The effect of polycythaemia on HbA1c is an important clinical consideration for any clinician managing patients with elevated red blood cell mass alongside diabetes or suspected hyperglycaemia. HbA1c — the standard marker for long-term glycaemic control — relies on predictable red blood cell lifespan and stable iron stores. In polycythaemia, particularly polycythaemia vera treated with therapeutic venesection, these assumptions may be disrupted, potentially rendering HbA1c misleading. Understanding when and why HbA1c may be unreliable, and which alternative glycaemic markers are appropriate within UK clinical practice, is essential for accurate diagnosis and safe ongoing monitoring.

How Polycythaemia Affects Red Blood Cell Turnover and HbA1c

Polycythaemia does not directly alter HbA1c through increased RBC mass; rather, treatments such as venesection causing iron deficiency, or concurrent changes in RBC lifespan, are the primary mechanisms by which HbA1c reliability is compromised.

Polycythaemia — characterised by an abnormally elevated red blood cell (RBC) mass — can alter the haematological environment in ways that influence glycated haemoglobin (HbA1c) measurements. To understand this, it is important first to appreciate how HbA1c is formed. Glucose in the bloodstream binds irreversibly to haemoglobin within circulating red blood cells through a non-enzymatic process called glycation. Because red blood cells have a lifespan of approximately 90–120 days, HbA1c reflects the proportion of haemoglobin that has been glycated over that period. Crucially, HbA1c is expressed as a percentage (or in mmol/mol) — it is a ratio, not an absolute quantity — and is therefore not directly affected by the total size of the RBC pool.

The principal factors that cause HbA1c to misrepresent true glycaemic control are changes in RBC lifespan and iron status, not RBC mass per se. Conditions that shorten RBC survival — such as haemolytic anaemia, recent significant blood loss, or recent blood transfusion — reduce the time available for glycation and tend to produce a falsely low HbA1c relative to actual blood glucose levels. Conversely, iron deficiency, vitamin B12 deficiency, or folate deficiency can prolong RBC lifespan or alter reticulocyte dynamics, and are associated with falsely elevated HbA1c values.

In polycythaemia vera (PV), a myeloproliferative neoplasm, RBC lifespan is often near-normal, meaning PV alone does not consistently bias HbA1c in a predictable direction. However, the treatments used for PV — particularly therapeutic venesection, which commonly induces iron deficiency — can significantly affect HbA1c reliability. Secondary polycythaemia, arising from chronic hypoxia, renal disease, or other systemic conditions, similarly does not automatically invalidate HbA1c; clinical judgement is required in each case.

Clinicians should therefore interpret HbA1c values cautiously in any patient with a known or suspected disorder of red cell mass, focusing on RBC lifespan, iron status, and assay-specific interferences rather than RBC pool size alone.

Why HbA1c Results May Be Unreliable in Polycythaemia

Venesection-induced iron deficiency is the most clinically significant confounder in polycythaemia vera, associated with falsely elevated HbA1c; hydroxycarbamide use, haemoglobin variants, and concurrent haemolysis can also distort results.

The reliability of HbA1c as a glycaemic marker rests on the assumption of a normal, predictable red blood cell lifespan and stable iron stores. In patients with polycythaemia, several factors can disrupt these assumptions, rendering HbA1c measurements potentially misleading.

The most clinically significant confounder in polycythaemia vera is iron deficiency resulting from therapeutic venesection. Venesection is a first-line treatment for PV and is frequently repeated; the resulting iron deficiency can prolong RBC survival and is associated with falsely elevated HbA1c values. This may lead to an incorrect diagnosis of diabetes or an overestimate of glycaemic burden in a patient already known to have diabetes. Clinicians should routinely check iron studies and reticulocyte count when interpreting HbA1c in patients undergoing venesection.

Additional confounding factors include:

• Haemoglobin variants: Structural haemoglobin variants (e.g., HbS, HbC, HbE) can interfere with certain HbA1c assay methodologies. Whilst not specifically associated with PV, they may coexist and should be considered if results appear discordant. The assay method used (e.g., HPLC, immunoassay, capillary electrophoresis) should be documented, as different methods have different susceptibilities to interference.

• Hydroxycarbamide (hydroxyurea): This cytoreductive agent, used in PV management, can cause macrocytosis and alter RBC indices. Whilst it does not directly change the kinetics of haemoglobin glycation, changes in RBC turnover associated with its use may indirectly affect HbA1c; clinical correlation is advised.

• Concurrent haemolysis or blood loss: If present alongside polycythaemia, these can shorten RBC lifespan and lower HbA1c independently of glycaemic control.

