Natural History of Glucose Intolerance in Patients with Cystic Fibrosis: Ten-Year Prospective Observation Program

Objective To determine prospectively the long-term natural history of glucose homeostasis in adult patients with cystic fibrosis (CF). Study design Between 1996 and 2005, a total of 971 modified oral glucose tolerance tests (OGTTs) were performed in 329 patients with CF without recognized CF-related diabetes (CFRD). Patients were classified as having normal glucose tolerance (NGT), impaired glucose tolerance (IGT), CFRD without fasting hyperglycemia (FH), or CFRD with FH. Data were collected at baseline from the Toronto Cystic Fibrosis database. Results On first OGTT, 63% of the 257 patients with pancreatic insufficiency (PI) had NGT, 23% had IGT, 11% had CFRD without FH, and 3% had CFRD with FH. Burkholderia cepacia complex colonization was correlated with worsening glucose tolerance category. There was a weak inverse relationship among weight, body mass index, forced expiratory volume in 1 minute, and 2-hour plasma glucose obtained during OGTT. Of the 168 PI patients who had a second OGTT, 17% improved their category of glucose tolerance, 70% remained unchanged, and 13% worsened. A similar trend was seen during the progression between any one test to a subsequent test. Conclusions Annual screening of glucose tolerance in patients with CF reveals highly variable results over time. Fluctuating levels of insulin resistance, probably with variable degrees of ongoing inflammation, affect the results and hinder prediction of future development of CFRD. Home glucose monitoring following abnormal OGTT results was essential in establishing the diagnosis of CFRD.