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Best Tests June 2006

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Laboratory Testing in Diabetes

Fasting blood glucose is the best initial test for diagnosing diabetes - Equivocal fasting glucose results, should be followed with a glucose tolerance test. Urine glucose and HbA1C are not appropriate for diagnosis.

Fasting blood glucose testing frequency:

  • For patients with normal glucose metabolism - test every 5 years (or 3 years for those at risk).
  • For patients with impaired glucose tolerance or impaired fasting glucose - test annually. Early identification is important because lifestyle interventions can delay and prevent the onset of diabetes.

HbA1C is the best test of glycaemic control in diabetes – Aim for less than 7%, over 8% indicates inadequate control.

HbA1C testing frequency:

  • For patients with stable diabetes - test 6 monthly.
  • For patients who have had changes in treatment - test no more than 3 monthly.

Preventing cardiovascular disease

  • Cardiovascular disease is a commom cause of morbidity in diabetes. Fasting lipid levels should be measured 3 monthly until stable and then 6 - 12 monthly.

Preventing diabetic renal disease

  • Use urinary albumin:creatinine ratio (ACR) and a serum creatinine with eGFR to test for diabetic renal disease. These tests should be performed at time of diagnosis and repeated at least annually.
  • Perform more frequently if there is microalbuminuria or reduced eGFR.

Testing for diabetes should be performed as part of a cardiovascular risk assessment

NZGG recommends this should begin at the following ages:

Age to commence testing
Population group Men Women
Asymptomatic people without known risk factors 45 years 55 years
Māori, Pacific peoples and people from the Indian subcontinent 35 years 45 years
People with cardiovascular risk factors or at high risk* of developing diabetes 35 years 45 years

*Factors associated with high risk of developing diabetes include:

  • Māori, Pacific or Indian ethnicity
  • Increasing age
  • Metabolic syndrome
  • Impaired glucose tolerance
  • Polycystic ovary syndrome
  • History of gestational diabetes or having a baby over 4 kg
  • Family history of diabetes
  • Physical inactivity
  • Increased BMI
  • Central Obesity
  • Hypertension
  • Adverse lipid profile
  • Elevated LFTs
  • Patients taking some drugs e.g. prednisone or anti-psychotic drugs (haloperidol, chlorpromazine, and newer/atypical antipsychotics)

Does testing serum glucose at the same time as HbA1C provide additional information?

Nationally on two thirds of occasions when GP’s requested HbA1C they also requested a glucose. This is probably higher than necessary because when a person has type 2 diabetes, is not taking insulin, is well and on a stable treatment regimen, there is seldom reason to test fasting glucose levels. HbA1C is the best test of glycaemic control and a better tool for managing treatment. In people with stable diabetes it is tested 6 monthly. Test no more than 3 monthly if their diabetes is unstable or following changes in treatment.

Glucose tests may often be performed because patients tend to focus more on fasting glucose levels as measures of adequacy of control because the concept of HbA1C is generally more difficult for patients to understand. Explaining and reinforcing the use of the HbA1C test, enables patients to then incorporate target levels of HbA1C into their goals.