Background: Spirometry reference values specifically designed for Asian Americans are currently unavailable. The performance of Global Lung Function Initiative 2012 (GLI-2012) equations on assessing spirometry in Asian Americans has not been evaluated. This study aimed to assess the fitness of relevant GLI-2012 equations for spirometry in Asian Americans. Methods: Asian subjects who never smoked and had qualified spirometry data were extracted from the National Health and Nutrition Examination Survey (NHANES) 2011–2012. Z-scores of forced expiratory volume in 1 s (FEV ), forced vital capacity (FVC), and FEV /FVC were separately constructed with GLI-2012 equations for North East (NE) Asians, South East (SE) Asians, and individuals of mixed ethnic origin (Mixed). In addition, Proportions of subjects with observed spirometry data below the lower limit of normal (LLN) were also evaluated on each GLI-2012 equation of interest. Results: This study included 567 subjects (250 men and 317 women) aged 6–79 years. Spirometry z-scores (z-FEV ,z- FVC, and z-FEV /FVC) based on GLI-2012 Mixed equations had mean values close to zero (− 0.278 to − 0.057) and standard deviations close to one (1.001 to 1.128); additionally, 6.0% (95% confidence interval (CI) 3.1–8.9%) and 6.4% (95% CI 3.7–9.1%) of subjects were with observed data below LLN for FEV /FVC in men and women, respectively. In contrast, for NE Asian equations, all mean values of z-FEV and z-FVC were smaller than − 0.5; for SE Asian equations, mean values of z-FEV /FVC were significantly smaller than zero in men (− 0.333) and women (− 0.440). Conclusions: GLI-2012 equations for individuals of mixed ethnic origin adequately fitted spirometry data in this sample of Asian Americans. Future studies with larger sample sizes are needed to confirm these findings. Keywords: Asian Americans, Lung function, LLN, Spirometry, Z-score Background Society (ATS) recommended spirometry reference values Accurate interpretation of pulmonary function test results, that were based on a sample from the third National which requires valid spirometry reference values, is of ma- Health and Nutrition Examination Survey (NHANES III) terial importance to respiratory medicine. In addition to for population aged 8–80 years in US [4, 5]. Nonetheless, gender, age, and height, race/ethnicity acts as another limited by race/ethnicity classification in NHANES III, major determinant of lung function [1–3]. Therefore, it is spirometry reference values for Asian Americans were un- recommended that spirometry reference values estab- able to be produced through Hankinson et al.’sstudy . lished with healthy people of similar race/ethnicity be ap- Previous studies showed that Asian Americans had clinic- plied to a certain population whenever possible. The ally significantly lower forced expiratory volume in 1 s European Respiratory Society (ERS)/American Thoracic (FEV ) and forced vital capacity (FVC) compared with Cau- casian people in US [6–11]. Accordingly, a correction factor * Correspondence: firstname.lastname@example.org for FEV and FVC has been developed and calibrated to be Department of Respiratory Medicine, Peking Union Medical College applied to NHANES III Caucasian equations when asses- Hospital, Peking Union Medical College & Chinese Academy of Medical sing spirometry in Asian Americans. Specifically, 0.94 and Sciences, Beijing 100730, China Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 2 of 9 0.88 have been sequentially proposed as the correction This study’s exclusion criteria were: 1) examinees who factor for FEV and FVC [4, 12, 13]. A recent systemic re- did not qualify for a baseline spirometry test; 2) current view suggested that a correction factor of 0.88 was more or past smokers (defined as those who had smoked at suitable than 0.94 to be applied to NHANES III Caucasian least 100 cigarettes in life); 3) participants who reported reference values for FEV and FVC evaluation in Asian respiratory illnesses (cough, cold, phlegm, runny nose, or Americans . other respiratory illnesses) seven days prior to the exam- In 2012, the Global Lung Function Initiative ination; 4) baseline spirometry effort quality attribute of (GLI-2012) published all-age-covering