The 1% Lie: Why HbA1c-Glycemia Discordance Is Breaking Your Clinical Algorithms

By: Dr. Pichakacheri Sureshkumar MD,PhD, FRCP

Date: 12/04/2026

Section: Clinical Practice & Diagnostic Accuracy

Let's play a diagnostic game.

A 58-year-old patient walks in. HbA1c is 8.2%. You diagnose uncontrolled diabetes, adjust their basal insulin, and schedule a follow-up.

But here's the twist: Their fasting glucose is 98 mg/dL. Their postprandial peaks are 140 mg/dL. By glucose meter, they are 'near normal'. By HbA1c, they are failing therapy. Who is right? The 90-day memory of glycated hemoglobin? Or the real-time glucose snapshot?According to a new comprehensive review—"Navigating the Discordance: A Comprehensive Review of HbA1c-Glycemia Mismatch in Clinical Practice"—this scenario isn't a rare curiosity. It occurs in 10–30% of patients depending on the population. And we are systematically mismanaging most of them. Today, we dissect this discordance: where it comes from, why your lab slip won't catch it, and what to do when hemoglobin lies to you.

The Sacred Cow of Diabetes Management : HbA1c is arguably the most trusted biomarker in chronic disease management. It doesn't require fasting. It reflects integrated glucose over 120 days. It correlates with microvascular complications in landmark trials (DCCT, UKPDS). Clinical guidelines are built around numeric targets: <7.0% for most adults, tighter or looser for others. But the foundational assumption—that HbA1c is a reliable surrogate for mean glucose—rests on two unstated premises:

1.  Normal red blood cell (RBC) lifespan (≈120 days)
2.  Normal hemoglobin structure and glycation rate

When either premise fails, the relationship between measured HbA1c and actual glycemia decouples. And because most clinicians never order a concurrent fructosamine or continuous glucose monitor (CGM) to validate the HbA1c, we treat the 'number', not the 'patient'.

The Discordance Matrix: Three Patterns, Three Pitfalls

The review organizes discordance into three clinically actionable patterns:

Pattern A: HbA1c Falsely High (Glycemia Normal)

Mechanisms:

• Iron deficiency anemia (prolonged RBC survival → more glycation time)
  Splenectomy (RBCs age longer)
• Uremia (carbamylated hemoglobin interferes with assays)
• Certain Hb variants (HbS, HbC, HbE—common in African, Mediterranean, and Southeast Asian populations)

Clinical consequence: You escalate therapy (insulin, sulfonylureas) into a patient who is actually 'euglycemic'. Iatrogenic hypoglycemia follows.

Pattern B: HbA1c Falsely Low (Glycemia Elevated)

Mechanisms:

• Hemolytic anemias (sickle cell disease, spherocytosis, G6PD deficiency)
• Recent blood loss or transfusion (young RBCs haven't had time to glycate)
• Chronic kidney disease on erythropoietin (accelerated RBC turnover)
  Pregnancy (expanded plasma volume + rapid RBC turnover)

Clinical consequence: You reassure a patient with mean glucose of 200 mg/dL because their HbA1c reads 6.5%. Microvascular complications progress silently.

Pattern C: True Biological Discordance

Mechanism: Genetic or acquired variation in the 'intrinsic glycation rate' of hemoglobin—independent of RBC lifespan or glucose exposure. African Americans, for example, have slightly higher average glycation rates than Caucasians at the same mean glucose, though the clinical significance remains debated.

Clinical consequence: Race-adjusted HbA1c algorithms? Personalized targets? The evidence is unsettled.

Why This Matters for Your Practice Right Now

The review offers three immediate takeaways:

1. Stop trusting discordant HbA1c without confirmation.
If HbA1c and glucose don't align (e.g., HbA1c 8.5% with fasting glucose 100 mg/dL), order a 'fructosamine' (2-week glycation of serum proteins, unaffected by RBC lifespan) or a CGM for 7–14 days. The cost is trivial compared to a year of misguided insulin.

2. Screen high-risk populations before committing to therapy.
Patients with:

• Unexplained HbA1c >9% without corresponding hyperglycemia
• Known hemoglobinopathy (even trait)
• CKD Stage 4–5
• Anemia (especially microcytic)

...deserve a baseline discordance check. Run HbA1c and fasting glucose simultaneously at diagnosis. Calculate the ratio. Document it.

3. Update your lab's methods.
Immunoassays (still common in smaller labs) are more vulnerable to Hb variant interference than high-performance liquid chromatography (HPLC) or capillary electrophoresis. If your lab reports "HbA1c: 7.2%" without a comment about variant hemoglobin, ask what method they use.

Where the Review Needs Scrutiny?

1. What is the gold standard for "true glycemia" when validating discordance?
Fructosamine/Glycated albumin? CGM mean glucose?  Each has its own confounders (albumin turnover in nephrotic syndrome, CGM accuracy at extremes). Without a perfect reference, how do we define which measure is correct?

2. How should we handle the grey zone?
A difference of 0.5% between predicted and measured HbA1c is noise. A difference of 2.0% is actionable. But what about 1.2%? The review doesn't propose a clinical decision threshold. Should it be absolute percentage? Percent difference? Z-score?

3. Is race-based adjustment scientifically valid or statistical overfitting?
The "African American glycation gap" has been replicated in multiple cohorts. But using race as a proxy for genetic variation is crude. Would a polygenic score for glycation rate be more ethical and accurate? Until then, do we adjust targets or not?

4. What about the cost-effectiveness of universal screening?
Ordering fructosamine or CGM for every anemic patient is cheap. Ordering it for every patient with an HbA1c >8% is not. What is the NNT (number needed to test) to prevent one severe hypoglycemic event from discordance-driven overtreatment?

The Bottom Line

HbA1c is a miracle of clinical chemistry—when it works. When it doesn't, it's a trap.

This review does not argue for abandoning HbA1c. It argues for 'triangulation': using multiple glycemia metrics (HbA1c + glucose + fructosamine/CGM) to build a robust clinical picture, especially when the numbers scream at each other.

The patient with the 8.2% HbA1c and normal glucose? After a week of CGM, their mean glucose was 115 mg/dL. Iron deficiency was the culprit. The insulin prescription was canceled.

That's not conservative medicine. That's precision medicine.

Over to you, SciPinion community. How often do you see discordance? What's your diagnostic algorithm? And most importantly—when was the last time you checked if your patient's HbA1c was 'lying'?

Read the full article: https://authors.elsevier.com/a/1modT_3rgrt817
Review and critique this paper on SciPinion

About the author: Dr.P.Sureshkumar is a renowned Diabetologist and researcher in the field with many publications in peer reviewed international journals 

Keywords: HbA1c, glycemia mismatch, discordance, diabetes diagnosis, hemoglobin variants, anemia, fructosamine, CGM, precision medicine, clinical laboratory errors.