Patient Guide

Complete Guide to Diabetes Blood Tests

Name: Complete Guide to Diabetes Blood Tests: HbA1c | HealthReview
Creator: Suman Konda
Published: 2026-07-05

Whether you're checking for diabetes or managing it, this guide explains every blood test your doctor might order: what each one measures and what your results mean.

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: 01 August 2026 · How we check our content

Tests used to diagnose diabetes

Test	Normal	Pre-diabetes	Diabetes
Fasting Blood Glucose	<100 mg/dL (<5.6 mmol/L)	100–125 mg/dL	≥126 mg/dL on 2 occasions
Post-meal (2hr OGTT)	<140 mg/dL (<7.8 mmol/L)	140–199 mg/dL	≥200 mg/dL
HbA1c	<5.7%	5.7–6.4%	≥6.5% on 2 occasions
Random Blood Glucose	<140 mg/dL	:	≥200 mg/dL with symptoms

HbA1c: the most important diabetes test

What is HbA1c?

HbA1c (glycated haemoglobin) reflects your average blood sugar over the past 2–3 months. Unlike a single fasting glucose reading which only shows one moment in time, HbA1c gives a fuller picture. Sugar in the blood binds to haemoglobin in red blood cells: the more sugar, the higher the HbA1c. It's used both to diagnose diabetes and to monitor how well blood sugar is controlled in people already on treatment. A target HbA1c below 7% is generally recommended for most diabetic adults, though your doctor may personalise this target.

When HbA1c Gives the Wrong Answer

HbA1c works by measuring how much glucose has stuck to your haemoglobin, and reports an average over roughly the previous eight to twelve weeks.¹ That average assumes two things: that your red cells live a normal length of time, and that your haemoglobin is the ordinary type. Where either assumption fails, the number can be misleading even though the test ran correctly.

Red cell lifespan comes first. Anything that shortens it — haemolysis, recent significant blood loss, some anaemias — gives glucose less time to attach, and HbA1c reads lower than your true control. Anything that lengthens it can push the reading up. Recent transfusion mixes someone else’s cells into the measurement. Pregnancy, advanced kidney disease and iron deficiency all shift it for related reasons. In these situations HbA1c is not the right test to judge control by, and fingerprick or continuous glucose monitoring gives a truer picture.

Haemoglobin Variants and Why the Method Matters

The second assumption fails more often than most people realise, particularly across South and Southeast Asia, the Mediterranean, Africa and the Middle East, where inherited haemoglobin variants are common. Variants such as HbE, HbS, HbC and beta-thalassaemia trait can interfere with the measurement itself, and the interference differs by laboratory method: ion exchange chromatography, capillary electrophoresis, immunoassay and boronate affinity are each affected differently.¹

How large this can be is easy to underestimate. In a study of 845 people with diabetes in a region where these variants are frequent, 34.3% carried an abnormal haemoglobin type — most commonly HbE trait. For most, two different measurement methods agreed closely. But in those with homozygous HbE or a complex variant, one method could not produce a usable HbA1c at all, while the other still returned a number.²

The practical pointTwo HbA1c results from different laboratories are not always comparable if you carry a haemoglobin variant, and a result that conflicts with your home glucose readings deserves questioning rather than acceptance. The study’s authors argue that in populations where haemoglobinopathies are common, knowing someone’s haemoglobin type is basic information to have before interpreting their HbA1c.²

If thalassaemia trait or another variant runs in your family, or your HbA1c has never matched your glucose meter, that is worth raising. The alternatives are established: fructosamine, or judging control from glucose monitoring directly, sidestep haemoglobin entirely.

OGTT: Oral Glucose Tolerance Test

When is OGTT done?

The Oral Glucose Tolerance Test (OGTT) is the most sensitive test for detecting diabetes and pre-diabetes. You fast overnight, have a fasting blood glucose taken, then drink a solution containing 75g of glucose. Your blood glucose is checked again at 2 hours. It's particularly useful for: diagnosing gestational diabetes in pregnancy (usually at 24–28 weeks), detecting pre-diabetes not picked up by fasting glucose, and confirming borderline results.

Tests to monitor diabetes complications

Diabetes affects many organs over time. The following tests should be done at least annually in people with diabetes:

Kidney monitoring

Test	Target for diabetics	What it checks
eGFR / Creatinine	eGFR >60	Kidney filtering ability
Urine ACR (albumin:creatinine ratio)	<30 mg/g	Earliest sign of diabetic kidney damage
Urine Routine	No protein / glucose	General kidney health screen

Cholesterol / cardiovascular monitoring

Test	Target for diabetics	Why it matters
LDL Cholesterol	<70 mg/dL (high-risk) or <100 mg/dL	Main statin target: diabetes doubles heart risk
Triglycerides	<150 mg/dL	High in poorly controlled diabetes
HDL Cholesterol	Men >40; Women >50 mg/dL	Protective: low HDL = higher heart risk

Other important checks

Test / Exam	Frequency	Why
Blood pressure	Every visit	Target <130/80 mmHg in diabetes
Eye exam (fundoscopy)	Yearly	Diabetic retinopathy: silent until advanced
Foot examination	Yearly (or more often)	Peripheral neuropathy and ulcer risk
TSH (thyroid)	At diagnosis and periodically	Thyroid disease is more common in diabetics
Vitamin B12	Yearly if on metformin	Metformin reduces B12 absorption

Annual diabetes health checklist

HbA1c: every 3–6 months until stable, then every 6 months
Fasting lipid profile (cholesterol): annually
Kidney function: eGFR + urine ACR, annually
Eye examination (diabetic retinopathy screen): annually
Foot examination: annually
Blood pressure check: every visit
Vitamin B12 if on metformin: annually
TSH: at diagnosis and every 1–2 years
Dental check: 6-monthly (gum disease is worse in diabetes)

Target blood sugar ranges for people with diabetes

Time of day	Target (mg/dL)	Target (mmol/L)
Fasting / before meals	80–130	4.4–7.2
2 hours after meals	<180	<10.0
Bedtime	100–140	5.6–7.8
HbA1c target	<7% (individualised)	:

Questions to ask your diabetes doctor

What is my HbA1c target?
Do I have any early kidney damage (urine ACR)?
When did I last have my eyes checked?
Should I be on a statin for heart protection?
Is my metformin affecting my Vitamin B12?
Am I eligible for newer diabetes drugs (GLP-1 or SGLT2 inhibitors) that protect the heart and kidneys?

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Yadav N, Kumar Mandal A. Interference of hemoglobin variants in HbA1c quantification. Clin Chim Acta. 2023;539:55–65. PMID 36476843 · doi:10.1016/j.cca.2022.11.031
Mitchai M, Suwansaksri N, Seanseeha S, et al. Misleading HbA1c measurement in diabetic patients with hemoglobin variants. Med Sci (Basel). 2021;9(2):43. PMID 34200315 · doi:10.3390/medsci9020043