Breastfeeding, growth, and lung disease in the first 3 years of life in children with cystic fibrosis.

BACKGROUND: The 2009 cystic fibrosis (CF) infant care guidelines recommend breastmilk as the initial feeding but do not address if/when it should be fortified or supplemented with formula to promote optimal growth and pulmonary health. METHODS: We conducted a prospective multi-center cohort study in breastfed and formula-fed infants that included 172 infants with CF who were born during 2012-17, enrolled after newborn screening at age 1.9 ± 1.0 months, and evaluated growth and lung disease manifestations in the first 3 years of life. RESULTS: Seventy-two percent of our study cohort was breastfed at birth, but 64 % transitioned to receiving fortified feedings (breastmilk, formula, or a combination) by 6 months of age to reverse the downward trajectory of their growth curves. Fortified feedings accelerated catch-up growth to normal weight-for-age (0.12 ± 0.80 z-score) and near normal height-for-age (-0.13 ± 0.90 z-score) at 3 years of age. Within the fortified group, breastmilk and formula were similarly effective in promoting catch-up growth, but proportionately fewer infants with CF fed predominantly breastmilk (30 %) experienced severe or moderate early-onset lung disease compared to those fed predominantly formula (62 %), p = 0.02. CONCLUSIONS: Most infants with CF require fortified feedings to recuperate from growth faltering and achieve normal growth at 3 years of age. For these infants, the proactive/preventive strategy of fortified breastmilk feedings starting soon after CF diagnosis, an alternative to the reactive/monitoring approach, can minimize the risk of prolonged postnatal growth faltering, accelerate the potential of attaining catch-up growth, and decrease the likelihood of experiencing more severe early-onset lung disease.

Vitamin D status and variable responses to supplements depend in part on genetic factors in adults with cystic fibrosis.

Vitamin D sufficiency has been difficult to achieve consistently in patients with cystic fibrosis (CF), even with robust oral supplements. To assess vitamin D status and resistance to supplementation, we studied 80 adults using 25-hydroxyvitamin D (25OHD) determinations and whole genome sequencing to construct polygenic risk scores (PRS) that aggregate variants associated with vitamin D status. The results revealed that 30 % of patients were below the threshold of 30 ng/mL and thus should be regarded as insufficient despite normal vitamin E status, a reflection of adherence to fat soluble vitamin supplementation. The PRS values were significantly correlated with 25OHD concentrations, confirming our results in children with CF, and indicating that genetic factors play a role and have implications for therapy.

Impact of intrinsic and extrinsic risk factors on early-onset lung disease in cystic fibrosis.

BACKGROUND: Although respiratory pathology is known to develop in young children with cystic fibrosis (CF), the determinants of early-onset lung disease have not been elucidated. OBJECTIVE: We aimed to determine the impact of potential intrinsic and extrinsic risk factors during the first 3 years of life, testing the hypothesis that both contribute significantly to early-onset CF lung disease. DESIGN: We studied 104 infants born during 2012-2017, diagnosed through newborn screening by age 3 months, and evaluated comprehensively to 36 months of age. Lung disease manifestations were quantified with a new scoring system known as CFELD for Cystic Fibrosis Early-onset Lung Disease. The variants in the cystic fibrosis transmembrane conductance regulator (CFTR) gene were determined and categorized. Whole genome sequencing was performed on each subject and the data transformed to polygenic risk scores (PRS) that aggregate variants associated with lung function. Extrinsic factors included socioeconomic status (SES) indicators and environmental experiences such as exposures to smoking, pets, and daycare. RESULTS: We found by univariate analysis that CFTR genotype and genetic modifiers aggregated by the PRS method were significantly associated with early-onset CF lung disease. Ordinal logistic regression analysis demonstrated that high and stable SES (maternal education ≥community college, stable 2-parent home, and not receiving Medicaid) and better growth (weight-for-age and height-for-age z-scores) reduced risks, while exposure to smoking and daycare ≥20 h/week increased the risk of CFELD severity. CONCLUSIONS: Extrinsic, modifiable determinants are influential early and potentially as important as the intrinsic risk factors in the onset of CF lung disease.

