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Best Tests September 2006

Including: Full colour PDF of ‘best tests’ September 2006. UTI PDF

Fasting vs Post Prandial glucose “no quick answer”

Following the bpacnz campaign ‘Laboratory Testing in Diabetes’, we were asked why fasting glucose was the best initial test for diagnosing diabetes, and the role of the post prandial glucose. We contacted Rick Cutfield for a comparison and explanation of the role of these tests.

Screening guidelines throughout the world recommend fasting glucose as the best test, mainly because of standardisation, reproducibility and reasonable correlation with the oral glucose tolerance test. However there are some problems with this. The specificity of a fasting glucose greater than 7.0 mmol/L is over 95% but sensitivity is only about 50%. It is less reliable and sensitive in the elderly and not reliable in all ethnic groups. As a result some people with fasting glucose of 5.5 ā€“ 6.0 mmol/L will have diabetes on further testing.

Post prandial tests are even more difficult to interpret because of the different glycaemic index of foods and different times after meals but they may be quite important prognostically and may predict cardiovascular disease better than the fasting levels. A casual glucose (without regard to time of last meal) of greater than 11 mmol/L on two occasions is diagnostic of diabetes and a casual glucose less than 6.9 mmol/L is probably normal. Tests in between these may end up in the impaired glucose tolerance area.

We have to balance the ease of random glucose, the slightly more inconvenient fasting test and the greatly more inconvenient oral glucose tolerance test with the importance of making an accurate diagnosis.

Despite all of this I suggest that most people can do a fasting glucose if it is explained and all international guidelines use this test.

I still stick with the principles below:
  • If the fasting glucose is greater than 7.0 mmol/L with classic symptoms (on two occasions if asymptomatic) then diabetes is diagnosed.
  • If the fasting test is between 6.1 - 6.9 mmol/L order an oral glucose tolerance test.
  • If the fasting test is between 5.5 - 6.0 mmol/L order an oral glucose tolerance test if the person is in a high risk group.
  • If the fasting test is less than 5.5 mmol/L, it is normal and the patient can be retested in 3 ā€“ 5 years depending again on risk factors.

HbA1C is not yet reliable enough to be used as a screening or diagnostic test.

I always see the glucose as part of the general cardiovascular screening package and as you know it is only one part of the screening program with fasting lipids, blood pressure and smoking all crucial. The screening targets are moving and we need to continually update them as more evidence is provided. But for now Iā€™d stick to the fasting glucose as our preferred screening tool.

Dr Rick G Cutfield MB CHB, FRACP
Physician and Endocrinologist
North Shore Hospital
Waitemata District Health Board