National Health and Nutrition Examination Survey

Spirometry, which means “the measuring of breath,” is a routinely used clinical pulmonary function test. It measures the amount and speed of air that a person can exhale after taking in the deepest possible breath. This allows estimates for lung volume and air flow rates to be made. Results from Spirometry testing are one key element (among others) needed to diagnose respiratory disorders such as asthma and chronic obstructive pulmonary disease (COPD). These diseases are typically characterized by a reduction in expiratory air flow rates. This is defined in spirometry as a reduction in the ratio of the expiratory volume measured in the 1st second of a forceful exhalation (FEV1) to the total volume of air exhaled in the forced expiration (FVC), the FVC being an estimate of the individual's effective lung volume. The "Spirometric Measurements Section" of the NHANES 2007-8 Respiratory Health Spirometry Procedures Manual contains a basic explanation of interpreting spirometry data. The manual is available at https://wwwn.cdc.gov/nchs/data/nhanes/2007-2008/manuals/spirometry.pdf

Spirometry is one of two NHANES 2007-8 components on respiratory health sponsored by the National Heart, Lung, and Blood Institute of the National Institute of Health, the Centers for Disease Control and Prevention, National Institute for Health Statistics and the National Institute for Occupational Safety and Health (NIOSH). Individual Exhaled Nitrous Oxide (ENO), a marker of airways inflammation, is also measured. The objective of the Spirometry data collection is to provide health examination data to estimate the current prevalence of asthma and COPD in the U.S. population. The data is also intended to help produce updated spirometric reference data for the U.S. National-level Spirometry data was collected in NHANES I (1971-1975), NHANES II (1976-1980) and most recently in NHANES III (1988-1994). Comparisons of the current data with that from previous studies will help to identify trends in respiratory conditions over time.

Eligible participants performed an initial or "baseline" 1st test spirometry examination. Then a selected subsample of participants whose Baseline 1st Test Spirometry results showed a FEV1/FVC ratio below the lower limit of normal and/or below 70% were asked to repeat spirometry after inhaling a β2-adrenergic bronchodilator medication to open up their airways. This helps differentiate asthma from COPD (Pellegrino, Viegi, Brusasco, et al. 2005). Asthma patients usually show improvements in post-bronchodilator spirometry testing, while patients with COPD exhibit little, if any, response to the medication. Spirometric testing using β2-adrenergic bronchodilator is routinely employed by clinicians to diagnose asthma in both children and adults, and current clinical practice guidelines (NHLBI Expert Panel Report, 2007) consider post-bronchodilator spirometry testing essential for the initial diagnosis of asthma. Second test post bronchodilator spirometry studies had not been performed in previous NHANES surveys.

Survey participants, ages 6 to 79 years of age, were eligible for the Spirometry Component. Specific exclusion criteria that were employed for Baseline Spirometry included criteria employed in earlier NHANES Spirometry surveys. Excluded from testing were examinees that had current chest pain or a physical problem with forceful expiration, were taking supplemental oxygen, had recent surgery of the eye, chest or the abdomen; had a recent heart attack, stroke, tuberculosis exposure or had recently coughed up blood. Adults with a personal history of detached retina or a collapsed lung and children with painful ear infections were also excluded. In the NHANES 2007-8, Baseline (1st test) Spirometry data was collected for 6,444 sample persons.

Participants completing Baseline 1st test Spirometry were selected for follow-up Bronchodilator 2nd Test Spirometry only if their baseline spirometry values indicated possible airflow obstruction that might indicate the presence of asthma or COPD. The participant's baseline spirometry results were evaluated by a computerized algorithm and examinees with an FEV1/FVC% less than the Lower Limit of Normal (LLN) determined for his or her age, sex, weight, height, and race/ethnicity, or those who had a FEV1/FVC% less than 70% were considered eligible for bronchodilator testing. These two criteria were used so that NHANES data could be compared with previous epidemiology studies that used these two different criteria. For the first criterion the LLN for the FEV1/FEC was calculated based on the sample person's age, gender, height and race/ethnicity in three major groups (non-Hispanic white, non-Hispanic black, Mexican-American). For ages 8-79 the FEV1/FVC% LLN was as defined by NHANES III data (Hankinson, Odencrantz, Fedan, 1999) and for children ages 6-7 years it was as defined by Wang et al. (1993). The administrative variable SPDBRONC indicates whether the participant was considered eligible for Bronchodilator 2nd Test Spirometry. The variable SPXBSTAT provides the bronchodilator study exam status.