Genetic factors help explain the variable responses of young children with cystic fibrosis to vitamin D supplements.

BACKGROUND & AIMS: Children with cystic fibrosis (CF) are susceptible to fat-soluble vitamin deficiencies unless supplemented, but even large doses of vitamin D may not prevent low 25-hydroxyvitamin D (25OHD) concentrations. The explanation for these vitamin D non-responders has been elusive. We utilized data from whole genome sequencing (WGS) to test the hypothesis that genetic variations predict responsiveness to vitamin D supplementation in a prospective cohort study of children with CF in the first 3 years of life. METHODS: One hundred and one infants born during 2012-2017 and diagnosed with CF through newborn screening were studied. Serum 25OHD concentrations and vitamin D supplement doses were assessed during early infancy and annually thereafter. WGS was performed, the resultant variant calling files processed, and the summary statistics from a recent genome-wide association study were utilized to construct a polygenic risk score (PRS) for each subject. RESULTS: Overall, the prevalence of vitamin D insufficiency (<30 ng/mL) was 21% in the first 3 years of life. Among the 70 subjects who always adhered to vitamin D supplement doses recommended by the US CF Foundation guidelines, 89% were responders (achieved vitamin D sufficiency) by 3 years of age, while 11% were transient or non-responders. Multiple regression analysis revealed that PRS was a significant predictor of 25OHD concentrations (p < 0.001) and the likelihood of being an earlier responder in the first 3 years of life (p < 0.01). A limited SNP analysis revealed variants in four important genes (GC, LIPC, CYP24A1, and PDE3B) that were shown to be associated with 25OHD concentrations and vitamin D responder status. Other determinants included vitamin D supplement dose, season at 25OHD measurement, and pancreatic functional status. CONCLUSIONS: Applying WGS in conjunction with utilizing a PRS approach revealed genetic variations that partially explain the unresponsiveness of some children with CF to vitamin D supplementation. Our findings suggest that a nutrigenomics strategy could help promote personalized treatment in CF.

Defining and identifying early-onset lung disease in cystic fibrosis with cumulative clinical characteristics.

BACKGROUND: Because of the heterogeneity in cystic fibrosis (CF) lung disease among young children, a clinical method to identify early-onset lung disease is needed. OBJECTIVE: To develop a CF early-onset lung disease (CFELD) scoring system by utilizing prospectively collected longitudinal data on manifestations in the first 3 years of life. DESIGN: We studied 145 infants born during 2012-2017, diagnosed through newborn screening by age 3 months, and followed to 36 months of age. Cough severity, pulmonary exacerbations (PEx), respiratory cultures, and hospitalizations were collected at each CF center visit (every 1-2 months in infancy and quarterly thereafter). These data were used to construct the CFELD system and to classify lung disease into five categories: asymptomatic, minimal, mild, moderate, and severe. RESULTS: The most frequent manifestation of CF early lung disease was MD-reported PEx episodes, PEx hospitalizations, and positive Pseudomonas aeruginosa cultures. Parent-reported cough severity was correlated with the number of respiratory hospitalizations (r = 0.48, p < 0.0001). The distribution of CFELD categories was 10% asymptomatic, 17% minimal, 29% mild, 33% moderate, and 12% severe. The moderate and severe categories occurred threefold higher in pancreatic insufficient (PI, 49%) versus sufficient subjects (16%), p < 0.0001. In addition to PI, gastrointestinal and nutrition-related hospitalizations, plasma cytokines interleukin (IL)-6 and IL-10, duration of CFTR modulator therapy, and type of health insurance were significant predictors of CFELD scores. CONCLUSION: The CFELD scoring system is novel, allows systematic evaluation of lung disease prognosis early, and may aid in therapeutic decision-making particularly in the initiation of CFTR modulator therapy.

