EnVision CF Multicenter Study of Glucose Tolerance in Cystic Fibrosis
EnVision CF Multicenter Study of Glucose Tolerance in Cystic Fibrosis
Cystic Fibrosis Related Diabetes has been identified by the CF community as one of the top
 ten priorities for CF research. In CF clinical decline due to dysglycemia begins early, prior
 to diagnosis of diabetes and increases mortality from pulmonary disease. There is presently
 no way to determine who, of those with dysglycemia, will experience clinical compromise.
 However, the CF Center in Milan has found that measurable age- and sex-dependent variables on
 oral glucose tolerance testing (OGTT) predict β-cell failure-the primary driver of decline in
 CF. the investigators propose a multi-center trial to develop nomograms of age and sex
 dependent reference values for OGTT-derived measures including glucose, insulin, c-peptide,
 and the resultant OGTT-derived estimates of β-cell function, β cell sensitivity to glucose,
 and oral glucose insulin sensitivity (OGIS) and to determine correlation of these with
 clinical status (FEV-1, BMI z score, number of pulmonary exacerbations over the past 12
 months). In a subset of the cohort the investigators will perform additional studies to
 determine possible mechanisms driving abnormal β cell function, including the role of lean
 body mass (as measured by DXA), impact of incretin (GLP-1, GIP) and islet hormones (glucagon,
 pancreatic polypeptide) on β cell function and the relationship of reactive hypoglycemia and
 catecholamine responses to β cell function, as well as the relationship of β cell sensitivity
 to glucose as determined by our model to abnormalities in blood glucose found in a period of
 free living after the study (determined by continuous glucose monitoring measures (Peak
 glucose, time spent >200 mg/dl, standard deviation). the investigators will also develop a
 biobank of stored samples to allow expansion to the full cohort if warranted and to enable
 future studies of dysglycemia and diabetes in CF. the investigator's eventual goal is
 utilization of the nomograms to determine the minimum number of measures to accurately
 predict risk for clinical decline from dysglycemia in CF.
A. Hypotheses and Specific Aims:

 Cystic Fibrosis Related Diabetes Mellitus (CFRD) has been identified by the CF community as
 one of the top ten priorities for CF research1. In CF, clinical decline due to dysglycemia
 begins early, prior to the diagnosis of CFRD2 and increases pulmonary decline and mortality3.
 However, current definitions of dysglycemia were derived from non-CF populations, and
 unfortunately, there is presently no way to determine who, of those with CF and dysglycemia,
 will experience morbidity, mortality or greatest risk for CFRD. However, the CF Center in
 Milan found that measurable age and sex dependent variables on oral glucose tolerance testing
 (OGTT) strongly predict β-cell failure 4- the primary driver of clinical decline. We propose
 a multi-center trial to determine age and sex dependent reference for fsOGTT-derived
 estimates of β-cell function and insulin sensitivity in the US CF population, and to
 investigate the relationship of these measures to CF-relevant clinical outcomes as well as
 exploring potential mechanisms that may be driving beta cell functional decline. Furthermore,
 we will collect these data in the new age of cystic fibrosis transmembrane conductance
 regulator (CFTR) modulator therapies, to define a model that will be most relevant for our
 current generation of CF patients.

 Specific Aim 1: To determine age and sex dependent reference values from OGTT and
 OGTT-derived estimates of β-cell function and insulin sensitivity in a diverse population of
 475 non-diabetic children and adults with CF, at 4 centers with EnVision Emerging Leaders as
 site-PIs.

 Hypothesis: Much like standard growth charts with means and standard-deviation based
 percentiles, fsOGTT values (glucose, insulin, c-peptide) and the resultant fsOGTT-derived
 estimates of β-cell function (beta cell sensitivity to glucose, oral glucose insulin
 sensitivity) will fall along an age and sex-dependent continuum which will be consistent
 across CF patients at four different US centers.

 Specific Aim 2: To determine the relationship between OGTT reference values and estimates of
 β-cell function and insulin sensitivity to clinically relevant outcomes.

