Diabetes GPSI Dr Roger Gadsby on diagnosing diabetes and ‘pre-diabetes’, and the potential of HbA1c as a diagnostic test
Diabetes is currently diagnosed on the basis of raised plasma glucose levels, as outlined in World Health Organization (WHO) guidelines from 1999.1 Blood glucose measurements are also used to define impaired glucose tolerance (IGT) and impaired fasting glucose (IFG), which together have been labelled as ‘pre-diabetes’.
An International Expert Committee is now suggesting that HbA1c can be used to diagnose diabetes and pre-diabetes,2 and this is being considered by the WHO.
Current diagnostic criteria
Based on fasting glucose, WHO criteria for diagnosing diabetes1 were adopted in the UK in 2000, and give the criteria for diagnosing diabetes on fasting or random blood glucose measurements.
Based on fasting blood results, diabetes is diagnosed as a fasting plasma glucose of 7mmol/l and above.
Based on random or post-glucose challenge results, the blood glucose level for diagnosing diabetes on a random plasma glucose or a post 75g glucose challenge is 11.1mmol/l and above.
In asymptomatic people, two abnormal blood glucose measurements diagnostic of diabetes must be obtained.
In IFG and IGT, patients with fasting glucose levels below 6mmol/l are classified as normal and those with levels between 6 and 7mmol/l are classified as having IFG.
If IFG is diagnosed, Diabetes UK3 recommends that an oral glucose tolerance test is done to find out whether the patient might have blood glucose levels diagnostic of diabetes after an oral glucose tolerance test (OGTT). To perform an OGTT, a person has a fasting blood specimen taken and is then given 75g of glucose orally. Blood glucose is measured again two hours later.
If the two-hour glucose level is:
• 11.1mmol/l or above, diabetes is diagnosed
• below 7.8mmol/l, it is classified as normal
• between 7.8 and 11.1mmol/l, a diagnosis of IGT is made.
IGT and IFG together are now being called pre-diabetes.
Do not use finger-prick testing alone to diagnose diabetes.
The diagnosis of diabetes has important medicolegal implications (such as for driving and insurance) so diagnostic blood glucose estimations must be from a laboratory with appropriate quality-control mechanisms, rather than handheld glucose oxidase stick-testing measurements.
HbA1c as a diagnostic tool
But what are the implications of using HbA1c? My current practice is usually to order an HbA1c test as well as a fasting glucose measurement when I suspect diabetes. I find that the added information given by an HbA1c reading at the time of diagnosis of diabetes is very helpful. Many other GPs and practice diabetes nurses also follow this approach, so I do not think that adopting the WHO HbA1c criteria for diagnosing diabetes will mean many extra tests will be ordered.
In my opinion, the area in which HbA1c as a diagnostic test for diabetes will have most impact is in screening. The glucose test in many screening strategies for diabetes is a fasting glucose measurement because random glucose measurements are very difficult to interpret. It therefore means that screening for diabetes with a fasting glucose measure has to be done between about 7.30 and 9.30 in the morning, which severely limits the number of people that can be screened each day.
HbA1c measurements can be taken any time of the day and if used as the test of glycaemia in a screening programme would speed up large-scale screening programmes.
People with IFG/IGT or an HbA1c at or above 6% but below 6.5% are at increased risk of developing diabetes, although the expert panel was keen to point out that not everyone in these categories will develop the condition – so pre-diabetes as a term is misleading and should be dropped.
It is good practice therefore to offer this group of patients (who are at highest risk) an annual screening test to see if they have developed diabetes.
The annual screen could now be a measurement of fasting glucose and HbA1c.
Diagnosing diabetes early in this way is likely to mean that the HbA1c at diagnosis is below 7% so helping the practice towards achieving the QOF target of 50% of people with diabetes having an HbA1c level at or below 7%.
Many practices keep a register of such people and have developed a call-and-recall system to enable them to offer this sort of annual diabetes screen. People who have had gestational diabetes are also at an increased risk of developing diabetes and they can be included in this register to ensure that they are offered annual screening.
Dr Roger Gadsby is a GP and associate clinical professor at Warwick Medical School, University of Warwick
Competing interests: none declared
Diagnosis Diagnosing diabetes