Study protocol and procedures:
The study participants first completed MBW according to American Thoracic Society (ATS) guidelines27. LCI was determined according to standardized protocols28. Subsequently spirometry followed by lung volumes (plethysmography) were performed according to ATS criteria and standards29,30. To measure airway hyper-responsiveness27(AHR), spirometry was repeated 20 minutes after administration of 4 puffs of albuterol via MDI. For healthy controls and SCD patients without a recent blood draw (3 months), whole blood samples were collected for processing on the day of testing.
Definitions: Obstructive lung disease was defined as 1 or more lung function measures (FEV1 or FEV1/FVC) < lower limit of normal (LLN) equivalent to a z-score of -1.645 based on Global Lung Initiative (GLI) normative data for age, height, weight and race. Restrictive lung disease was defined as Total Lung Capacity (TLC) or Forced Vital Capacity (FVC) <LLN based on GLI normative data for age, height, weight and race. AHRwas defined as having a positive bronchodilator response after repeat spirometric measures with an increase in FEV1 and/or FVC of ≥12% from baseline31.
Statistical Analysis: The sample size was based on results from Zwitserloot et al.32where the comparison of LCI between patients with asthma and healthy children (controls) showed a LCI difference of 0.27 with standard deviations of 0.48 and 0.38 for the two groups respectively. There are no defined normative LCI values in African American children. We assumed a similar LCI difference and standard deviation between SCD and healthy control children. In order to achieve 80% power and a 5% error rate, we needed to enroll 42 children in each group. Demographics, clinical events, lab results, and pulmonary function data were compared across groups using Student’s t-tests for continuous variables. Wilcoxon tests were performed for non-linear data. Chi-Square tests were performed for categorical variables and Fisher’s Exact tests were used when cell counts were low. Correlation analyses were performed to analyze the associations between clinical and pulmonary function data. To check for possible moderating effects due to participant age, interaction terms were assessed with age groups 6-12 years of age and 13-18 years, as well as performing analyses stratified by age groups. Multivariable models were then used to determine if associations with LCI would become attenuated in the larger model. Variables in the multivariable model were chosen due to statistical significance at the bivariate level and for clinical meaningfulness. All analytic assumptions were verified, with non-parametric tests (i.e. Wilcoxon tests and Spearman correlations) used when data were non-linear. Collinearity was assessed in the multivariable model with variance inflation factors. All analyses were performed using SAS v9.4 (SAS Institute, Cary, NC).