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Diagnosing Diabetes

Diagnosing Diabetes

Criteria for the diagnosis of diabetes have been described by the American Diabetes Association (1) and WHO (2) and are widely adopted.

The use of HbA1c for diagnosing Type 2 diabetes has now been approved in New Zealand. It is now the tool of choice for opportunistic screening with advantages including:

No need for fasting
HbA1c is less afected by day-to-day variation in blood glucose levels
Reduced biological variability (therefore fewer false positives and negatives with HbA1c testing compared to plasma glucose testing)
Simpler laboratory requirements

Although HbA1c testing is expensive, since the long term consequences of undiagnosed diabetes are also expensive this cost is offset by (hopefully) savings made further down the line. Forward thinking!

Criteria for the diagnosis of diabetes mellitus

Symptoms of diabetes plus random plasma glucose > 11.1 mmol/l OR Fasting Plasma Glucose (FPG) > 7.0 mmol/l OR Oral Glucose Tolerance Test (OGTT) 2-hr post glucose load > 11.1 mmol/l

HbA1c test > 50* mmol/mol

* NOTE A diagnostic value of 48 mmol/mol is the international standard – it is anticipated that New Zealand will align with this in the near future.

In the absence of symptoms two separate diagnostic tests are recommended.

All tests should be performed in an accredited laboratory. This means that point-of-care testing – whilst useful – cannot be used definitively for diagnosis.

HbA1c is unsuitable as a diagnostic tool in certain circumstances including:

- For the diagnosis of Type 1 diabetes
- For the diagnosis of diabetes in pregnancy (see ‘Gestational Diabetes‘ below)
- In patients who are acutely unwell

In patients who have had diabetes symptoms for less than 2 months
In patients with certain haemoglobinopathies or anaemias [see page on the HbA1c test]

Pre-diabetes and Impaired Glucose Tolerance

In the context of Type 2 diabetes, it is important to point out that there is an intermediate between the “definitely diabetic” and “definitely not diabetic” states. The term ‘borderline diabetes‘ has been used to describe this. Many will vehemently cry that surely you either have diabetes or you do not; but unfortunately there is a grey area…

The term ‘glucose tolerance‘ describes the ability of the body to keep blood glucose levels within the normal range. There is a grey area within the diagnostic criteria for diabetes in which people with ‘not-quite-normal’ glucose tolerance fall. Such people are said to have ‘impaired glucose tolerance’, or IGT. Some may go on to develop full blown diabetes, others may not. People with IGT are also at risk of developing diabetes related complications.

Another term sometimes used is ‘impaired fasting glucose’ or IFG. This describes the borderline condition in which fasting blood glucose levels are slightly high.

Recently, IGT and IFG have both been reclassified as ‘pre-diabetes‘. People with ‘pre-diabetes’ are at high risk of developing diabetes.

Identifying ‘Pre-diabetes’
Method of testing	FPG	OGTT	HbA1c
Normal	< 5.6 mmol/l	2-hr post glucose load <7.8 mmol/l	< 5.7% <39 mmol/mol
Pre-diabetes	5.6 – 6.9 mmol/l	2-hr post glucose load 7.8 – 11.1 mmol/l	5.7 – 6.5% 39 – 48 mmol/mol
Diabetes	> 7.0 mmol/l	2-hr post glucose load > 11.1 mmol/l	> 6.5% > 48 mmol/mol

Diagnosis of Gestational Diabetes

Diagnosis of diabetes in pregnancy – Gestational DIabetes or GDM – has slightly different criteria; the text below has been taken from the MOH Clinical Practice Guideline (3)

Early pregnancy

Universal screening using glycated haemoglobin (HbA1c), as part of ‘booking’ antenatal blood tests (ideally before 20 weeks), will identify women with probable undiagnosed diabetes or prediabetes. Women with an HbA1c ≥ 50 mmol/mol should be under the care of a service that specialises in diabetes in pregnancy. Women with HbA1c values in the range of 41–49 mmol/mol should be offered the diagnostic oral glucose tolerance test at 24–28 weeks as they are at an increased risk of gestational diabetes. Some local policies currently treat women with HbA1c values in the range of 41–49 mmol/mol.

At 24–28 weeks’ gestation

At 24–28 weeks’ gestation, all women not previously diagnosed with diabetes who are at high risk of gestational diabetes (HbA1c of 41–49 mmol/mol) should be offered the diagnostic two-hour, 75 g oral glucose tolerance test. (If fasting glucose ≥ 5.5 mmol/L or two-hour value ≥ 9.0 mmol/L, refer to services that specialise in diabetes in pregnancy.) All other women should be offered screening for gestational diabetes using the one-hour, 50 g, oral glucose challenge test known as the polycose test. (If glucose ≥ 11.1 mmol/L, refer directly to services that specialise in diabetes in pregnancy without further testing; if glucose ≥ 7.8–11.0 mmol/L, arrange a 75 g, two-hour oral glucose tolerance test (OGTT) without delay). Offer enrolment in the randomised trial of different diagnostic criteria. For further details of the New Zealand GEMS Trial contact gems@auckland.ac.nz or go to www.ligginstrials.org/GEMS

Metabolic Syndrome

Metabolic syndrome is a term often used in relation to diabetes and heart disease. It refers to the cluster of conditions that are the most dangerous of the heart attack risk factors.

A quarter of the world’s adults have metabolic syndrome
People with metabolic syndrome are twice as likely to die from, and three times as likely to have a heart attack or stroke compared with people without the syndrome
People with metabolic syndrome have a five-fold greater risk of developing type 2 diabetes
Up to 80% of the 200 million people with diabetes globally will die of cardiovascular disease
This puts metabolic syndrome and diabetes way ahead of HIV/AIDS in morbidity and mortality terms yet the problem is not as well recognised

Earlier diagnosis is needed to stop this global time bomb. This new International Diabetes Federation (IDF) Worldwide Definition of the Metabolic Syndrome provides physicians with the tools to quickly identify those at risk and also to compare the impact across nations and ethnic groups. The new IDF worldwide definition was developed during a unique consensus workshop

According to the new IDF definition, for a person to be defined as having the metabolic syndrome they must have the following:

Central obesity (defined as waist circumference ≥ 94cm for Europid men and ≥ 80cm for Europid women, with ethnicity specific values for other groups) plus any two of the following four factors:

Raised TG level: ≥ 150 mg/dL (1.7 mmol/L), or specific treatment for this
lipid abnormality
Reduced HDL cholesterol: < 40 mg/dL (1.03 mmol/L*) in males and < 50
mg/dL (1.29 mmol/L*) in females, or specific treatment for this lipid
abnormality
Raised blood pressure: systolic BP ≥ 130 or diastolic BP ≥ 85 mm Hg, or
treatment of previously diagnosed hypertension
Raised fasting plasma glucose (FPG) ≥ 100 mg/dL (5.6 mmol/L), or
previously diagnosed type 2 diabetes

If above 5.6 mmol/L or 100 mg/dL, OGTT is strongly recommended but is not
necessary to define presence of the syndrome.

Page updated: Jan 2025

Explore this section Introducing Diabetes :

Diagnosing Diabetes