Vitamins A, D, E status as related to supplementation and lung disease markers in young children with cystic fibrosis.

BACKGROUND: The variable response to fat-soluble vitamin supplementation in young children with cystic fibrosis (CF), and factors contributing to this variability, remain under-investigated. OBJECTIVE: To determine if recommended supplement doses normalize serum vitamins A (retinol), D (25-hydroxy-vitamin D, 25OHD), and E (α-tocopherol), and identify factors predictive of achieving sufficiency, in children with CF in the first 3 years of life. DESIGN: We studied 144 infants born during 2012-2017 and diagnosed with CF through newborn screening. Serum retinol, 25OHD, α-tocopherol and plasma cytokines interleukin (IL)-6, IL-8, IL-10, and tumor necrosis factor (TNF)-α were measured in early infancy and yearly thereafter. Vitamin supplement intakes and respiratory microbiology were assessed every 1-2 months in infancy and quarterly thereafter. RESULTS: The prevalence of vitamin D insufficiency (<30 ng/ml) at all ages combined was significantly higher (22%) compared to vitamin A (<200 ng/ml, 3%) and vitamin E (<5 µg/ml, 5%). All children were vitamin A sufficient by age 2 years. Vitamin E insufficiency was rare. Only 42% were early responders of vitamin D and 17% remain insufficient despite high supplement intakes. IL-6 was positively correlated, while IL-8, IL-10, and TNF-α were negatively correlated, with retinol and 25OHD. Multiple regression analysis revealed that supplement dose, season, α-tocopherol, pancreatic insufficiency, respiratory infections and IL-10 were significant predictors of 25OHD. CONCLUSION: Diagnosis through newborn screening coupled with supplementation normalized serum retinol and α-tocopherol in almost all infants with CF by age 3 years. However, response to vitamin D supplements in young children with CF occurred later and variably despite early and sustained supplementation.

Assessing the Reproducibility of Microbiome Measurements Based on Concordance Correlation Coefficients.

Evaluating the reproducibility or agreement of microbiome measurements is often a crucial step to ensure rigorous downstream analyses in microbiome studies. In this paper, we address this need by developing adaptations of Lin's concordance correlation coefficient (CCC) tailored to microbiome studies. We introduce a general formulation of the new CCC measures upon the use of a distance function appropriately characterizing the discrepancy between microbiome compositional measurements. We thoroughly study the special cases that adopt Euclidean distance and Aitchison distance. Our proposals appropriately account for the unique features of microbiome compositional data, including high-dimensionality, dependency among individual relative abundances, and the presence of many zeros. We further investigate a practical compound approach to help better understand the sources of data inconsistency. Extensive simulation studies are conducted to evaluate the utility of the proposed methods in realistic scenarios. We also apply the proposed methods to a microbiome validation dataset from the Feeding Infants Right.. from the STart (FIRST) study. Our analyses offer useful insight about the extent of data variations resulted from two different experiment procedures as well as their heterogeneous patterns across genera.

Zinc status and growth in infants and young children with cystic fibrosis.

BACKGROUND: Zinc deficiency is associated with poor growth in children without cystic fibrosis (CF), but its impact on growth in children with CF is unknown. OBJECTIVE: To determine the prevalence of low serum Zn (sZn) and its relationship with growth in the first 3 years of life in children with CF. METHODS: We utilized data from infants with CF who were enrolled in a longitudinal study of nutrition and lung health and had sZn measured as part of clinical care. Cross-sectional correlations between sZn levels and growth z scores were assessed by Pearson's correlation coefficient. To identify factors associated with sZn status and its association to longitudinal growth patterns, multiple regression analysis with repeated measures were performed using generalized estimating equations. RESULTS: A total of 106 sZn measurements from 53 infants were identified. Seventeen infants (32%) had intermittent Zn insufficiency, defined as at least one sZn <70 mcg/dl in their first 3 years of life. There were no significant cross-sectional associations between sZn and growth z scores. However, analysis of longitudinal growth patterns revealed that weight- and length-for-age z scores in children with intermittent Zn insufficiency were lower during early infancy and their weight-for-length z scores at age 3 years were also lower compared to those who were always Zn sufficient. CONCLUSION: Low sZn occurs in one-third of children with CF in the first 3 years of life. Cross-sectional and longitudinal analyses revealed discrepant associations between sZn and growth. Therefore, prospective studies are needed to understand the role of Zn in growth in CF.

