Fasting blood glucose (FBG) in diagnosis of diabetes

Diabetes is diagnosed on the basis of history (ie polyuria, polydipsia and unexplained weight loss) PLUS

• a random venous plasma glucose concentration >= 11.1 mmol/l
• OR a fasting plasma glucose concentration >= 7.0 mmol/l (whole blood >= 6.1 mmol/l)
• OR 2 hour plasma glucose concentration >= 11.1 mmol/l 2 hours after 75g anhydrous glucose in an oral glucose tolerance test (OGTT)

With no symptoms diagnosis should not be based on a single glucose determination but requires confirmatory plasma venous determination. At least one additional glucose test result on another day with a value in the diabetic range is essential, either fasting, from a random sample or from the two hour post glucose load (1,2). If the fasting or random values are not diagnostic the 2-hour value should be used.

These diagnostic criteria for diagnosing and classifying diabetes were applied to the management of diabetes in the UK from June 1st 2000 (1). The new criteria included lowering the threshold for diagnosing diabetes from a fasting glucose level of 7.8 mmol/l to 7.0 mmol/l.

It should be noted that children usually present with severe symptoms and diagnosis should then be based on a single raised blood glucose result, as above. Immediate referral to a Paediatric Diabetes Team should not be delayed.

A diagnosis should never be made on the basis of glycosuria or a stick reading of a finger prick blood glucose alone, although such tests may be useful for screening purposes.

HbA1c in the diagnosis of diabetes mellitus (3)

• HbA1c can be used as a diagnostic test for diabetes providing that stringent quality assurance tests are in place and assays are standardised to criteria aligned to the international reference values, and there are no conditions present which preclude its accurate measurement
• an HbA1c of 6.5% (48 mmol/mol) is recommended as the cut point for diagnosing diabetes (i.e. HbA1c >= 6.5% (48 mmol/mol) is sufficient for the diagnosis of diabetes)
• a value of less than 6.5% (48 mmol/mol) does not exclude diabetes diagnosed using glucose tests
  • diagnosis of diabetes in an asymptomatic person should not be made on the basis of a single abnormal plasma glucose or HbA1c value. At least one additional HbA1c or plasma glucose test result with a value in the diabetic range is required, either fasting, from a random (casual) sample, or from the oral glucose tolerance test (OGTT)
  • advisable to use one test or the other but if both glucose and HbA1c are measured and both are 'diagnostic' then the diagnosis is made. If one only is abnormal then a further abnormal test result, using the same method, is required to confirm the diagnosis
    
    
• Situations where HbA1c is not appropriate for diagnosis of diabetes (4):
  • ALL children and young people
    
    
  • Patients of any age suspected of having Type 1 diabetes
    
    
  • Patients with symptoms of diabetes for less than 2 months
    
    
  • Patients at high diabetes risk who are acutely ill (e.g. those requiring hospital admission)
    
    
  • Patients taking medication that may cause rapid glucose rise e.g. steroids, antipsychotics
    
    
  • Patients with acute pancreatic damage, including pancreatic surgery
    
    
  • In pregnancy
    
    
  • Presence of genetic, haematologic and illness-related factors that influence HbA1c and its measurement

Reference:

1. CMO's Update, 26, May 2000.
2. MeReC Briefing 2004; 25:1-8.
3. WHO (2011).Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus (Abbreviated Report of a WHO Consultation). WHO/NMH/CHP/CPM/11.1
4. Diabetes UK. New diagnostic criteria for diabetes (Jan 2011) - Use of Haemoglobin A1c (HbA1c) in the diagnosis of diabetes mellitus in the UK