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Yung et al present important data on cystic fibrosis related diabetes (CFRD) and suggest a selective approach for screening and diagnosis.1 Although the majority of patients with CFRD may be identified using this approach, over 8% would remain undiagnosed.
CFRD is associated with substantial morbidity and mortality. Analysis of 21 000 patients followed by the Cystic Fibrosis Foundation Registry shows a sixfold increase in mortality for CFRD with more severe pulmonary disease.2
Once insulin treatment begins, FEV1 and FVC increase and are comparable to non-diabetic patients by two years. The number of pulmonary infections with Haemophilus influenzae andStaphylococcus aureus fall and body mass index increases sharply within three months of starting treatment.3 Consequently, many may judge the risk for patients undiagnosed by the selective approach to be too high.
Recommendations from the 1998 Consensus Conference on CFRD2 state that the fasting glucose and oral glucose tolerance test (OGTT) should be performed in all patients with symptoms of diabetes, particularly as measurement of glycosylated haemoglobin (HbA1c) has been shown to be unreliable in the diagnosis of new CFRD. The major discrepancy between the findings of Lannget al 3 and the Brompton group1 emphasises this point.
Clear distinction needs to be made between screening and diagnosis. Many tests of glucose control in CFRD lack the sensitivity and specificity to identify most new cases. The approach used by Yunget al may prove to be a suitable screening test since the majority of cases were identified.
The OGTT is currently recommended as the test of choice in the diagnosis of CFRD which aids in prompt and accurate identification of the disease. Otherwise, an entirely treatable cause of pulmonary decline may be missed.
author’s reply I thank Drs Pirzada and Wales for their interest in our paper and would like to make two points in response. Firstly, I agree entirely with Drs Pirzada and Wales that the oral glucose tolerance test (OGTT) should be the test of choice in the diagnosis of cystic fibrosis related diabetes (CFRD). The selective use of OGTT in the diagnosis of CFRD as described by us may indeed miss a few cases of CFRD, but it represents an alternative approach to the diagnosis of CFRD in large cystic fibrosis centres where it may not be practical to perform OGTTs on all patients. As stated in our paper, patients with CFRD missed by our selective approach are those with normal random blood glucose levels, glycosylated haemoglobin, and those asymptomatic of hyperglycaemia. We speculate that the clinical and metabolic consequences of their diabetes may not be as great as those diabetics with one or more of the abnormal criteria cited above. As most adult patients are reviewed at least three monthly, these patients are likely to be identified at a later date.
Secondly, whether the development of CFRD is associated with a deterioration of clinical status remains unresolved. The evidence in the literature on this issue is conflicting.1-1-1-5 CFRD may cause a decline in patients’ clinical status or it may merely be a marker of poor clinical and nutritional status. The latter may explain the apparent high mortality of CFRD patients as reported by the Cystic Fibrosis Foundation. In the management of patients with CFRD, every effort should continue to be made to improve the clinical and nutritional status of patients so that the impact of the development of CFRD is kept to a minimum.