Evaluating Pulmonary Function Change Over Time in the Occupational Setting

Periodic spirometry testing is performed in medical screening and surveillance programs for workers with various occupational exposures and for cigarette smokers. The recent American College of Occupational and Environmental Medicine (ACOEM) evidence-based statement, Spirometry in the Occupational Setting, comprehensively reviewed the issues involved in conducting and interpreting standardized spirometry tests in occupational medicine.1 However, interpreting change over time was only briefly discussed in that statement, and little other guidance on assessing longitudinal change in lung function is available for health professionals. As a result, many practitioners do not evaluate change in lung function over time, but instead repeatedly determine whether each year's test results fall within the normal range. Other practitioners evaluate change over time or “trending” but are unaware of the pitfalls that can distort their evaluations. Although health professionals must determine whether evaluating lung function change over time effectively screens for a specific outcome disease,2–4 the ACOEM Occupational and Environmental Lung Disorder Committee recognized the need to provide guidance in the selection and use of simple measures of change over time. The Committee developed this separate ACOEM position statement to: 1) explain the need for longitudinal analysis of pulmonary function when evaluating employee respiratory health; 2) describe the pitfalls to be avoided when collecting serial measurements for longitudinal analysis; and 3) recommend simple criteria to use for flagging abnormal change in pulmonary function over time. The statement's key points are summarized in Table 1. Real-life examples illustrate the pitfalls to be avoided and the application of longitudinal methods for evaluating pulmonary function.TABLE 1: Evaluating Change Over TimeWhy Examine Change Over Time? Spirometry in Medical Screening and Surveillance Programs Spirometry is performed periodically in screening and surveillance programs for a variety of occupational exposures. Occupational Safety and Health Administration (OSHA) regulations require periodic spirometry testing for certain workers exposed to asbestos,5 coke oven emissions,6 cadmium,7 cotton dust,8 benzene,9 and formaldehyde.10 Many companies mandate medical surveillance with periodic spirometry testing for additional exposures as well as incorporating spirometry into their respirator medical clearance screening programs. In each case, health professionals must evaluate current and previous test results to determine whether an employee is at increased risk of impairment from further occupational exposure or if any limitations should be placed on the employee's activities or use of personal protective equipment. However, the details of the evaluation of current and previous results are usually not specified. The OSHA Cotton Dust Standard is only slightly more explicit than the other OSHA regulations listed here, stating that “a determination [shall be] made by the physician as to whether there has been a significant change [between the current examination results and those of previous examinations].”8 Comparing Observed With Cross-sectional Predicted Values Traditionally, an individual's measured lung function has been compared with a “predicted” value, ie, the average expected for an asymptomatic nonsmoker of the subject's age, height, race/ethnicity, and sex. Many sources of predicted values have been derived from studies of asymptomatic nonsmoking populations, and some applications require the use of specific sets of prediction equations. Selection of reference values has been reviewed elsewhere.1,11–15 The comparison of observed with predicted values is usually summarized in a numeric index, the percent of predicted (% Pred). Lower limits of normal should be determined for the prediction equations in use, and the individual's measured results are then interpreted relative to the normal range as normal or abnormal and possibly impaired. Because lower limits of normal generally decrease with age, the use of 80% Pred as a lower limit of normal for all age groups is no longer recommended.11 Definitions of the lower limit of normal, choice of measurements for evaluation, and definitions of airways obstruction have evolved over time,1,11 but the approach of comparing an individual with the average “predicted” from an asymptomatic population has been widely used for decades. Need for Longitudinal Lung Function Evaluation in Occupational Settings In the clinical setting, patients with lung disease are often tested to determine the severity of their disease.3,16 In contrast, in the occupational setting, many healthy workers are tested periodically, not because they have abnormal lung function, but to monitor their response to potentially harmful occupational exposures. Because of their health, working populations usually have higher levels of pulmonary function than clinic populations, and many workers have lung function that is above average, ie, >100% Pred. Such individuals may lose their lung function at an excessive rate but still remain in the normal range throughout their working lifetime and into retirement. Remaining in the normal range does not indicate respiratory health, because their function may drop from the top to the bottom of the normal range, but these individuals must lose large fractions of their lung function before they will fall below the normal range. For these workers, the widespread practice of repeatedly comparing serial test results with the traditional normal range may not detect serious pulmonary function deterioration. Longitudinal evaluation that compares current measured values with previously measured values, “using the subject as his/her own control,” is needed especially for this group, as summarized in Table 2.11,17,18TABLE 2: Why Examine Change Over TimeEvaluating Longitudinal Change to Screen for Specific Diseases Although this statement provides a method for evaluating change over time, health professionals must decide whether screening for excessive loss of function is appropriate for specific outcome diseases of interest. Monitoring pulmonary function longitudinally may be more