GENERALIZED ACCELERATED RECURRENCE TIME MODEL IN THE PRESENCE OF A DEPENDENT TERMINAL EVENT.

Recurrent events are commonly encountered in longitudinal studies. The observation of recurrent events is often stopped by a dependent terminal event in practice. For this data scenario, we propose two sensible adaptations of the generalized accelerated recurrence time (GART) model (Sun et al., 2016) to provide useful alternative analyses that can offer physical interpretations while rendering extra flexibility beyond the existing work based on the accelerated failure time model. Our modeling strategies align with the rationale underlying the use of the survivors' rate function or the adjusted rate function to account for the presence of the dependent terminal event. For the proposed models, we identify and develop estimation and inference procedures, which can be readily implemented based on existing software. We establish the asymptotic properties of the new estimator. Simulation studies demonstrate good finite-sample performance of the proposed methods. An application to a dataset from the Cystic Fibrosis Foundation Patient Registry (CFFPR) illustrates the practical utility of the new methods.

Early life growth patterns persist for 12 years and impact pulmonary outcomes in cystic fibrosis.

BACKGROUND: In children with cystic fibrosis (CF), recovery from growth faltering within 2 years of diagnosis (Responders) is associated with better growth and less lung disease at age 6 years. This study examined whether these benefits are sustained through 12 years of age. METHODS: Longitudinal growth from 76 children with CF enrolled in the Wisconsin CF Neonatal Screening Project was examined and categorized into 5 groups: R12, R6, and R2, representing Responders who maintained growth improvement to age 12, 6, and 2 years, respectively, and I6 and N6, representing Non-responders whose growth did and did not improve during ages 2-6 years, respectively. Lung disease was evaluated by % predicted forced expiratory volume in one second (FEV1) and chest radiograph (CXR) scores. RESULTS: Sixty-two percent were Responders. Within this group, 47% were R12, 28% were R6, and 25% were R2. Among Non-responders, 76% were N6. CF children with meconium ileus (MI) had worse lung function and CXR scores compared to other CF children. Among 53 children with pancreatic insufficiency without MI, R12 had significantly better FEV1 (97-99% predicted) and CXR scores during ages 6-12 years than N6 (89-93% predicted). Both R6 and R2 experienced a decline in FEV1 by ages 10-12 years. CONCLUSIONS: Early growth recovery in CF is critical, as malnutrition during infancy tends to persist and catch-up growth after age 2 years is difficult. The longer adequate growth was maintained after early growth recovery, the better the pulmonary outcomes at age 12 years.

Generalized accelerated recurrence time model for multivariate recurrent event data with missing event type.

Recurrent events data are frequently encountered in biomedical follow-up studies. The generalized accelerated recurrence time (GART) model (Sun et al., 2016), which formulates covariate effects on the time scale of the mean function of recurrent events (i.e., time to expected frequency), has arisen as a useful secondary analysis tool to provide meaningful physical interpretations. In this article, we investigate the GART model in a multivariate recurrent events setting, where subjects may experience multiple types of recurrent events and some event types may be missing. We propose methods for the GART model that utilize the inverse probability weighting technique or the estimating equation projection strategy to handle event types that are missing at random. The new methods do not require imposing any parametric model for the missing mechanism, and thus are robust; moreover, they enjoy easy and stable implementation. We establish the uniform consistency and weak convergence of the resulting estimators and develop appropriate inferential procedures. Extensive simulation studies and an application to a dataset from Cystic Fibrosis Foundation Patient Registry (CFFPR) illustrate the validity and practical utility of the proposed methods.