spirometry pre- “B”, “C” or “D”, or baseline FEV or FVC quality attri- dictive equations for multiple ethnicities, including bute of “D (questionable results, use with caution)” or “F North East (NE) Asian and South East (SE) Asian . (results not valid)” [26, 27]. A detailed study sample in- In addition, a set of GLI-2012 equations were designed clusion and exclusion process is shown in Fig. 1. for individuals of mixed ethnic origin (Mixed) . Al- though with mixed results, GLI-2012 equations showed Spirometry measurements clinically acceptable generalisability to spirometry in sev- Participants aged 6–79 years were eligible for spirometry eral validation samples [16–21]. Therefore, relevant tests in NHANES 2011–2012. Examinees who had breath- GLI-2012 equations are potentially useful for evaluating ing problem requiring oxygen/taking deep breath, current lung function of Asian Americans. Nonetheless, per- ear infection, eye/chest/abdominal surgery, or stroke/heart formance of GLI-2012 reference equations on assessing attack in the past three months, tuberculosis in the past spirometry in Asian Americans has not been evaluated. year, or coughing up blood in the past month were ex- Asian people, including Asian alone and in combin- cluded from a baseline spirometry. Technicians received ation with other races, account for more than 17.3 mil- formal training and used an Ohio 822/827 dry-rolling seal lion (5.6%) of total American population in 2010 . volume spirometer (Ohio Medical, Gurnee, IL, USA) for Of note, the total US Asian population increased by 5.4 spirometry tests. Regular calibration of spirometry equip- million (45.6%) from 2000 to 2010, and is projected to ment and rigorous spirometry curves quality control were grow to 48.6 million by 2060 [23, 24]. Owing to the re- conducted by health technicians and were subsequently markable quantity and rapid growth of Asian population verified by supervisory staff . in US, it is clinically important to assess spirometry ref- erence values that have been recommended for or can Statistical analysis be potentially used in that population. Herein, we con- The fitness of GLI-2012 reference equations designed ducted this study to assess the fitness of relevant for NE Asians, SE Asians, and individuals of mixed eth- GLI-2012 equations and NHANES III reference values nic origin and Caucasians were evaluated for spirometry for spirometry in Asian Americans. in this sample. GLI-2012 equations were designed using the “Generalized Additive Models for Location, Scale, and Shape (GAMLSS)” method, which permitted the fit- Methods ness of mean (M), coefficient of variance (S), and skew- Study design ness (L) of spirometry data [15, 29]. Z-scores of FEV Asian subjects from NHANES 2011–2012, where spir- (z-FEV ), FVC (z-FVC), and FEV /FVC (z-FEV /FVC) 1 1 1 ometry data were available, were included in this study. were calculated using the formula: z-score = ((observed The NHANES utilized a complex, multistage, probability value/M) ^ L - 1) / (L*S). The z-score is defined as how sampling design to collect health and nutrition data many standard deviations (SDs) a measured value is from from a nationally representative sample of civilian, predicted value (z-score = (observed - predicted)/SD). One non-institutionalized people in US each year. Since the may argue that the z-score is a more appropriate approach year of 2011, NHANES has started to oversample Asian to reporting lung function data than using % predicted by population in US and code them as “non-Hispanic considering lung function related variables (age, height, Asian” for race/ethnicity, which provided opportunity ethnicity, etc.) . The proportion of subjects with ob- for investigating health conditions specifically on Asian served spirometry data below lower limit of normal Americans . NHANES 2011–2012 finally released (LLN), which corresponds to the 5th percentile of pre- demographic, nutritional, and health data of 1282 dicted values, were also evaluated for FEV ,FVC, and non-Hispanic Asian participants, which served as the FEV /FVC on each GLI-2012 equation of interest. The basis for this study. NHANES protocols were reviewed cutoff z-score of LLN was calculated with the formula: and approved