Blood Tests for Diabetes
How diabetes is diagnosed in Australia — the four key blood tests, what each threshold means, and what your GP does with the results.
The Four Diabetes Blood Tests
Diabetes is diagnosed through blood tests that measure how your body handles sugar (glucose). There are four main tests, each with different strengths. Your GP will choose based on your symptoms, risk factors, and clinical context.
Fasting Blood Glucose (FBG)
The concentration of glucose in your blood after 8-12 hours of fasting. This reflects your baseline blood sugar level when not influenced by recent food.
3.5 – 5.4 mmol/L
5.5 – 6.9 mmol/L (Impaired Fasting Glucose)
≥ 7.0 mmol/L on two separate occasions
Best for: Initial screening and routine monitoring. Quick, inexpensive, widely available. The most commonly ordered diabetes screening test in Australian general practice.
Limitations: Only a snapshot of one moment. Affected by stress, illness, medications, poor sleep, and whether you truly fasted. A single result is not diagnostic — must be confirmed.
HbA1c (Glycated Haemoglobin)
The percentage of haemoglobin in red blood cells that has glucose permanently attached. Since red blood cells live approximately 120 days, HbA1c reflects your average blood sugar over 2-3 months.
< 42 mmol/mol (< 6.0%)
42 – 47 mmol/mol (6.0 – 6.4%)
≥ 48 mmol/mol (≥ 6.5%) on two separate occasions
Best for: Confirming diagnosis, monitoring treatment effectiveness, assessing long-term control. Not affected by a single bad day. Preferred for patients who cannot fast reliably.
Limitations: Unreliable in conditions that affect red blood cell lifespan: iron deficiency anaemia, haemoglobinopathies (thalassaemia), recent blood transfusion, chronic kidney disease, and pregnancy (first two trimesters). Also not validated in children under 10.
Oral Glucose Tolerance Test (OGTT)
How your body processes a standardised glucose load (75g) over 2 hours. Blood is drawn fasting, then at 1 and 2 hours after drinking the glucose solution.
2-hour glucose < 7.8 mmol/L
2-hour glucose 7.8 – 11.0 mmol/L (Impaired Glucose Tolerance)
2-hour glucose ≥ 11.1 mmol/L
Best for: Diagnosing gestational diabetes (mandatory screening at 24-28 weeks in Australia). Also useful for borderline cases where fasting glucose and HbA1c disagree. The most sensitive test for detecting impaired glucose tolerance.
Limitations: Time-consuming (2+ hours at the pathology lab). Unpleasant glucose drink can cause nausea. Must be performed correctly — results invalid if you vomit, eat during the test, or had inadequate carbohydrate intake in the days before.
Random Blood Glucose
Blood sugar at any random point in time, regardless of when you last ate. Usually done in emergency departments, hospital wards, or opportunistically during other blood tests.
4.0 – 7.7 mmol/L (varies with meals)
Not applicable — random glucose is not used for pre-diabetes screening
≥ 11.1 mmol/L with classic symptoms (thirst, polyuria, weight loss)
Best for: Emergency screening when diabetes is clinically suspected. Quick result. Useful when a patient presents with classic symptoms and immediate assessment is needed.
Limitations: Highly variable depending on recent meals. Only diagnostic when 11.1 mmol/L or above AND the patient has classic diabetes symptoms. Cannot detect pre-diabetes. Not suitable for routine screening.
Types of Diabetes — How Blood Tests Distinguish Them
Type 1 Diabetes
Key tests: Random or fasting glucose (usually dramatically elevated at diagnosis, often above 15 mmol/L). Autoantibody testing: GAD antibodies, IA-2 antibodies, ZnT8 antibodies. C-peptide (low or absent — indicates the pancreas is not producing insulin).
Usually diagnosed in children, teens, or young adults (but can occur at any age). Onset is rapid — weeks to months. Presents with extreme thirst, frequent urination, weight loss, and sometimes diabetic ketoacidosis (DKA). The immune system has destroyed insulin-producing beta cells. Requires lifelong insulin injections from diagnosis.
Type 2 Diabetes
Key tests: Fasting glucose of 7.0 mmol/L or above and/or HbA1c of 48 mmol/mol or above on two occasions. No autoantibodies. C-peptide is normal or elevated (the pancreas is still producing insulin, but the body is resistant to it).
