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Q: Is an OGTT good enough for CF diabetes testing?

23 Jul, 08 | by Bob Phillips

Glucose Tolerance DrinkJamie, a 13-year-old girl with cystic fibrosis (CF), has been referred to the Paediatric Diabetes clinic because of an abnormal oral glucose tolerance test (OGTT) in her recent CF Annual Review. It showed impaired glucose tolerance. Continuous glucose monitoring system (CGMS) over three days showed normal fasting and pre-prandial glucose but frequent post-prandial glucose excursions between 11.1 mmol/l and 16.0 mmol/l. In retrospect, although Jamie’s previous OGTT carried out at Annual Review were normal, her lung function had gradually deteriorated over the past three years. This had been attributed to increasing episodes of infective exacerbations. You wondered for how long Jamie has had abnormal glucose metabolism which had gone undetected in her previous OGTT.The cut-offs which turn a test, especially one with a ‘continuous outcome’ from negative to positive are frequently debated… and with good reason. Those who see the ‘late’ diagnoses wish for greater sensitivity, but those who repeatedly see the fear produced by a false positive wish for better specificity. Underlying it all are values which are often decided by the most statistically attractive cut point, rather than an appreciation the the costs and benefits (in financial, clinical resource and patient experience)
of the effects the statistics imply.

What about here – is the OGTT just too coarse a measure for the job of detecting CF induced diabetes? Ngee Lek and Carlo L Acerini from Cambridge (UK) have set themselves the task of answering the question.

Acknowledgement: Image from gisarah under CreativeCommons2.0

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  • Matthew Babirecki

    The patient experience is a very important issue when looking at chronic, mult-system diseases such as CF. We know that having CF related diabetes is associated with worsening lung function and its incidence is increasing as CF patients live longer. When I worked in a CF clinic a lot of patients weren’t keen on coming to clinic early with no breakfast for on OGTT. In fact I know that 1 in 3 adult patients in the same centre didn’t attend for an annual OGTT. Continuous monitoring may well be preferred by many patients.
    I don’t think it is clear cut when, and how, to treat CF related diabetes. So when discussing the cut-offs for sensitivity/specificity we don’t necessarily know what we will do with a positive result. As the patient progresses from one annual review to another the other aspects of their disease will be changing. The diagnosis could be seen as a ‘welcome’ explanation for a drop in lung function that gives the patient something to tackle, or it could be adding to the strain of having a serious disease that already requires multiple medications/nebs/gastrostomy feeds/physio, etc.
    When we are performing a test we should be explaining why are doing it, and what is meant by a positive or negative result. This is tricky when doing annual reviews involving multiple tests in conditions such as CF.

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