 Hypothesis: The fsOGTT-derived values of insulin, glucose and C-peptide in combination with
 estimates of β-cell function (beta cell sensitivity to glucose, OGIS) will correlate with
 important markers of clinical status (FEV-1, BMI Z score, number of pulmonary exacerbations
 over the past 12 months) in an age and sex dependent manner.

 Specific Aim 3 (exploratory): To gather additional measures in a subset of this large cohort
 to illuminate the relationship between normal ranges of beta cell function (as determined
 above) to possible mechanisms driving abnormal beta cell function, including: abnormal lean
 body mass, abnormalities of hormones that regulate beta cell function (specifically incretins
 (glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP)), and
 islet hormones (glucagon, pancreatic polypeptide (PP)), abnormalities of counter-regulation
 to abnormal insulin secretion (catecholamine levels), and free-living glucose measures
 collected by continuous glucose monitoring. Additionally, stored serum from 475 patients with
 exceptionally well characterized metabolic profiles provides an unprecedented opportunity to
 help inform future research questions.

 Hypothesis 3a: Lower lean body mass as assessed by DXA is associated with beta cell glucose
 sensitivity in an age and sex dependent manner.

 Hypothesis 3b: Patterns of islet hormone and incretin secretion are age and sex dependent and
 β cell glucose sensitivity will correlate with PP and GIP, but not with glucagon and GLP-1
 Hypothesis 3c: Reduced β cell glucose sensitivity is associated with higher rates of reactive
 hypoglycemia with inappropriate catecholamine and glucagon response during OGTT.

 Hypothesis 3d: identifiable CGM measures (ex. Peak glucose, time spent >200 mg/dl, standard
 deviation) will correlate with model derived outcomes specifically β cell glucose sensitivity
 in an age and sex-dependent manner

 B. Background and Significance:

 Cystic Fibrosis Related Diabetes (CFRD) is an independent cause of increased rates of
 premature death in people with cystic fibrosis (CF)3. Because of this, the CF community has
 recognized CFRD as one of their top ten research priorities1. Importantly, abnormalities of
 blood glucose metabolism (dysglycemia) are present in CF likely from birth5 and cause
 increasingly rapid lung function decline and loss of healthy body mass years before the
 diagnosis of CFRD is made2. However, it is unclear at what point individuals might benefit
 from treatment of dysglycemia, as the treatment itself can be an additional burden.
 Furthermore, with the increasing use of cystic fibrosis transmembrane conductance regulator
 (CFTR) modulators, the natural history of CFRD development and progression is likely to
 change. Thus, it is imperative that we better understand which CF patients with dysglycemia
 are at the greatest risk of excess mortality due to dysglycemia and progression to CFRD. The
 primary defect leading to dysglycemia in CF is inadequate insulin secretion secondary to
 abnormal function of the beta cell6. Importantly, understanding beta cell dysfunction is
 essential to addressing every form of diabetes mellitus, not just CFRD. However, the gold
 standard tests for beta cell function (hyperglycemic clamp and tracer studies) are complex
 and therefore they are not easily utilized in large-scale studies. Due to this, there has
 been extensive effort on the part of many centers to develop methods to assay beta cell
 function from more readily accessible data points and, ideally, clinically useful ones. Dr.
 Andrea Mari, from the Institute of Neuroscience in Italy, has developed a model, which can
 derive multiple indices of beta cell function from frequently sampled OGTT, with excellent
 concordance to the gold standard hyperglycemic clamp. This is especially relevant in the CF
 population where OGTT testing is the recommended modality for diabetes screening. Dr. Mari's
 model has been well-validated in normal subjects7, subjects with type 1 and type 2 diabetes
 mellitus and in subjects with pre-diabetes8 and is an accepted alternative for the assessment
 of beta cell function in large studies where clamp protocols are difficult or impossible9.
 Importantly, Dr. Alberto Battezzatti, a pediatric endocrinologist at the CF center in Milan,
 Italy, with a long dedicated career interest in CFRD, has expanded Dr. Mari's modeling to the
 CF population. Dr. Battezzati has shown that there are strong relationships between the
 insulin, glucose and C-peptide levels measured during 3 hour OGTT testing and the age and sex
 of CF subjects4. Our goal, in concert with Dr. Battezzati and colleagues is to develop normal
 ranges for these measures or, in essence, a "growth chart" for the OGTT in CF patients that
 can accurately predict beta cell dysfunction and eventual progression to CFRD. Next steps are
 to simplify the model to so that 2-3 discrete samples will accurately predict progression to
 CFRD and morbidity in CF subjects.

