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August 25, 2022

Tests for Prediabetes

There are three suggested blood tests that can be used to detect and diagnose prediabetes:

  • The most common test in the UK is HbA1c (Glycated haemoglobin test)
  • FPG (Fasting plasma glucose)
  • OGTT (Oral glucose tolerance test)

These tests also identify type 2 diabetes before diagnosis, as there is an intermediate range between normal and type 2 diabetes. It is this range that is considered diagnostic for prediabetes.

The HbA1c blood test has advantages as it doesn’t involve fasting or lengthy lab visits. It simply uses average sugar levels over the last two to three months.

The FPG test involves fasting for eight hours, or overnight, before the blood test. You also need to fast for OGTT. The test consists of two blood tests, one at the beginning of the appointment, and the other two hours later after having a sugary drink.

Prediabetes diagnosis thresholds

The results indicating prediabetes are:

  • HbA1c - 2mmol/mol to 47mmol/mol or 6% to 6.4% or 4
  • FPG - 5.6mmol/L-6.9mmol/L or 100mg/dL to 125 mg/dL
  • OGTT - 7.8mmol/L and 11.1mmol/L or 140 mg/dL to 199 mg/dL (Two hours after a sugary drink)

A fasting plasma glucose (FPG) test includes fasting for eight hours, or overnight, and having a blood test. A blood sugar level of 5.6mmol/L-6.9mmol/L or 100–125 mg/dL indicates prediabetes.

For an oral glucose tolerance test (OGTT), blood is taken once and then again two hours later after drinking a sugary drink. Blood sugar level between 7.8mmol/L and 11.1mmol/L (140–199mg/dL) indicates prediabetes or impaired glucose tolerance (IGT). 

If results are below these figures, there is still a possibility of developing type 2 diabetes in the future; however the risk of developing type 2 diabetes increases when results lie within these prediabetes ranges.

The National Institutes of Health (NIH)[1] suggests taking the same test twice to confirm a diagnosis.

It is important to remember that these tests do not always identify prediabetes in the same way for some people. Some may have prediabetes results from a Hb1Ac test, but their FPG may be within the ‘normal’ range.

If prediabetes is suspected but results are ‘normal’, it is important to repeat a test in a short period of time, such as within a year, or to confirm the ‘normal’ result with one of the other tests.

For those with prediabetes under the age of 60 and with other risk factors, metformin (a medication used to treat and prevent type 2 diabetes) should be considered.



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