breathing, confusion, or if your blood glucose is persistently above 20 mmol/L. These may be signs of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), both of which are medical emergencies. If you are taking an SGLT2 inhibitor, be aware that DKA can occur even when your glucose reading is not very high.

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When speaking to your GP, bring a record of your home glucose readings or CGM data, along with a list of your current medicines. This provides objective evidence that may support a request for further investigation. Your GP may arrange a full blood count, reticulocyte count, fructosamine test, or fasting plasma glucose, or refer you to a diabetes specialist or haematologist if appropriate.

If you suspect that a medicine is causing an unwanted effect — for example, signs of haemolysis such as unusual fatigue, pallor, or yellowing of the skin — you or your healthcare professional can report this through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Your diabetes management should always be based on the full clinical picture — not a single number. Open, honest communication with your healthcare team is the most effective way to ensure your treatment plan truly reflects your needs. Never adjust your medication based on an HbA1c result alone without first discussing it with a qualified healthcare professional.

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