 The investigators propose a collaboration with Dr. Battezzati and the Italian CF centers. the
 investigators will validate the Italian model in a large and diverse US population, and
 correlate it to specific clinical outcomes relevant to morbidity and mortality in CF, which
 will allow for development of predictors in the model specific to US subjects and ensure
 clinical relevance to our patients. Furthermore, the investigaros will collect these results
 and follow patient outcomes in this new era of CFTR modulator therapy, developing a model
 that will be most relevant for the current generation of patients with CF. This will
 establish age and sex-dependent "normal ranges" for glucose, insulin and c-peptide values
 from OGTT as well as determinants of beta cell function derived from OGTT. Establishment of
 normal ranges of these measures will allow the determination of which patients with CF are
 the lost abnormal and eventually determine who, within a given glucose tolerance category in
 CF will experience clinical compromise and progress to CFRD. The long-term goal will be to
 develop from these normal ranges, including normal ranges for indices of beta cell function
 and insulin resistance, the most predictive time points and assays to develop a simplified
 model that will optimally predict risk of clinical decline in a CF patient of a given age and
 sex. The ideal model will consist of 2- 3 discrete time points to accurately predict clinical
 deterioration.

 All four of the investigators on this grant have established or are developing a long term
 career focus on Cystic Fibrosis Related Diabetes Mellitus and dysglycemia in Cystic Fibrosis.
 This project represents a significant opportunity to characterize a large cohort of subjects
 with well-defined dysglycemia and beta cell dysfunction in collaboration with an
 equally-large international cohort with matching data sets. It represents an unprecedented
 opportunity to fully explore the depth and breadth of dysglycemia in CF as well as to
 investigate the impact dysglycemia has on the health of people with CF. The long term value
 of this project is also enhanced by the establishment of a biorepository of stored samples
 that will be obtained from this cohort. This will allow the ability in the future to query
 these samples to further investigate the natural history and pathophysiology of CFRD, and
 potentially biomarkers with in a large cohort with clearly and accurately defined beta cell
 function. This group has unique assets to complete this task as we represent four unique CF
 centers; all with dedicated CF endocrinologists recognized as Emerging Leaders in CF
 Endocrinology by the EnVision program, with coordinated mentorship though that program by the
 leading expert in the field of CFRD, Dr. Antoinette Moran.

 C. Preliminary Studies:

 Currently the recommended screening test for CFRD is the oral glucose tolerance test
 (OGTT)10. Although abnormalities on this test predict long term outcomes in CF, the
 diagnostic endpoints we currently use are based on data from other adults with type 2
 diabetes and are not specific to CF. There is emerging evidence that CF patients who do not
 meet formal criteria for glucose abnormalities by OGTT are still experiencing clinical
 decline11,12. Therefore, there remains a knowledge gap regarding how we can best predict
 clinical decline. Given that the primary pathology in CF is insulin deficiency, it is
 reasonable to extrapolate that measures of beta cell function would be the best indicators of
 outcomes in CF patients. Typical simplified methods of assessing beta cell function (such as
 HOMA-B) utilize primarily fasting measurements, which are inappropriate in CF where fasting
 abnormalities develop after the onset of frank CFRD and long after clinical decline is
 established.

