Diagnosis of cystic fibrosis related diabetes: a selective approach in performing the oral glucose tolerance test based on a combination of clinical and biochemical criteria.

BACKGROUND: Cystic fibrosis related diabetes (CFRD) has become increasingly common with the increasing longevity of patients with cystic fibrosis. The diagnosis of CFRD is important as its development may lead to a clinical deterioration which may be reversed with treatment. The oral glucose tolerance test (OGTT) is the method of choice in the diagnosis of CFRD, but performing OGTTs on all patients is inconvenient for patients and labour intensive for staff. The aim of this study was to identify a more selective approach in performing OGTTs in the diagnosis of CFRD based on the use of a combination of clinical and biochemical criteria.
METHODS: Clinically stable adult patients with cystic fibrosis not known to be diabetic attending the Royal Brompton Hospital Cystic Fibrosis Clinic for their annual review were invited to return within a month to have an OGTT. The result of the OGTT was compared with the results of tests performed during the annual review. The sensitivities and specificities of various methods used in the screening or diagnosis of CFRD were determined using OGTT as the "gold standard" diagnostic method. The combination of clinical and biochemical criteria which resulted in the highest sensitivity and specificity in the diagnosis of CFRD was determined.
RESULTS: Between August 1996 and May 1997 122 patients became eligible for the study, 91 of whom agreed to take part. The number of patients with normal, impaired, and diabetic glucose tolerance was 58 (64%), 21 (23%), and 12 (13%), respectively. When used alone, abnormal glycosylated haemoglobin (HbA1c) was found to have the highest sensitivity (83%; 95% CI 62 to 100) in the diagnosis of CFRD. The combination of an abnormal random blood glucose and/or abnormal HbA1c and/or symptoms of hyperglycaemia or weight loss was found to have the highest sensitivity (92%; 95% CI 76 to 100) in the diagnosis of CFRD. The specificity of this combination in the diagnosis of CFRD was 79% (95% CI 70 to 88). By selectively performing OGTTs in patients with one or more of the criteria cited above, 11 of the 12 patients with OGTT defined diabetes would have been identified.
CONCLUSIONS: Patients with cystic fibrosis already have to undergo a large number of routine investigations. The selective approach in performing OGTTs described here has the potential to identify the majority of patients with CFRD without the need to perform this investigation on all patients. This approach is likely to be welcomed by patients and will lead to significant savings in terms of time and resources for patients and staff. Further larger studies are warranted to validate this selective approach in the diagnosis of CFRD.