Participants eligible for Bronchodilator 2nd Test Spirometry were referred to the Mobile Examination Center (MEC) Physician for further evaluation as documented in the Respiratory Health Bronchodilator Procedures Manual. The Bronchodilator testing protocol was explained and the participant was further screened for the presence of any exclusion criteria for bronchodilator administration. These included certain cardiovascular conditions (uncontrolled blood pressure, irregular pulse on examination, taking medication for major arrhythmia, having an implanted defibrillator, or history of congenital heart disease) or taking certain prescription medications (a monamine oxidase inhibitor, an anticonvulsant, a tricyclic antidepressant plus current treatment for cardiac disease, potassium lowering drugs). Examinees were also excluded from bronchodilator administration if they had already recently taken a β2-adrenergic bronchodilator, or had a previous adverse reaction to albuterol. Women who were currently pregnant or breastfeeding were also excluded.

Following eligibility screening, the MEC physician obtained written informed consent and administered two puffs of the β2-adrenergic bronchodilator albuterol. The examinee was then required to wait a minimum of ten minutes before proceeding to the Spirometry exam room to obtain a repeat 2nd Test Bronchodilator spirometry study. A total of 611 sample persons performed Bronchodilator (2nd Test) Spirometry.

Testing procedures met the recommendations of the American Thoracic Society (ATS) (Miller, Hankinson, Brusasco el al. 2005). The protocol was similar to that used in NHANES III (1998-1994), with the minor change in the goal for the number of acceptable quality spirometry maneuvers, from five in NHANES III, to three in NHANES 2007-8. Also, to ensure data comparability between NHANES survey cycles, the same spirometers that were employed for the NHANES III spirometry examination (1988-1994) were used for NHANES 2007-8 data collection. These were Ohio 822/827 dry-rolling seal volume spirometers. The devices were reconditioned for use in the current survey and were maintained by the NIOSH laboratories. In NHANES 2007-8, unlike NHANES III, a new in-line biological filter (A-M Systems PFT Filter Kit B) was fitted to each spirometry hose to minimize infection risks to the participant.

Spirometry was performed in the standing position unless the participant was physically impaired. The participant was asked to elevate the chin and extend the neck slightly and place a nose clip on his or her nose during testing, the latter to prevent air leaks. He/she was then instructed to perform a series of maximal forced expiratory maneuvers. In each maneuver, the participant took the deepest breath possible to fill the lungs with air, then put the mouthpiece into his/her mouth making a tight seal and then blew the air out as hard and fast as possible. Participants ages 6-10 years were asked to blow out the air for a minimum of 3 seconds of exhalation, whereas those ages 11-79 years were asked to blow out the air for a minimum of 6 seconds of exhalation.

Spirometry testing for an individual continued until he/she was able to achieve a reproducible spirogram, or until a maximum of eight spirometry curves had been obtained, or until the participant could not continue. The overall goal was for the participant to achieve three acceptable exhalation maneuvers by ATS criteria in which the two highest values for the FVC and the FEV1 (each taken from an acceptable forced expiratory maneuver) showed minimal variability: i.e. the two largest FVC values taken from 2 acceptable curves should agree within 150 ml, and similarly for the two largest values for the FEV1. The procedure for the Bronchodilator (2nd Test) Spirometry, performed after the participant had taken the β2-adrenergic bronchodilator albuterol, was identical to the procedure for the Baseline 1st test. Please refer to the NHANES 2007-8 Repiratory Health Spirometry Procedure manual for full details on the testing protocol, and for detailed criteria for a reproducible spirogram.

The NIOSH Division of Respiratory Disease Studies served as the NHANES Spirometry training and quality control consultant. Each MEC Health Technician received formal training and satisfactorily completed the NIOSH-approved spirometry course. Additionally, all NHANES spirometry data were reviewed by expert reviewers at the NIOSH quality control center on an ongoing basis. MEC Technicians received continuous feedback on their performance throughout the survey.

The Respiratory Health Spirometry Procedures Manual provides detailed descriptions of the quality assurance and quality control measures that were used for the NHANES 2007-8 Spirometry examination. The Chief Health Technician at each MEC monitored ongoing staff performance in the field. Health technician performance was also monitored using direct observation at periodic site visits by NIOSH consultants and project officers from Westat (the data collection contractor) and NHANES. Spirometry equipment was kept in calibration via regularly scheduled quality control checks by the Health Technicians and these were verified by supervisory staff.

All spirometry data collected were reviewed by expert consultants at the NIOSH quality control center where data processing and editing were performed to ensure internal data consistency.