 Dr. Andrea Mari has extensive data showing the effectiveness of his model in evaluating beta
 cell glucose sensitivity (the ability of the beta cell to respond to respond to changes in
 glucose concentration)7,13,14. The OGTT itself has poor reproducibility, however beta cell
 glucose sensitivity as measured from frequently sampled OGTT has significantly better
 reproducibility, with a coefficient of variation around 15-20%7,15. This model of beta cell
 glucose sensitivity is an extremely accurate parameter to assess glucose control and has been
 found to be a significant predictor for the development of type 1 and type 2 diabetes in
 non-diabetic and pre- diabetic populations. This model allows, as well, measurement of first
 phase insulin secretion (termed "rate sensitivity" - which is directly comparable to the
 acute insulin response obtained from the gold standard IVGTT)16. This is significant, as loss
 of first phase insulin secretion is the initial abnormality in the progression to CFRD. This
 model is highly reproducible, and has also been studied longitudinally15,17 (see figure 1).

 Preliminary data -islet and incretin hormones:

 Dr. Larson Ode has devoted her career to understanding the origins of dysglycemia in CF. Her
 work has upended previous dogma, finding that up to 39% of CF children 3 months to 5 years of
 age may have dysglycemia18. However, it is not yet known what underlying mechanisms drive
 dysglycemia in CF. In a previous small dataset (n=25 CF, n=9 control), 9 of 25 CF subjects
 had dysglycemia (all controls were normal). Stored blood was assayed for islet and incretin
 hormones to interrogate the difference between children with and without dysglycemia. CF
 children with abnormal glucose tolerance had higher GIP levels (p<0.03) and a trend toward
 lower GLP-1 levels compared to normal CF subjects (p= 0.06). All CF subjects had abnormally
 low PP levels (p<0.01). Glucagon levels were uninformative (unpublished data- see Figure 1).
 This implies that incretin and islet hormones may play a role in dysglycemia in CF. However,
 data from larger and older cohorts are needed to fully understand incretin and islet hormone
 patterns in people with CF. This proposed project will establish a biobank which will allow
 the study of age based progression in biomarkers of islet dysfunction throughout a diverse
 cohort including a large age-range of subjects. The deep understanding of CF typical beta
 cell function changes with age that will be established by this cohort will allow greatly
 improved understanding of the progression of dysglycemia in CF.

 Experience with CGM:

 Dr. Chan currently has a CFFT Shwachman grant to investigate the role of CGM in CF. She also
 has previously used CGM in other studies, enrolling over 100 obese youth to undergo OGTTs and
 wear CGM in order to better understand simpler measures of prediabetes and type 2 diabetes
 screening in the obese adolescent population19,20. Although her CGM in CF study is currently
 active, recruitment will be ending soon and will not overlap with the timing in this proposed
 study. The present proposal, with its target enrollment of 475 individuals with CF, offers an
 unprecedented opportunity to further our understanding of CGM and its relationship with the
 proposed model and its novel age- and sex-specific outcomes

 Experience with hypoglycemia research:

 Dr. Moheet has extensive experience in diabetes research with special expertise in the gold
 standard tests for beta cell function (insulin clamp studies). His research utilizes methods
 relevant to the study of in vivo metabolism in human subjects, particularly use of both
 hyperglycemic and hypoglycemic insulin clamps methodologies. His research has examined the
 underlying pathogenesis in development of defective counterregulatory response to
 hypoglycemia in people with type 1 diabetes. Dr. Moheet's substantial background in
 hypoglycemia research would be invaluable in the conduct of this study.