Pubertal Height Growth and Adult Height in Cystic Fibrosis After Newborn Screening.

BACKGROUND: To examine long-term growth benefit of newborn screening (NBS), adolescent peak height velocity (PHV), and adult height were compared between the screened (diagnosed early via NBS) and the control (identified generally by symptoms) in the Wisconsin Randomized Clinical Trial. METHODS: Data from 107 children born in 1985-1994 and followed through 2012 were analyzed. PHV was estimated by a semiparametric growth curve model and compared with Tanner reference. RESULTS: Meconium ileus (MI; n = 25) was associated with the worst pubertal growth and adult height, including 1 child who did not experience apparent PHV; children with pancreatic sufficiency (n = 18) achieved the best growth (normal PHV and adult height). In children with pancreatic insufficiency without meconium ileus (n = 64), the subgroup most likely to benefit from NBS, screened children had similar PHV but better adult height compared with controls. Specifically, in boys, the screened group (n = 22) achieved normal PHV (9.5 cm at 13.5 years); the control group (n = 19) had similar onset age (13.6 years) but 0.6-cm lower magnitude (P = .08). In girls, the screened group (n = 10) had somewhat later (12.5 years vs 11.7 years, P = .12) and lower PHV (7.3 cm vs 7.9 cm, P = .33) than the controls (n = 13), coinciding with later menarche (13.6 years vs 12.2 years, P = .10). Adult height was taller in the screened than the control (50th vs 29th percentile, P = .02), even after adjusted for genetic potential (32nd vs15th percentile, P = .006). Differences in adult height were primarily attributable to NBS and better prepubertal growth. CONCLUSIONS: Early linear growth benefits of NBS were sustained through puberty, leading to better adult height in cystic fibrosis.

Generalizing Quantile Regression for Counting Processes with Applications to Recurrent Events.

In survival analysis, quantile regression has become a useful approach to account for covariate effects on the distribution of an event time of interest. In this paper, we discuss how quantile regression can be extended to model counting processes, and thus lead to a broader regression framework for survival data. We specifically investigate the proposed modeling of counting processes for recurrent events data. We show that the new recurrent events model retains the desirable features of quantile regression such as easy interpretation and good model flexibility, while accommodating various observation schemes encountered in observational studies. We develop a general theoretical and inferential framework for the new counting process model, which unifies with an existing method for censored quantile regression. As another useful contribution of this work, we propose a sample-based covariance estimation procedure, which provides a useful complement to the prevailing bootstrapping approach. We demonstrate the utility of our proposals via simulation studies and an application to a dataset from the US Cystic Fibrosis Foundation Patient Registry (CFFPR).

Nonparametric association analysis of bivariate left-truncated competing risks data.

We develop time-varying association analyses for onset ages of two lung infections to address the statistical challenges in utilizing registry data where onset ages are left-truncated by ages of entry and competing-risk censored by deaths. Two types of association estimators are proposed based on conditional cause-specific hazard function and cumulative incidence function that are adapted from unconditional quantities to handle left truncation. Asymptotic properties of the estimators are established by using the empirical process techniques. Our simulation study shows that the estimators perform well with moderate sample sizes. We apply our methods to the Cystic Fibrosis Foundation Registry data to study the relationship between onset ages of Pseudomonas aeruginosa and Staphylococcus aureus infections.

Association Between Alcohol Use Among College Students and Alcohol Outlet Proximity and Densities.