by the Research Ethics Review Board of LLN z-score = − 1.6445 * (SD of z-scores). National Center for Health Statistics, and written in- Student’s t-tests were used to examine the difference formed consent was obtained from each NHANES between the mean of z-scores and zero. Bland-Altman participant. plots of spirometry predictions based on NHANES III Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 3 of 9 Fig. 1 Flowchart of study sample selection Caucasian equations with 0.88 as the correction factor women were 164.1 (15.6) cm and 154.2 (11.4) cm, re- for FEV and FVC against GLI-2012 Mixed equations spectively. In this sample, there were 17 (6.8%) men and were generated (difference = NHANES III prediction – 19 (6.0%) women who had a BMI ≥ 30 kg/m . Addition- GLI-2012 prediction). Bland-Altman plots are used to ally, 38.4% of men and 31.6% of women were born in describe agreement between two quantitative methods US. Among those who were not born in US, 31.6% of of measurement by calculating the mean difference and men and 37.3% of women had lived in US for more than 95% limits of agreement (1.96*SD of the difference) be- 20 years, whereas 27.0% of men and 20.6% of women tween the two measurements . A two-sided P < 0.05 had been in US for less than 5 years. was considered statistically significant for all tests. Data analyses were performed with SAS 9.4 (SAS Institute, Performance of GLI-2012 equations (Table 2) Cary, NC, USA) and R version 3.4.0 (R Foundation for For NE Asian equations, all mean (median) values of Statistical Computing, Vienna, Austria). z-FEV and z-FVC were smaller than − 0.5 in both men and women, with the lowest as − 0.743 (− 0.819) for Results z-FVC in women. For SE Asian equations, mean values Sample characteristics (Table 1) of z-FEV /FVC were − 0.333 in men and − 0.440 in Five hundred and sixty-nine Asian participants (250 women, all significantly different from zero. In terms of men and 317 women) were finally included in this ana- the Mixed equations, all mean values of z-FEV , z-FVC, lysis. The mean (SD) age were 28.4 (17.8) years for men and z-FEV /FVC were not significantly different from and 34.3 (19.7) years for women; and the age range for zero in men; and in women, although statistically signifi- men and women were 6 to 75 years and 6 to 79 years, cantly different from zero, all absolute differences were respectively (Fig. 2). The mean (SD) height for men and within 0.3. SDs of z-scores based on GLI-2012 SE Asian Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 4 of 9 Table 1 Baseline characteristics of sample subjects by gender Mixed) against age in men and women were showed in Additional file 2: Fig. S2 and Additional file 3: Fig. S3, Characteristics Gender respectively. Men (n = 250) Women (n = 317) Regarding proportion of observed spirometry data Age (year) 28.4 ± 17.8 34.3 ± 19.7 below LLN (% < LLN), the Mixed equations showed a 23 (14, 41) 32 (16, 51) satisfactory overall performance. Specifically, 6.0% (95% Height (cm) 164.1 ± 15.6 154.2 ± 11.4 confidence interval (CI): 3.1–8.9%) and 6.4% (95% CI: Weight (kg) 62.8 ± 19.4 53.9 ± 14.4 3.7–9.1%) of z-FEV /FVC were below LLN for men and BMI (kg/m ) 22.7 ± 4.7 22.3 ± 4.6 women, respectively. In contrast, according to SE Asian equations, 9.2% (95% CI: 5.6–12.8%) of z-FEV /FVC in Born in the U.S. 96 (38.4%) 100 (31.6%) b men and 10.0% (95% CI: 6.7–13.3%) of z-FEV /FVC in Length of time in U.S. women were below LLN; for NE Asian equations, all % Less than 5 years 41 (27.0%) 44 (20.6%) < LLN for z-FEV and z-FVC were significantly larger 5 to 10 years 28 (18.4%) 26 (12.2%) than 5% (11.2 to 16.2%). 