 Fasting and reactive hypoglycemia after meals is common in CF patient without diabetes21.
 Reactive hypoglycemia can be particular bothersome and difficult to manage in some patients
 with CF. Hypoglycemia has also been frequently reported during OGTT in CF with reported rates
 for 3 hour OGTT between 15-59%22,23. A recent systematic review noted that, however, that the
 pathophysiology is poorly understood and highlighted the need for high quality studies to
 examine the possible etiologies underlying reactive hypoglycemia in CF24. This proposed study
 will address this knowledge gap by evaluating counterregulatory hormones including glucagon
 and catecholamines during hypoglycemia and assessing symptomatic hypoglycemia via a validated
 and structured questionnaire25. This proposed study will provide an unprecedented opportunity
 to examine the possible mechanisms underlying reactive hypoglycemia in CF in a large cohort
 of patients

 Preliminary data-body composition:

 Dr. Granados has previously investigated the association between body composition (% fat and
 LBM by DXA) and measurements of insulin resistance (HOMA-IR), insulin sensitivity (Insulin
 Sensitivity Index - Matsuda (ISIMatsuda)) and β-cell insulin secretion (Φtotal) in 15 CF
 patients with pancreatic insufficiency between 12-24 years old (mean BMI 19.5 ± 2.87 kg/m2).
 Her preliminary data showed that insulin resistance significantly correlated with weight
 z-scores (HOMA-IR:r=0.48, p=0.003) and ISIMatsuda (r=-0.46, p=0.004). She also found a
 positive correlation between Φtotal, (measurement of insulin secretion in response to
 glucose) and fat % in the subjects with CF without diabetes (r=0.69, p=0.004). Moreover,
 there was a trend in those with lower fat % and higher LBM to have better insulin sensitivity
 (ISIMatsuda:r=-0.45, p=0.09).

 D. Experimental Design and Methods:

 Study Hypothesis Primary hypothesis: Much like standard growth charts with means and
 standard-deviation based percentiles, fsOGTT values (glucose, insulin, c-peptide) and the
 resultant fsOGTT-derived estimates of β-cell function (beta cell sensitivity to glucose, oral
 glucose insulin sensitivity) will fall along an age and sex-dependent continuum which will be
 consistent across CF patients at four different US centers.

 Secondary hypothesis: The fsOGTT-derived values of insulin, glucose and C-peptide in
 combination with estimates of β-cell function (beta cell sensitivity to glucose, OGIS) will
 correlate with important markers of clinical status (FEV-1, BMI Z score, number of pulmonary
 exacerbations over the past 12 months) in an age and sex dependent manner.

 Exploratory hypotheses:

 Hypothesis 3a: Lower lean body mass as assessed by DXA is associated with beta cell glucose
 sensitivity in an age and sex dependent manner.

 Hypothesis 3b: Patterns of islet hormone and incretin secretion are age and sex dependent and
 β cell glucose sensitivity will correlate with PP and GIP, but not with glucagon and GLP-1
 Hypothesis 3c: Reduced β cell glucose sensitivity is associated with higher rates of reactive
 hypoglycemia with inappropriate catecholamine and glucagon response during OGTT.

 Hypothesis 3d: Identifiable CGM measures (ex. Peak glucose, time spent >200 mg/dl, standard
 deviation) will correlate with β cell glucose sensitivity in an age and sex dependent manner.

 Screening and Enrollment The University of Minnesota CF center has 332 CF patients eligible
 for this study. The University of Iowa has 144 eligible subjects. The University of Colorado
 pediatric center has approximately 225 eligible subjects. Washington University in St. Louis
 has 236 eligible subjects. Experience from recruiting previous OGTT based studies in these
 centers suggests 40-60% of CF subjects will agree to participate. It is expected that
 approximately 160 subjects/year will participate, collectively over all the sites, to reach a
 total of 475 subjects by the end of the 3 years of the study.

 Experimental design and procedures:

 Study Assessments:

 Data to be collected at enrollment

 1) Age, sex, date of birth, diagnosis of CF (genotype and sweat test results), any diagnosis
 of diabetes mellitus, date of last hospitalization for pulmonary exacerbation and initiation
 of any systemic steroid therapy. We will collect a list of the subject's current and active
 medications from the parent (if the subject is a minor) or from the subject (if the subject
 is an adult).