BACKGROUND: Alcohol consumption is common on college campuses and is associated with negative consequences. Factors associated with availability of alcohol are not completely understood. OBJECTIVE: To describe how proximity and density of alcohol outlets are associated with any drinking and binge drinking in students at the University of Wisconsin-Madison. METHODS: Participants were full-time students enrolled in the Young Adults Eating and Active for Health, a multisite, randomized intervention that assessed a variety of health behaviors. Geographic information systems were used to calculate proximity and enumerate alcohol outlet densities. Participants were categorized as "drinkers" or "nondrinkers" based on self-reported alcohol consumption. Binge drinking was categorized as "non-binge drinker," "frequent binge drinker," and "excessive binge drinker." Analysis included regression, t tests, and chi-square tests. RESULTS. Among the 166 participants, 126 (76%) were drinkers. Among drinkers, 80 (63%) were either frequent or excessive binge drinkers. Drinkers lived closer to an alcohol outlet than non-drinkers (0.18 +/- 0.15 vs. 0.61 +/- 1.59 miles, respectively, P=0.005). Within a 1-mile walking radius, there were 47% more establishments for drinkers (153 +/- 47 compared to 104 +/- 55 outlets for nondrinkers, P<0.0001). At distances of 0.10-0.25 and 0.25-0.50 miles, twice as many outlets were available to drinkers (19 +/- 19 and 43 +/- 25, respectively) compared to nondrinkers (7 +/- 11 and 20 +/- 22, respectively), P<0.001. Proximity and density were hot associated with binge drinking frequency. CONCLUSION: Drinkers lived closer to alcohol outlets and had significantly more outlets available at a distance of up to 1 mile. Municipal and college administrators could consider limiting alcohol license distributions in municipalities with high alcohol consumption.

Comparing the Use of Centers for Disease Control and Prevention and World Health Organization Growth Charts in Children with Cystic Fibrosis through 2 Years of Age.

OBJECTIVE: To examine differences between use of World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) growth reference in children with cystic fibrosis (CF) up to 2 years of age. STUDY DESIGN: Growth from 1-24 months in 2587 children, born 2003-2006 and recorded in the US CF Foundation Registry, was evaluated using WHO and CDC references. RESULTS: In both boys and girls with CF aged 1-24 months, use of WHO charts resulted in ∼8 percentile lower length-for-age and ∼13% higher short stature rate (length-for-age <5th percentile). WHO weight-for-age was ∼9 percentile lower prior to age 6 months, crossed at 6-7 months, and remained ∼14 percentile higher at 8-24 months. WHO weight-for-length (WFL) percentile (WFLp) was similar before 12 months but ∼10 percentile higher at 12-24 months compared with CDC. When using WHO charts, 9% of children had underweight (WFLp <50th) classified differently and this rate varied with age: 4% in the first year, 7% at 12, 13% at 15, and 16% at 18 months, respectively. Weight status assessed by WHO body mass index (BMI) charts was different from WHO WFL charts. At 24 months when switching back to CDC, 26% of children with normal WFLp on WHO charts appeared underweight on CDC charts. A 70th percentile of WHO BMI percentile was equivalent to the 50th percentile CDC BMI percentile. CONCLUSIONS: Growth status in children with CF differed when using WHO and CDC references, particularly during the second year of life. These differences need to be considered for all uses of growth assessment in CF.

Pubertal height velocity and associations with prepubertal and adult heights in cystic fibrosis.

OBJECTIVES: To test the hypothesis that pubertal peak height velocity (PHV) in cystic fibrosis (CF) has improved and is influenced by prepubertal growth and genetic potential. STUDY DESIGN: PHV from 1862 children born in 1984-87 and documented in the 1986-2008 US CF Foundation Registry was determined by statistical modeling and classified into normal, delayed (2-SD > average age), attenuated (magnitude <5th percentile), or both delayed and attenuated (D&A). Genetic potential for height was estimated by parental stature. RESULTS: PHV averaged 8.4 cm/year at age 14.0 years in boys and 7.0 cm/year at age 12.1 years in girls, ∼6-month delay and ∼15% reduction compared with healthy children. PHV was normal in 60%, delayed in 9%, attenuated in 21%, and D&A in 5%. Patients with delayed PHV reached similar adult height percentile (boys: 34th, girls: 46th) to those with normal PHV (boys: 33rd, girls: 34th); both were significantly taller than the attenuated (boys: 11th, girls: 19th) and D&A PHV subgroups (boys: 8th, girls: 14th). Pancreatic-sufficient patients had taller prepubertal and adult heights but similar PHV compared with pancreatic-insufficient or meconium ileus patients. Adjusting for genetic potential reduced adult height percentiles more in boys (from 25th to 16th) than girls (from 28th to 24th). Height at age 7 years, PHV age and magnitude, and parental stature significantly predicted adult height. CONCLUSIONS: Pubertal PHV has improved in children with CF born after mid-1980s compared with older cohorts but remains below normal. Suboptimal prepubertal and pubertal growth led to adult height below genetic potential in CF.