Where HbA1c results appear discordant with capillary blood glucose readings, continuous glucose monitoring (CGM) data, or clinical symptoms, clinicians should consider whether one or more of these confounders is operative. Results should never be interpreted in isolation; the haematological context, current treatments, and assay methodology should all be documented clearly in the patient record.

Clinical Implications for Diabetes Diagnosis and Monitoring

When HbA1c is unreliable in polycythaemia, NICE NG28 supports plasma glucose-based tests — fasting plasma glucose (≥7.0 mmol/L) or OGTT (≥11.1 mmol/L) — as diagnostic alternatives to avoid misclassification of diabetes.

The diagnostic thresholds for diabetes and prediabetes using HbA1c — as defined by NICE guideline NG28 and aligned with World Health Organisation (WHO) criteria — are 48 mmol/mol (6.5%) for diabetes and 42–47 mmol/mol (6.0–6.4%) for non-diabetic hyperglycaemia (prediabetes). These thresholds assume a normal haematological background. Importantly, in asymptomatic adults, a diagnosis of diabetes requires confirmation with a second abnormal result, either from the same sample or a repeat test on a separate occasion.

When polycythaemia is present, HbA1c should not automatically be considered invalid; rather, its reliability should be assessed on a case-by-case basis, taking into account RBC lifespan, iron status, and assay methodology. Where HbA1c is judged to be unreliable, relying on it to diagnose or exclude diabetes carries a meaningful risk of misclassification:

• Falsely low HbA1c (e.g., due to haemolysis or recent blood loss) may result in a missed or delayed diagnosis of diabetes in a patient with genuinely elevated blood glucose.

• Falsely elevated HbA1c (e.g., due to venesection-induced iron deficiency) may result in an incorrect diagnosis of diabetes and unnecessary treatment initiation.

For ongoing monitoring in patients already diagnosed with diabetes, the same principles apply. An HbA1c that appears reassuring may mask consistently elevated blood glucose levels if iron deficiency or altered RBC dynamics are present, potentially delaying appropriate intensification of diabetes management.

Where HbA1c is considered unreliable for diagnostic purposes, NICE NG28 supports the use of plasma glucose-based tests as alternatives. The relevant diagnostic thresholds are:

• Fasting plasma glucose (FPG): ≥7.0 mmol/L

• 2-hour plasma glucose on a 75 g oral glucose tolerance test (OGTT): ≥11.1 mmol/L

• Random plasma glucose: ≥11.1 mmol/L in the presence of symptoms of hyperglycaemia

Clinicians should document clearly in the patient record that polycythaemia has been identified as a potential confounder, the rationale for selecting an alternative diagnostic approach, and the result obtained, to ensure continuity of care across primary and secondary settings.

Alternative Glycaemic Markers: Guidance for UK Practice

Fasting plasma glucose and OGTT are the recommended diagnostic alternatives to HbA1c; for monitoring, CGM or SMBG are preferred, with fructosamine reserved for specialist settings when other approaches are impractical.

When HbA1c is judged unreliable due to haematological conditions such as polycythaemia, the choice of alternative depends on whether the clinical need is diagnostic or for ongoing monitoring.

For diagnosis, NICE NG28 recommends plasma glucose-based tests — fasting plasma glucose (FPG) or a 75 g oral glucose tolerance test (OGTT) — as the primary alternatives to HbA1c. These provide a direct measure of blood glucose at a specific point in time and are not influenced by red cell dynamics or iron status. Diagnostic thresholds are as described in the section above.

For monitoring in patients with established diabetes, NICE guidance supports self-monitored blood glucose (SMBG) using a calibrated glucometer and, where eligible, continuous glucose monitoring (CGM) or flash glucose monitoring. These approaches are independent of haemoglobin and provide real-time or trend glucose data that are unaffected by RBC lifespan or iron status. CGM eligibility in the NHS is defined by NICE: it is recommended for all adults with type 1 diabetes (NICE NG17), and should be considered for selected adults with type 2 diabetes, including those on insulin or at risk of hypoglycaemia, in line with current NICE guidance and local commissioning arrangements.

Fructosamine — which measures glycation of total serum proteins, predominantly albumin, reflecting average blood glucose over approximately 2–3 weeks — may be considered in specialist settings when HbA1c is unreliable and plasma glucose testing is impractical for ongoing monitoring. It is available in many NHS laboratories, but its use should be guided by local pathology services. Interpretation can be affected by conditions that alter serum protein levels, including nephrotic syndrome, liver disease, hypoalbuminaemia, and thyroid dysfunction; these must be taken into account.

Glycated albumin specifically measures the proportion of albumin that has been glycated and offers a similar 2–3 week window of glycaemic information. However, it is not widely standardised or routinely available across NHS trusts in the UK, and its use should be considered only in specialist settings with appropriate laboratory support.

Clinicians should document clearly why an alternative test has been selected, which test was used, and the result obtained, to ensure consistent interpretation across care settings.

