Risk factors

Experts caution against using HbA1c as a screening tool in CF


Despite a large prospective study recently suggesting that HbA1c is effective as a screening tool for cystic fibrosis related diabetes (CFRD), a group of Australian experts have warned against its widespread use for now.

In April this year a study published in the Journal of Cystic Fibrosis showed that at a cut off level of ≥5.8% HbA1c had a high sensitivity of 93.8% with respect to detecting CF patients at risk of a positive oral glucose tolerance test (OGTT), and reduced the need to perform an “inconvenient and time-consuming” OGTT by around 51%.

But in a Letter to the Editor Dr John Widger, a respiratory physician from Sydney Children’s Hospital, and colleagues raised concern that using the proposed cut off for HbA1c would fail to detect patients with early glucose abnormalities.

Using this criterion, 27 patients with impaired glucose tolerance (IGT) were missed in the study, they noted.

Speaking to the limbic, Dr Widger says that using a screening tool that is less sensitive than the OGTT is moving in the wrong direction when all the evidence so far suggests that CFRD needs to be diagnosed and treated earlier than it is now.

“What we’ve noticed is that when we diagnose CFRD, patients have been declining in health for three or four years prior to that, so the obvious conclusion from this is that the test [OGTT] isn’t sensitive enough. So in other words we’re picking them up too late.”

Research also indicates that insulin started at the diagnosis of CFRD does not completely reverse the clinical decline seen before the diabetes diagnosis, he added.

“Clearly early diagnosis and treatment of early glucose abnormalities may be desirable to prevent this decline,” Dr Widger said, noting that his group is conducting a randomised clinical trial to determine whether starting insulin in this early stage of insulin deficiency will prevent clinical deterioration.

“Therefore until we can clarify if insulin treatment should be offered to patients with early insulin deficiency in CF, we would caution against the widespread use of HbA1c as a screening tool in this population”.

Dr Widger also argues that the two-hour OGTT was never really designed to look at CFRD and is not the right test to be using as the gold standard.

He says a more accurate diagnostic test or algorithm is needed, and believes it may important to look at intermittent hyperglycaemia in people with CF.

In a recent pilot study his group showed that a peak glucose of 8.2 mmol/L or above found during a 30 minutely sampled OGTT was predictive of weight and lung function decline, irrespective of what the two hour glucose level was.

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