10 to 20 years 35 (23.0%) 64 (29.9%) In addition, we confirmed that the NHANES III Cau- More than 20 years 48 (31.6%) 80 (37.3%) casian reference values with a correction factor of 0.88 for FEV and FVC satisfactorily fitted the spirometry FEV (L) 3.159 ± 0.904 2.344 ± 0.634 c data (FEV and FVC) of this sample (data not shown). FVC (L) 3.761 ± 1.052 2.785 ± 0.710 FEV /FVC 0.84 ± 0.07 0.84 ± 0.08 BMI body mass index, FEV forced expiratory volume in 1 s, FVC forced Agreement between NHANES III and GLI-2012 predictions vital capacity Overall, lung function predictions based on NHANES data were presented as mean ± standard deviation, median (interquartile III Caucasian reference values with a correction factor of range), or as number (percentage) for participants who were not born in the United States; 2 missing these data 0.88 for FEV and FVC were smaller than those based for men and 3 missing these data for women c on the GLI-2012 equations for FEV , FVC, and FEV / 1 1 1 missing these data for men and 6 missing these data for women FVC (Fig. 4). The average differences in FEV (L), FVC (L), and FEV /FVC (%) predictions were − 0.187, − 0.130, equations and the Mixed equations ranged from 1.002 and − 2.46 for men, and − 0.131, − 0.095, and − 2.12 for to 1.089 and 1.001 to 1.128, respectively, indicating that women, respectively. those equations adequately fitted variations of our spir- ometry data. In contrast, SDs of z-FEV and z-FVC based on GLI-2012 NE Asian equations were 1.512 and Discussion 1.517, respectively. Distributions of z-scores based on In this population-based cross-sectional analysis of lung GLI-2012 equations were showed in Fig. 3. For Cauca- function, we were the first to assess the generalisability sian equations, mean values of z-FEV and z-FVC were of relevant GLI-2012 reference equations to spirometry substantially smaller than zero in both men and women in Asian Americans. In addition, we evaluated the agree- (Additional file 1: Fig. S1). Also, plots of spirometry ment of lung function predictions between the NHANES z-scores for GLI-2012 reference eqs. (NE, SE, and the III Caucasian values with a correction factor of 0.88 for Fig. 2 Age distribution of study subjects by gender Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 5 of 9 Table 2 Spirometry z-scores of the present study population based on the GLI-2012 equations for North East Asians, South East Asians, and individuals of mixed ethnic origin GLI-2012 equations Statistics z-FEV z-FVC z-FEV /FVC 1 1 Men Women Men Women Men Women North East Asians Mean ± SD −0.571 ± 1.512 − 0.703 ± 1.029 −0.695 ± 1.517 −0.743 ± 1.223 0.021 ± 1.177 −0.135 ± 1.089 95% CI of mean (−0.759, − 0.382) * (−0.817, − 0.589) * (−0.884, − 0.506) * (−0.880, − 0.607) * (−0.124, 0.167) (− 0.257, − 0.014) * Median −0.558 − 0.730 −0.619 − 0.819 0.148 − 0.152 (5th, 95th percentile) (−3.143, 1.855) (−2.313, 0.799) (− 3.458, 1.909) (− 2.797, 1.099) (− 1.928, 1.798) (− 1.999, 1.716) N (%) < LLN 28 (11.2%) 51 (16.1%) 33 (13.3%) 42 (13.5%) 12 (4.8%) 20 (6.4%) 95% CI of % < LLN (7.3, 15.1%) (12.1, 20.1%) (9.1, 17.5%) (9.7, 17.3%) (2.1, 7.5%) (3.7, 9.1%) South East Asians Mean ± SD −0.037 ± 1.052 0.105 ± 1.080 0.177 ± 1.002 0.312 ± 1.089 −0.333 ± 1.055 −0.440 ± 1.010 95% CI of mean (−0.094, 0.168) (− 0.014, 0.225) (0.051, 0.302) * (0.190, 0.433) * (−0.465, − 0.201) * (−0.552, − 0.327) * Median 0.017 0.126 0.251 0.290 −0.203 −0.462 (5th, 95th percentile) (−1.736, 1.733) (−1.619, 1.667) (− 1.610, 1.899) (− 1.502, 1.994) (− 2.073, 1.264) (− 2.154, 1.239) N (%) < LLN 13 (5.2%) 8 (2.5%) 12 (4.8%) 10 (3.2%) 23 (9.2%) 31 (10.0%) 95% CI of % < LLN (2.4, 8.0%) (0.8, 4.2%) (2.1, 7.5%) (1.2, 5.2%) (5.6, 12.8%) (6.7, 13.3%) Individuals of mixed Mean ± SD −0.101 ± 1.051 − 0.278 ± 1.079 −0.112 ± 1.050 −0.212 ± 1.128 −0.057 ± 1.020 −0.152 ± 1.001 ethnic origin 95% CI of mean (−0.232, 0.030) (− 0.397, − 0.159) * (−0.229, 0.033) (− 0.338, − 0.086) * (−0.185, 0.070) (− 0.264, − 0.040) * Median −0.111 − 0.284 −0.051 − 0.266 0.055 − 0.167 (5th, 95th percentile) (−1.881, 1.591) (−1.975, 1.282) (−2.007, 1.702) (− 2.096, 1.508) (− 1.745, 1.484) (− 1.865, 1.547) N (%) < LLN 18 (7.2%) 20 (6.3%) 20 (8.0%) 25 (8.0%) 15 (6.0%) 20 (6.4%) 95% CI of % < LLN (4.0, 10.4%) (3.6, 9.0%) (4.6, 11.4%) (5.0, 11.0%) (3.1, 8.9%) (3.7, 9.1%) GLI Global Lung Function Initiative, SD standard deviation, CI confidence interval, FEV forced expiratory volume in 1 s, FVC forced vital capacity, z-FEV FEV z-score, z-FVC FVC z-score, z-FEV /FVC FEV /FVC z-score, LLN 1 1 1 1 1 lower limit of normal *P < 0.05 for student’s t-tests comparing mean values of z-scores and zero Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 6 of 9 Fig. 3 Distributions of z-scores of FEV , FVC, and FEV /FVC based on GLI-2012 equations for North East Asians, South East Asians, and individuals 1 1 of mixed ethnic