Managing Diabetes Monitoring in Patients With Polycythaemia

A tailored monitoring plan — incorporating SMBG, CGM where eligible, fructosamine in specialist settings, and routine iron studies — should be agreed, documented, and reassessed whenever the haematological picture changes significantly.

Effective diabetes monitoring in patients with polycythaemia requires a tailored, multidisciplinary approach that accounts for the limitations of standard glycaemic markers. At the outset of care, clinicians should assess whether HbA1c is likely to be reliable for that individual, taking into account the type and severity of polycythaemia, current treatments (particularly venesection or hydroxycarbamide), iron status, and any concurrent haematological abnormalities.

For patients in whom HbA1c is considered unreliable, a structured monitoring plan should be agreed and documented. Key components may include:

• Self-monitored blood glucose (SMBG) using a calibrated glucometer, particularly for patients on insulin or sulfonylureas where hypoglycaemia risk is relevant.

• Continuous glucose monitoring (CGM) or flash glucose monitoring, where the patient meets NICE eligibility criteria (see above). These devices provide real-time and trend glucose data independent of haemoglobin and are the preferred monitoring approach where available and appropriate.

• Fructosamine testing, considered in specialist settings when CGM and SMBG are insufficient or impractical. The frequency of testing should be guided by clinical need, local laboratory advice, and the timing of treatment changes, rather than a fixed interval. Fructosamine's short reflective window (approximately 2–3 weeks) means it is most useful when assessing glycaemic control during periods of active treatment change or when discordance between HbA1c and clinical findings is suspected.

• Iron studies and reticulocyte count should be checked routinely when interpreting any glycaemic marker in patients undergoing venesection, as iron deficiency is a common and clinically significant confounder.

• Haematological review at a frequency determined by the haematology team's management plan — which for PV is typically more frequent than annually, particularly during active treatment phases or at any major treatment change. HbA1c reliability should be reassessed whenever the haematological picture changes significantly.

Consistency of assay method over time is important; if the glycaemic marker used changes (e.g., from HbA1c to fructosamine), this should be clearly documented in the patient record with the clinical rationale.

Patient education is a critical component of safe management. Patients should be informed clearly why their HbA1c may not accurately reflect their glucose control, and should understand the importance of attending alternative monitoring appointments. Written information, supported by resources from Diabetes UK and NHS patient information leaflets, can reinforce verbal explanations and support informed self-management.

When to Seek Specialist Haematology or Endocrinology Advice

Specialist referral is indicated when polycythaemia is newly diagnosed, when diabetes cannot be reliably confirmed due to haematological confounders, or when glycaemic control appears to be deteriorating despite stable HbA1c values.

Given the complexity of managing glycaemic monitoring in the context of polycythaemia, there are several clinical scenarios in which referral to — or liaison with — specialist services is strongly advisable. Timely specialist input can prevent diagnostic errors, optimise treatment, and ensure patient safety.

Urgent same-day assessment should be sought if a patient with polycythaemia develops any of the following, which may indicate serious haematological complications:

• New focal neurological deficit, severe headache, or visual disturbance in the context of a markedly elevated haematocrit (possible hyperviscosity or cerebrovascular event)

• Chest pain or features consistent with acute coronary syndrome or pulmonary embolism

• Signs or symptoms of deep vein thrombosis

• Rapidly progressive splenomegaly

These presentations require emergency assessment and should not be delayed for outpatient review.

Referral to haematology should be considered when:

• Polycythaemia is newly suspected or diagnosed and requires further investigation (e.g., JAK2 V617F mutation testing, erythropoietin level, bone marrow biopsy)

• The patient's haematological condition is unstable or undergoing active treatment changes that may further affect glycaemic marker reliability

• There is uncertainty about the degree to which venesection or cytoreductive therapy is influencing laboratory results

Referral to or liaison with endocrinology or diabetology is appropriate when:

• Diabetes is suspected but cannot be reliably confirmed or excluded using standard diagnostic criteria due to polycythaemia

• Glycaemic control appears to be deteriorating despite apparently stable HbA1c values, suggesting the need for alternative monitoring strategies

• Insulin therapy is being considered, where precise glycaemic data are essential for safe dose titration

In primary care, GPs should maintain a low threshold for seeking advice via NHS Advice and Guidance services or local integrated care board (ICB) e-referral pathways when managing patients with both conditions concurrently. Patients should be advised to contact their GP promptly if they experience symptoms suggestive of hyperglycaemia — such as increased thirst, frequent urination, or unexplained fatigue — or hypoglycaemia, particularly if their monitoring regimen has recently changed. A collaborative, proactive approach between haematology, diabetes services, and primary care remains the cornerstone of safe and effective management.

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