Usually diagnosed in adults over 40 (but increasingly seen in younger people). Develops gradually over years. Often detected incidentally on routine blood tests with no symptoms. Strongly associated with excess weight, inactivity, family history, and ethnicity. Initial management is lifestyle modification and metformin.
Gestational Diabetes (GDM)
Key tests: OGTT at 24-28 weeks gestation. Diagnostic thresholds (ADIPS 2014): fasting of 5.1 mmol/L or above, 1-hour of 10.0 mmol/L or above, or 2-hour of 8.5 mmol/L or above. Any ONE abnormal value is diagnostic.
Develops during pregnancy due to placental hormones increasing insulin resistance. Usually resolves after delivery. However, 50% of women with GDM develop Type 2 diabetes within 5-10 years. All women with GDM should have an OGTT at 6-12 weeks postpartum, then annual screening.
Pre-Diabetes
Key tests: Fasting glucose 5.5-6.9 mmol/L (Impaired Fasting Glucose) or HbA1c 42-47 mmol/mol, or OGTT 2-hour glucose 7.8-11.0 mmol/L (Impaired Glucose Tolerance).
Not yet diabetes, but significantly increased risk. Without intervention, 5-10% of people with pre-diabetes progress to Type 2 diabetes each year. With lifestyle changes (5-7% weight loss, 150 min/week exercise), progression can be prevented or delayed by 58%. Fully reversible in many cases.
Gestational Diabetes Testing
In Australia, all pregnant women are offered gestational diabetes screening with an OGTT at 24–28 weeks gestation. Women with high-risk factors (previous GDM, BMI above 35, PCOS, ATSI background, family history) may be tested earlier in pregnancy.
The ADIPS (2014) criteria are stricter than general diabetes thresholds because even mildly elevated glucose during pregnancy increases risks to both mother and baby. Only one abnormal value on the OGTT is needed for diagnosis:
Fasting
≥ 5.1 mmol/L
1-hour
≥ 10.0 mmol/L
2-hour
≥ 8.5 mmol/L
How Often Should You Be Tested?
Healthy adult, no risk factors
Tests: Fasting glucose
Frequency: Every 3 years from age 40 (RACGP/Diabetes Australia)
High-risk (family history, obesity, ATSI)
Tests: Fasting glucose or AUSDRISK
Frequency: Every 1-3 years from age 18 (ATSI) or age 40 (others)
Pre-diabetes diagnosed
Tests: Fasting glucose + HbA1c
Frequency: Every 6-12 months. OGTT if transitioning
Type 2 diabetes, stable
Tests: HbA1c
Frequency: Every 3-6 months. Annual comprehensive check (kidneys, eyes, lipids, feet)
Type 2 diabetes, changing treatment
Tests: HbA1c
Frequency: Every 3 months until target achieved (usually below 53 mmol/mol)
Type 1 diabetes
Tests: HbA1c + daily self-monitoring (CGM or finger prick)
Frequency: HbA1c every 3 months. CGM provides real-time continuous data
Pregnancy (no prior GDM)
Tests: OGTT
Frequency: 24-28 weeks gestation. Earlier if high risk
Post-GDM
Tests: OGTT at 6-12 weeks postpartum, then fasting glucose annually
Frequency: Lifelong annual screening — 50% develop Type 2 within 10 years
Medicare Coverage for Diabetes Testing
Australia has comprehensive Medicare coverage for diabetes screening, diagnosis, and ongoing management. Here is what is covered.
Fasting blood glucose
Fully bulk-billed with GP referral. No out-of-pocket cost at any bulk-billing pathology lab.
HbA1c
Bulk-billed for diagnosis and monitoring. Recommended every 3-6 months for people with diabetes.
OGTT (75g)
Bulk-billed. Universal screening recommended at 24-28 weeks pregnancy. Also covered for non-pregnant patients when clinically indicated.
C-peptide
Covered when investigating diabetes type (distinguishing Type 1 from Type 2).
Autoantibodies (GAD, IA-2)
Covered when Type 1 diabetes is suspected, particularly in adults who may have latent autoimmune diabetes (LADA).