origin. Panels A and B showed z-score distributions based on GLI-2012 equations for North East Asians in women and men, respectively; panels C and D showed z-score distributions based on GLI-2012 equations for South East Asians in women and men, respectively; and panels E and F showed z-score distributions based on GLI-2012 equations for individuals of mixed ethnic origin in women and men, respectively. In this graph, red dot denotes 5th and 95th percentiles of observed spirometry data; blue diamond denotes median of observed values; solid line represents a z-score of zero; and dotted line represents z-scores of ±1.96. FEV : forced expiratory volume in 1 s; FVC: forced vital capacity; GLI: Global Lung Function Initiative Fig. 4 Bland-Altman plots of spirometry predictions using NHANES III Caucasian values with a correction factor of 0.88 for FEV and FVC against those with GLI-2012 equations for individuals of mixed ethnic origin (difference = NHANES III prediction – GLI-2012 prediction). In this graph, dashed line represents the mean difference; dotted line represents 95% confidence interval of the mean difference; solid line represents the value of zero. NHANES III: The Third National Health and Nutrition Examination Survey; FEV : forced expiratory volume in 1 s; FVC: forced vital capacity; GLI: Global Lung Function Initiative Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 7 of 9 FEV and FVC and the GLI-2012 equations for individ- for spirometry in Asian Americans aged outside 6 to uals of mixed ethnic origin. 79 years. Secondly, GLI-2012 equations were designed Our findings showed that GLI-2012 Mixed equations with a semiparametric predictive modelling method, adequately fitted FEV , FVC, and FEV /FVC data of our which was able to fit variance and skewness of spirom- 1 1 sample for both gender. GLI-2012 Mixed equations were etry data in addition to the mean value . Moreover, designed for people of mixed ethnic origin, which we be- splines used in GLI-2012 equations modeled age-related lieve current Asian Americans could be categorized into variations for spirometry data. NHANES III equations due to the following several reasons. First, in the year were built based on quadratic function for FEV and 2010, around 16% of Asian Americans were Asian in FVC and linear function for FEV /FVC. Thus, compared combination with one or more other races, among with GLI-2012 equations, NHANES III equations were whom Asian in combination with White were the major- less likely to reflect actual patterns of spirometry data ity . Second, US Asian population consists of more due to their fixed function formats. Thirdly, NHANES than twenty subgroups, with Chinese, Indian, Filipino, III equations for FEV /FVC LLN and equations for Vietnamese, Korean, and Japanese accounting for the FEV /FVC are same as each other except different inter- most in quantity . Third, due to diversities of birth cepts. Therefore, according to NHANES III equations, country and years living in US, which is readily trans- LLN for FEV /FVC differs from FEV /FVC by a constant 1 1 lated into difference in environmental exposures and so- magnitude regardless of a subject’s age. However, since cioeconomic status, Asian Americans may have quite LLN theoretically corresponds to the 5th percentile of different lung function development [32–37]. Therefore, spirometry data and lung function varies with age, it is Asian Americans are genetically, environmentally, and conceptually insufficient to define LLN as a constant dif- socioeconomically heterogeneous in nature, which may ference to the mean for the entire age range. GLI-2012 explain the satisfactory performance of GLI-2012 Mixed reference values address this issue by defining LLN with equations in fitting spirometry data in this sample. spirometry z-scores, a way comprehensively taking GLI-2012 NE Asian equations were built based on two mean, variance, and skewness of spirometry data into datasets, one collected from North China and the other consideration. from South Korea; whereas the GLI-2012 SE Asian The GLI-2012 equations have been proposed to be equations were derived from a collated dataset consist- adopted worldwide in order to standardise the interpret- ing of five