Summary results datasets are provided for both 1st and 2nd test Spirometry. These give the single best (highest) overall value for the following parameters taken from reproducible curves. Note that in this dataset, an individual best value for a specific variable could be taken from any of the individual acceptable curves that the participant performed. The summary data set format is similar to that previously released for the NHANES III data. The spirometry summary data release variables are: 

In addition, each summary dataset contains the following administrative variables:

The Number of Acceptable Curves
FVC Quality Rating
FEV1 Quality Rating
Examinee Effort Quality Rating

The following administrative variables are provided to support analysis of the summary best-test spirometry datasets.

FEV1 and FVC Quality Codes: These variables rate the quality of the summary best-test FEV1 and FVC values. The variables are SPXNQFVC and SPXNQFV1 for 1st Test Spirometry and SPXBQFVC and SPXBQFV1 for 2nd Test Spirometry. Quality of the FEV1 and FVC estimates was rated on a scale from A-F defined as follows:

A= Exceeds ATS data collection standards: 3 acceptable curves present and 2 reproducible curves; 2 observed values within 100 ml.
B= Meets ATS data collection standards: 3 acceptable curves present and 2 reproducible curves; 2 observed values within 150 ml.
C= Potentially usable value, but does not meet all ATS standards. Estimates usually based on 2 curve results with values within 200 ml. of each other.
D= Questionable result, use with caution
F= Results not valid

For those who want to perform analytic studies using data that excludes subjects that may not have perfect data, they should simply use data only with FEV1 and FVC quality grades A and B. Grade C indicates less than a perfect test, for example instances where there are only 2 acceptable curves achieved but where the data most likely represents a value where the subject made a maximal effort. Generally data with quality grades D and F are not used for data analysis. In the NHANES 2007-8 data collection 87% of summary best-test FVC estimates were of quality A or B, and 94% were of quality A, B, or C. For FEV1, 91% of summary best-test FVC estimates were of quality A or B, and 96% were of quality A, B, or C. When assessing data results, analysts may also want to take into consideration whether an examinee had a history of a recent respiratory illness such as a cold, which could potentially cause transient changes in spirometric values. The questionnaire variable ENQ100 captures a participant’s history of a recent respiratory illness.

Additional Variables Relating to Data Quality:
The summary spirometry datasets also provide a variable for the number of acceptable spirometry curves achieved by the participant judged by ATS criteria. These are the variables SPDNACC for Baseline 1st Test Spirometry and SPDBACC for Bronchodilator 2nd Test Spirometry. Also, data providing a summary rating of the overall spirometry curve quality for the participant are provided by the variables SPXNQEFF and SPXBQEFF.

Spirometry Exam Status Codes: Baseline 1st Test Spirometry has an overall exam status code (SPXNSTAT) which provides analysts with a single variable to identify those survey participants with complete or partial spirometry exams, those who were excluded from spirometry, or those who were eligible for spirometry but for whom the exam was not done. A similar variable (SPXBSTAT) is provided for the subset of participants who were selected for Bronchodilator 2nd Test Spirometry. For these variables a complete exam was defined as one in which both the best-test FEV1 and FVC estimates had quality ratings of A, B, or C. Partial exams were exams with any other spirometry data collected that was of lesser quality.

Spirometry Exam Comment Codes: Comment codes were added by the MEC Health Technicians, during data collection, to document the reasons why it was only possible to collect partial exam data for participants, and to document the reasons why Spirometry exams were not done if a participant was otherwise eligible. The comment code variable for Baseline 1st Test Spirometry is SPXNCMT, and for Bronchodilator 2nd test Spirometry it is SPXBCMT. Analysts may wish to review the comment code information for the Spirometry data prior to specific types of data analysis.

Sample Weights: The NHANES examination sample weights, i.e. the full sample 2-Year MEC Exam Weight (WTMEC2YR) should be used to analyze the 2007-8 spirometry data. Please refer to the NHANES Analytic Guidelines (https://wwwn.cdc.gov/nchs/nhanes/analyticguidelines.aspx) and the on-line NHANES Tutorial (https://www.cdc.gov/nchs/tutorials/) for further details on the use of sample weights and other analytic issues.

Additional 2007-8 NHANES Data on Respiratory Health:
The raw data for the spirometry curves attempted by each participant in the NHANES 2007-8 Spirometry Component is released in a separate dataset (SPXRAW_E). The raw data permits users to graph a participant's spirometry test curves. It is also intended for the use of data analysts who may want to perform more advanced modeling studies with the raw data. The NHANES 2007-8 Respiratory Health Component also included testing for Exhaled Nitric Oxide (ENX_E), and Household Inteview Questionaire data on a prior diagnosis of asthma and COPD (MCQ_E), recent respiratory symptoms such as cough and wheezing (RDQ_E), data for the participant's current prescription medications (RXQ_RX_E; RXQ_DRUG), and data for occupational risk factors for COPD (OCQ_E).