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OGTT - there are more gems in there than you think

Dr Chee L Khoo

An oral glucose tolerance test (OGTT) is a challenge test. Examining how the body handles a significant glucose load (75g of glucose) gives us an insight of how tired the body is in its attempt in maintaining glucose haemostasis. As you are aware, glucose dysmetabolism is more than just diabetes. Diabetes, obesity, CV disease, fatty liver and gout are all interlinked.

Have you notice that there is a whole variety of patterns to the OGTT results? Some patients have normal fasting glucose but fail the 2 hour reading. Others have high fasting glucose but normal (or close to normal) 2 hour readings. Yet others fail both fasting and 2 hour readings. If you add a 30 mins or 60 mins readings to the standard 0 and 120 mins glucose readings, the combinations and permutations of glucose patterns becomes even more varied. Epidemiological studies have shown that 30 min glucose readings may be more predictive of risk for future diabetes than FPG, 2hPG or HbA1c. (2-5).

Do these different patterns have any bearing on risk of future diabetes (if they do not fail the OGTT) or CV mortality (irrespective of the diagnosis of diabetes)? Well, someone has done a study looking into those glucose patterns.

Adam Hulman et al interrogated the Danish Inter99 study data and looked at 5861 non-diabetic patients who underwent a OGTT with measurements of plasma glucose at 0, 30, 120 mins. Follow up time on the subjects were 11-13 years. Incident diabetes, CV disease and all cause mortality information were extracted from the Danish national registry. Cardiometabolic risk factors were adjusted for in the modelling. Insulin sensitivity index were also calculated using the equation by Gutt.(1)OGTTPattern

They were able to describe 4 distinct patterns of glucose profiles. Not unexpectedly, patients with increasingly high 30 min glucose also had high glucose at 120 mins. One unusual pattern (so called Class 3 pattern) had very high 30 min glucose reading but low 120 min glucose levels. (see Fig 1).

Diabetes Risk

Not unexpected, the higher the 30 min glucose levels are, the higher the risk of diabetes. For example, participants with Class 3 glucose patterns (third highest 30 min readings) have a 3 fold risk of developing T2D compared with participants with Class 1 patterns even though Class 3 had the same glucose levels at 120 mins. (see fig 2). But Class 3 had half the risk of developing diabetes compared with Class 4 even though Class 4 had a slightly elevated 120 min glucose levels.

DiabetesRiskThis is in line with previous studies that report high risk of diabetes amongst those whose glucose excursion did not return to baseline after 2 hours.

Cardiovascular Disease

Despite the progressively higher 30 min glucose, there were no significant differences in CV disease between the classes. Participants with Class 3 patterns had high level of insulin sensitivity and it is thought perhaps, that this might explained the lower CVD risk. This is in line with previous studies including previous Finnish study finding 2hPG to be a more important marker of CVD risk than FPG and 1hPG among people with impaired fasting plasma glucose or impaired glucose tolerance (7).

All cause mortality

Both Class 3 and 4 had significantly higher all cause mortality than Class 1 and 2 eventhough Class 3 had the same or lower fasting and 120 min glucose levels than Class 1 and 2.

There’s more to OGTT than the 2 hour readings.

Reference:

  1. Gutt M, Davis CL, Spitzer SB et al Validation of the insulin sensitivity index0, 120: comparison with other measures. Diabetes Res Clin Pract 47:177–184
  2. Abdul-Ghani MA, Williams K, DeFronzo RA, Stern M (2007) What is the best predictor of future type 2 diabetes? Diabetes Care 30:1544–1548
  3. Alyass A, Almgren P, AkerlundMet al (2015)Modelling of OGTT curve identifies 1 h plasma glucose level as a strong predictor of incident type 2 diabetes: results from two prospective cohorts. Diabetologia 58:87–97
  4. Jagannathan R, Sevick MA, Li H et al (2016) Elevated 1-hour plasma glucose levels are associated with dysglycemia, impaired beta-cell function, and insulin sensitivity: a pilot study from a real world health care setting. Endocrine 52:172–175
  5. Bergman M, Chetrit A, Roth J, Dankner R (2015) Dysglycemia and long-term mortality:  observations from the Israel study of glucose intolerance, obesity and hypertension. DiabetesMetab Res Rev 31: 368–375
  6. Lind M, Tuomilehto J, Uusitupa M et al (2014) The association between HbA1c, fasting glucose, 1-hour glucose and 2-hour glucose during an oral glucose tolerance test and cardiovascular disease in individuals with elevated risk for diabetes. PLoS One 9:e109506
  7. Adam Hulman, Dorte Vistisen, Charlotte Glümer, Michael Bergman, Daniel R. Witte, Kristine Fĉrch. Glucose patterns during an oral glucose tolerance test and associations with future diabetes, cardiovascular disease and all-cause mortality rate. Diabetologia

The views expressed are views of the authors and do not necessarily represent the views of the board of SSW GP Link

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