Nutritional status is associated with health-related quality of life in children with cystic fibrosis aged 9-19 years.

BACKGROUND: The impact of improved nutritional status on health-related quality of life (HRQOL) is unknown for children with cystic fibrosis (CF). METHODS: Associations between nutritional status and HRQOL were examined over 2 years in 95 children, aged 9-19 years, who were followed in the Wisconsin Newborn Screening Project. HRQOL was assessed using the Cystic Fibrosis Questionnaire (CFQ). Associations between height z-score (HtZ), BMI z-score (BMIZ) and seven CFQ dimensions were evaluated. RESULTS: Mean values of at least 80 were observed for all CFQ dimensions except respiratory symptoms and treatment burden. Treatment burden was significantly worse in patients with meconium ileus (57) compared to pancreatic insufficient (65) and sufficient (78) subjects, p<0.0001. HtZ and BMIZ were positively associated with physical functioning and body image (p<0.05). CONCLUSIONS: Better nutritional status was associated with increased HRQOL scores. Early diagnosis through newborn screening and improved nutrition provides an opportunity to enhance quality of life and body image perception.

Nonparametric estimation of the mean function for recurrent event data with missing event category.

Recurrent event data frequently arise in longitudinal studies when study subjects possibly experience more than one event during the observation period. Often, such recurrent events can be categorized. However, part of the categorization may be missing due to technical difficulties. If the event types are missing completely at random, then a complete case analysis may provide consistent estimates of regression parameters in certain regression models, but estimates of the baseline event rates are generally biased. Previous work on nonparametric estimation of these rates has utilized parametric missingness models. In this paper, we develop fully nonparametric methods in which the missingness mechanism is completely unspecified. Consistency and asymptotic normality of the nonparametric estimators of the mean event functions accommodate nonparametric estimators of the event category probabilities, which converge more slowly than the parametric rate. Plug-in variance estimators are provided and perform well in simulation studies, where complete case estimators may exhibit large biases and parametric estimators generally have a larger mean squared error when the model is misspecified. The proposed methods are applied to data from a cystic fibrosis registry.

Comparing age of cystic fibrosis diagnosis and treatment initiation after newborn screening with two common strategies.

BACKGROUND: Newborn screening (NBS) for CF has become widespread, although there are multiple strategies. Little is known about outcomes such as age of diagnosis after different NBS methods. METHODS: We used the U.S. Cystic Fibrosis Foundation Patient Registry to identify infants with CF born between 2001 and 2008 in states that utilized NBS. We compared ages at diagnosis, genotyping, sweat test, and first visit to a CF Centre between states that used serial immunoreactive trypsinogen (IRT/IRT) levels and states that used IRT and DNA analysis (IRT/DNA). RESULTS: We identified 1288 infants with CF. Compared to infants born in IRT/IRT states, infants born in IRT/DNA states were younger at the time of diagnosis (median 2.3 weeks versus 4.0 weeks in IRT/IRT states, p<0.001), genotyping (0.7 weeks versus 5.3 weeks, p<0.001), and initial CF Centre visit (5.9 weeks versus 7.7 weeks, p=0.008). CONCLUSIONS: Although there is room to improve outcomes with both strategies, infants born in IRT/DNA states have treatment initiated at a younger age than infants born in IRT/IRT states.