A colleague recently approached one of the authors with questions about the diagnosis of diabetes in patients with cystic fibrosis (CF). Here are the questions:
Q1. Can A1c be used to screen for diabetes in CF patients?
Q2. Can an oral glucose tolerance test (OGTT) be run on a POCT instrument?
Q3. Should the OGTT in CF patients include a 1 hour time point in addition to the zero and 2 hour time points?
The best article concerning CF diabetes (including evidence-based recommendations) was published in 2010 by Moran et al. (Moran A, Brunzell C, Cohen RC, et al.; CFRD Guidelines Committee. Clinical care guidelines for cystic fibrosis-related diabetes: a position statement of the American Diabetes Association and a clinical practice guideline of the Cystic Fibrosis Foundation, endorsed by the Pediatric Endocrine Society. Diabetes Care. 2010 Dec;33(12):2697-708).
Here are our responses:
Q1. The article states that the "use of A1C as a screening test for
CFRD is not recommended." This is based on studies of the sensitivity of A1c testing for the detection of diabetes. The OGTT is recommended for screening for diabetes in CF patients.
Q2. The OGTT samples for glucose measurement should not be run on a POCT device because these devices lack sufficient accuracy. Recall that some POCT devices exhibit biases of 15% to 20%.
Q3. The OGTT recommended in CF patients is identical to the routine OGTT used in nonpregnant subjects. The diagnosis of hyperglycemia is based upon the fasting and/or 2 hour result. The cutoffs for hyperglycemia are identical to those used in non-CF, nonpregnant individuals: fasting hyperglycemia is defined as a plasma glucose ≥126 mg/dL and an elevated 2 hour value is ≥200 mg/dL.
The diagnosis of diabetes in any patient leads to serious medical, social, and economic consequences. Diabetes is a chronic illness that can be fatal if untreated. Treatment often involves lifestyle changes, and the diagnosis of diabetes is expensive for patients who must disclose it when seeking healthcare insurance. The diagnosis of diabetes should be based on reliable laboratory criteria that have been validated by clinical studies. If you want to explore CF diabetes more thoroughly, please read the above reference. A PDF of the article is available free of charge through PubMed and other online services.