Annual diabetes cycle of care
GPs receive a Medicare incentive (SIP item 2517/2521) for completing the annual cycle: HbA1c, lipids, kidney function, eye check referral, foot exam. Ensures comprehensive monitoring.
NDSS registration
National Diabetes Services Scheme provides subsidised blood glucose strips, insulin needles, and CGM sensors. Registration is free through your GP or diabetes educator.
Chronic Disease Management plan
Medicare-funded GP Management Plan (item 721) + Team Care Arrangement (item 723) provides 5 allied health visits per year — dietitian, exercise physiologist, podiatrist, diabetes educator.
What Your Results Mean at Each Stage
Understanding where you fall on the spectrum is critical because the actions are very different at each stage. Early intervention in the pre-diabetes stage is one of the most evidence-based preventive measures in all of medicine.
Normal
No action needed. Continue healthy lifestyle. Rescreen in 3 years (or as per risk profile).
Pre-Diabetes
Lifestyle intervention is the priority: 5-7% weight loss, 150 min/week exercise, dietary modification. Consider referral to Life!/Get Healthy program. Retest every 6-12 months. Metformin may be considered if lifestyle changes insufficient.
New Diabetes
Confirm with second test. Register with NDSS. Start metformin (unless contraindicated). Arrange diabetes educator consultation. Begin annual diabetes cycle of care. Assess for complications (eyes, kidneys, feet, cardiovascular risk).
Frequently Asked Questions
Which blood test is best for diagnosing diabetes?
HbA1c and fasting glucose are both recommended by Australian guidelines. HbA1c has the advantage of not requiring fasting and not being affected by one bad day. Fasting glucose is more sensitive to early changes. In practice, many GPs order both together. Diagnosis requires two abnormal results — either two of the same test or one of each.
Can I have a normal fasting glucose but still be diabetic?
Yes, though it is uncommon. Some people have normal fasting glucose but abnormal post-meal glucose (impaired glucose tolerance). This is only detected by an OGTT. Also, in early Type 2 diabetes, fasting glucose may be borderline while HbA1c is already in the diabetic range because HbA1c captures post-meal spikes that fasting glucose misses.
How accurate is HbA1c?
HbA1c is highly reliable for most people. However, it can be falsely low in iron deficiency anaemia (common in Australian women), haemolytic anaemia, or recent blood loss/transfusion. It can be falsely high in some haemoglobin variants (rare) and very high iron levels. If your HbA1c does not match your fingerprick readings or clinical picture, your GP may investigate further.
My fasting glucose was 6.1 mmol/L. Is that pre-diabetes?
Yes, a fasting glucose between 5.5 and 6.9 mmol/L is classified as Impaired Fasting Glucose (IFG), which is a form of pre-diabetes. This means your body is starting to lose its ability to regulate blood sugar. The evidence is clear: lifestyle changes (5-7% weight loss, 150 min/week of moderate exercise) reduce the risk of progressing to diabetes by 58%. Ask your GP about a referral to the Life! program (Victoria) or Get Healthy (NSW) for free coaching.
Do I need to fast for all diabetes blood tests?
No. Only fasting glucose and OGTT require fasting (8-12 hours for fasting glucose, overnight for OGTT). HbA1c and random glucose do not require fasting. This is one reason HbA1c is increasingly preferred for both diagnosis and monitoring — patients can have blood drawn at any time without special preparation.
How is gestational diabetes different from other types?
Gestational diabetes (GDM) develops during pregnancy and usually resolves after delivery. It is caused by placental hormones that increase insulin resistance. GDM uses different, stricter diagnostic thresholds than general diabetes (ADIPS criteria). A single abnormal OGTT value is enough for diagnosis, whereas non-pregnant adults need two abnormal tests. The key concern is that GDM significantly increases the risk of developing Type 2 diabetes later in life.
Related Reading
Track Your Diabetes Markers
Upload your blood test results and see your glucose and HbA1c trends charted over time — giving you and your GP a clear picture of your diabetes management progress.
This information is based on guidelines from the Australian Diabetes Society (ADS), the RACGP, the Australasian Diabetes in Pregnancy Society (ADIPS), and Diabetes Australia. Diagnostic thresholds follow current Australian practice. SmarterBlood provides educational information only and is not a substitute for professional medical advice.