 2) Medical Records: consent will be obtrained to request the following information from
 clinic and hospital medical records for participants, with HIPAA authorization:

 a. medications, results of pulmonary function testing, specifically most recent FEV1%
 predicted, FVC, CFTR genotype and seat testing results,. history of steroid use, number of
 hospitalizations for pulmonary exacerbation in the last year (defined as admission for iv
 antibiotics or steroid therapy), fecal elastase Data to be collected at Study visits

 Frequently sampled 3-hour oral glucose tolerance test (OGTT):

 1. Subjects are admitted to the clinical research unit

 2. Fasting status is confirmed

 3. An IV is placed for sequential laboratory draws. Topical lidocaine cream will be
 provided for the IV start site if requested by the family.

 4. The oral glucose solution (Glucola) at the dose of 1.75 g/kg is administered by mouth at
 time zero.

 5. Insulin, glucose, c-peptide, and additional serum to store will be obtained at 0, 15,
 30, 60, 90, 120, 150 and 180 minutes. Additional blood will be stored for future
 testing.

 1. Each sample should require no more than 1-2 tbsp of blood. Sample amount will be
 based on patient weight and will not exceed 3 ml/kg/day total blood withdrawn.

 2. Samples will be processed, batched, and shipped to the University of Iowa for
 analysis.

 3. If the IV is lost at any point during the study, we will request permission from
 the family to obtain samples by finger-stick or heel poke.

 6. Anthropometrics at each visit.

 a. Height on stadiometer for all subjects b. Weight on standing scale for all subjects.
 c. These measurements to be used to calculate BMI and BMI z-score if applicable.

 7. Additional data collection.

 1. Medication list will be recorded and verified with the subject/parents at the start
 of each visit.

 2. From the medical record:

 i. Last hospitalization, number of hospitalizations in the past year ii. Last systemic
 steroid therapy (if ever) iii. Last Antidiabetic medication use (if ever) iv. Diagnosis
 of diabetes- yes/no, date v. Fecal elastase values vi. CFTR mutation vii. Results of
 sweat testing viii. Use of CFTR correctors/modulators (if any)- Starting date A subset
 of the subjects will complete additional assessments evaluating the exploratory
 endpoints. The goal is that 1/3 of the total subjects will participate in these
 protocols.

 DXA scan 1) A DXA scan will be obtained the date of the study visit or within a 2 week period
 from the OGTT.

 a. Hologic Inc. (Bedford, MA, USA) machines will be used to standardize results CGM

 1) A blinded CGM Freestyle LibrePro (Abbott Diabetes Care Inc) will be placed on the back of
 the upper arm. Participants will be asked to wear the CGMS for up to 10 days.

 1. The CGMS does not require calibration, but participants will be asked not to wear the
 monitor in water deeper than three feet, or to keep it submerged longer than 30 minutes.

 2. Participants will also be instructed not to change any of their current dietary or
 activity habits so that free-living glucose data are collected

 3. They will also be asked to complete a simple log of their activity and dietary intake.

 4. Subjects will be given the option of personally returning the CGM and log-sheet to the
 CTRC or be provided a self-addressed, stamped envelope to return the device and
 log-sheet at the completion of the recording period.

 5. Using software that accompanies the CGM recorder, reports will be generated for each
 subject providing a summary of the sensor glucose data.

 6. These results will be converted into raw data in an excel spreadsheet and saved on a
 password-protected, secure network server, via a local computer, in a locked office to
 which only the investigators will have access.

 i. These raw data include sensor glucose values obtained every 15 minutes during device wear.

 g. We will review CGM tracings to analyze additional variables including: i. area under the
 total CGM curve ii. number of excursions >140 mg/dl and >200 mg/dl iii. maximum sensor
 glucose after first meal of the day, maximum sensor glucose, sensor glucose mean iv. standard
 deviation and other measures of glycemic variability v. Measures of hypoglycemia specifically
 % time spent <60 and <70mg/dl. The Freestyle Libre Pro system has been approved in Europe and
 has been studied in children as young as 4 years of age for up to 14 days of sensor wear. It
 does not display blood glucose information in real time, and participants will not be given
 the capability to view the CGMS data during the study. This CGM does not require any
 calibration or training and can easily be removed and returned Laboratory assays

 1. Clinical research assays

 a. plasma Glucose is run via YSI analyzer in order to return results quickly to
 subjects, all other assays are batched.