subsamples from South Asia and a subsample ation of lung function . Admittedly, the application from US . Quanjer et al. found that the two subsam- of a correction factor to the NHANES III Caucasian ref- ples of NE Asians had significantly larger lung function erence values offers a practical solution to assessing spir- than the six subsamples of SE Asians, and therefore they ometry in Asian Americans. However, the rationale constructed spirometry predictive equations separately behind the development of a correction factor, which is for NE Asians and SE Asians . Not surprisingly, only for temporary use, is not conceptually and meth- GLI-2012 NE Asian equations led to substantially larger odologically ideal. Based on the current findings and FEV and FVC predictions compared with observed data what has been discussed above, it is reasonable to regard in our sample for both gender, strongly suggesting GLI-2012 Mixed equations as superior to the NHANES against the application of those equations to assessing III Caucasian reference values with a correction factor spirometry in Asian Americans. GLI-2012 SE Asian for evaluating spirometry in Asian Americans. In par- equations, while performed satisfactorily in fitting FEV ticular, the ready availability of spirometry z-scores and and FVC, contributed to significantly larger FEV /FVC LLN from the GLI-2012 equations could possibly pro- predictions compared with the observed data, which will vide a convenient approach to the diagnosis and severity potentially result in an overdiagnosis of chronic ob- stratification of obstructive lung diseases. Therefore, structive pulmonary disease in Asian Americans. with the rapid increase of Asian population in US, the Generally, both the GLI-2012 Mixed equations and application of GLI-2012 Mixed equations to Asian the NHANES III Caucasian reference values with a cor- Americans is clinically important. rection factor of 0.88 adequately fitted the lung function This study has several limitations. First, the sample data in this sample. However, GLI-2012 equations pos- size of this study is relatively small. However, we would sess several potential advantages over the NHANES III argue that our sample sizes of men and women are both reference values. First, as all-age-covering spirometry large enough for validating spirometry reference values, reference values, GLI-2012 equations are valid for which requires at least 150 subjects for each gender . people aged 3 to 95 years old ; the NHANES III Second, as shown in Fig. 2, the distributions of age are equations, in contrast, have a comparably narrower valid right skewed in both men and women. Especially for age range of 8 to 80 years. Of note, in this study we were men, the proportion of adults and elderly people is rela- not able to evaluate the fitness of GLI-2012 equations tively small, which may limit the power of this study in Zhang et al. BMC Pulmonary Medicine (2018) 18:95 Page 8 of 9 that population. This issue is clinically relevant in that Competing interests The authors declare that they have no competing interests. obstructive lung diseases, where lung function references are widely used, are most prevalent in elderly people. As the accrual of NHANES data of Asian Americans, the Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in fitness of GLI-2012 equations could be better evaluated published maps and institutional affiliations. in the near future. Author details Department of Respiratory Medicine, Peking Union Medical College Conclusions Hospital, Peking Union Medical College & Chinese Academy of Medical In this cross-sectional analysis of lung function from a na- Sciences, Beijing 100730, China. Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY, USA. Yale tionally representative sample of US Asian population, we School of Public Health, Yale University, New Haven, CT, USA. showed that the GLI-2012 reference equations for individ- uals of mixed ethnic origin performed adequately on fit- Received: 16 January 2018 Accepted: 21 May 2018 ting spirometry data of this sample. 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BMC Pulmonary Medicine – Springer Journals
Published: May 31, 2018
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