 2. Research assays will be performed by Ms. Yi in the flow core at the University of Iowa,
 all assays except glucose use the same platform, which allows multiple samples to be run
 accurately on microliters of blood a. GLP-1 (active &total), GIP, PP, glucagon,
 c-peptide, insulin i. HMHEMAG-34K | MILLIPLEX luminex xMAP Human Metabolic Hormone
 Magnetic Bead Panel - Metabolism Multiplex Assay (Millipore Sigma, Burlington, MA, USA)
 ii. No cross-reactivity is seen within the panel, the assays show high recovery
 (97-105%), good precision (all assays have intra-assay coefficient of variation<10% and
 inter-assay coefficient of variation <15%, except GLP-1 total which has an inter-assay
 coefficient of variation of <20%) iii. Insulin an c-peptide are measured on this
 platform as well to allow for quality control comparison with clinical samples

 Sample size estimates/ Power calculation:

 Primary outcomes:

 Using 187 subjects from a single Italian center, the Italian group has previously defined the
 75.0 quantile for various glucose tolerance and beta cell function parameters, including
 beta-cell glucose sensitivity. As the goal of the present project is to develop the 5th to
 95th quantiles, due to the clinical relevance of cut offs based on the 5th and 95th
 percentile, sample size for the present project was calculated to estimate the 95.0 quantile
 of a covariate-dependent quantile curve with a pre-specified precision. The primary outcome
 variable of the quantile curve is beta-cell sensitivity to glucose and the predictor variable
 is age. On the basis of the available data 4 we expect a linear relationship between
 beta-cell glucose sensitivity and age with no heteroskedasticity. Therefore the minimum
 sample size needed to estimate the 95.0 quantile of beta-cell glucose sensitivity -for-age
 with a 90% probability for it to lie between the 90.5 and 97.4 quantiles (with respect to
 symmetric tolerance evaluated on a logit scale) is 475 subjects31. For the other outcomes
 (glucose, insulin, c-peptide) the minimum sample size is smaller than the 475 subjects needed
 for primary endpoint of beta cell glucose sensitivity. The University of Minnesota CF center
 has 332 CF patients eligible for this study. The University of Iowa has 144 eligible
 subjects. The University of Colorado pediatric CF center has 225 eligible subjects.
 Washington University in St. Louis has 236 eligible subjects. Historically at the University
 of Minnesota and the University of Iowa, we have had excellent enrollment rates of CF
 subjects in CFRD studies, so we expect about 40-60% of eligible subjects to enroll at some
 point over the 3 year duration of the study.

 Safety evaluation and quality control A formal DSMP will be in place for this project. Please
 see attached DSMP. There will be no need for efficacy analysis as this is not an
 interventional study.
Inclusion Criteria:

 1. Age >/= 6 years

 2. Diagnosis of cystic fibrosis

 3. CF patients regularly attending the CF centers

 4. Clinically stable in previous 3wks:

 -  absence of major clinical events including pulmonary exacerbations,

 -  no change in their habitual treatment regimen including introduction of
 antibiotics or steroids in the past 3 weeks

 Exclusion Criteria:

 1. Diagnosis of type 1 diabetes, type 2 diabetes, or MODY

 2. Organ transplantation

 3. new diagnosis of CFRD in the past 6 months

 4. antidiabetic treatment in past 6 mos (insulin or oral hypoglycemic agents)

 -patients with previous CFRD diagnosis, but not currently taking
 insulin/glucose-lowering medications for at least 6 months should be included

 5. pulmonary exacerbation associated with systemic steroid requirement in the last 6
 months

 6. on CFTR corrector less than 6 months prior to enrollment