justified for some exposures, eg, smoking-related chronic obstructive pulmonary disease (COPD), than for others.2–4 The sensitivity, specificity, and positive and negative predictive values of excessive loss of pulmonary function relative to the outcome disease of interest should be investigated. Screening for excessive loss of function is recommended if the prevalence and severity of the outcome disease are significant and if the effectiveness of the intervention or treatment balances the financial and nonfinancial costs of the intervention.19 Pitfalls in Collecting Serial Measurements Although “using the subject as own his/her own control” may detect pulmonary function declines that are missed by comparisons with predicted values, practitioners who analyze longitudinal spirometry data are often unaware of the pitfalls that can invalidate their conclusions. Because both technical and biologic factors affect spirometry results at each test session, practitioners should attempt to hold these factors constant if longitudinal analysis is anticipated.20–25 Failure to control these factors produces extraneous variability, which may be interpreted as an excessive loss or gain of lung function. Therefore, users of spirometry data should appreciate the effects of technical and biologic factors on measurements and be prepared to evaluate test quality and reject inadequate tests before evaluating change over time. Sources of technical and biologic variability are summarized in Table 3.TABLE 3: Pitfalls in Collecting Serial MeasurementsTechnical Variability Standardization and Documentation of Testing Technique and Equipment. Spirometry testing procedures, type of spirometer, and spirometer maintenance and quality assurance checks should be standardized across location and time, particularly if longitudinal analysis of lung function measurements is anticipated. American Thoracic Society (ATS) recommendations were summarized in the recent ACOEM spirometry statement.1 That statement will be revised in 2006 to reflect the newly published ATS Guidelines.26 Testing procedures and equipment used should be fully documented, and the documentation should be updated whenever changes occur. Standardization and documentation are particularly important if testing is contracted out to multiple vendors over time. In fact, frequent changes in vendor and/or poor vendor quality control of testing may preclude any meaningful longitudinal evaluation of results. Equipment malfunction and errors in testing technique can cause measurements to be falsely elevated or reduced.1,26–32 Some technical errors cause increased variability that is random, although many problems cause results to be biased. When a series of erroneous measurements is examined, healthy workers may appear to “decline,” whereas others' deteriorating function may be masked by the noise in the measurements. Figures 1 and 2, discussed subsequently, illustrate the difficulty in evaluating change over time using technically flawed measurements. A summary of technical errors that raise and lower test results is available on the Internet.31Fig. 1.: Poor coaching reduced the forced vital capacity and forced expiratory volume in 1 second.Fig. 2.: Incorrect calibration reduced the forced vital capacity.Testing Technique. Health professionals should develop a written testing protocol and ensure that technicians understand and follow the specified procedures. The many details involved in conducting tests and maintaining equipment may be easily misunderstood, resulting in nonstandardized testing procedures. The details of the testing procedures should be spelled out in the written protocol, eg, the definition of end-of-test (recording to a forced vital capacity [FVC]) plateau vs recording for a specified number of seconds), testing posture (standing vs sitting), minimum number of acceptable maneuvers to be recorded, criteria for rejecting a maneuver, ie, what makes a maneuver “unacceptable,” and whether to print out curves during a test for coaching if there is no real-time graphic display. Changes in the testing procedures over time should be documented. Figure 1 illustrates the effect of poor coaching, which elicited only submaximal inspirations from an employee who had recovered from pleurisy but appeared not to have returned to his baseline level of pulmonary function. When an experienced technician urged the employee to inhale maximally, his FVC and forced expiratory volume in 1 second (FEV1) results increased by 0.8 and 0.5 L, respectively, returning to their baseline levels. The variability introduced by inconsistent testing technique such as that shown in Figure 1 probably precludes meaningful evaluation of change over time.18 Spirometry training courses such as those approved by the National Institute for Occupational Safety and Health (NIOSH) are recommended, and NIOSH has developed a course-approval web page and is reorganizing its program to ensure better standardization among courses.32 A single vendor should provide training for all technicians at a location, if feasible, and training should be followed by supervised on-the-job testing experience26 and quality assurance review of spirograms for technical quality.26,33,34 Periodic refresher courses are recommended,1 and quality assurance reviews of spirograms should be continued indefinitely, perhaps conducted at least on a quarterly basis. Equipment. When longitudinal evaluation is anticipated, equipment variability should be minimized across locations and time. Variability may be increased if different spirometers are used, if calibrations or calibration checks are not performed correctly and consistently, or if spirometer temperatures vary widely.35 Recommendations to minimize equipment variability are presented subsequently. Minimize Unnecessary Equipment Changes Unnecessary equipment changes should be avoided if longitudinal analysis of results is anticipated, although excessively variable spirometers should be replaced by instruments with greater precision. The ATS recommends that spirometers should be accurate to within ±3.5% of the volume introduced into a spirometer, so a spirometer meets minimum criteria for accuracy if it records between 2.90 and when a volume is However, because variability both within and between a subject on spirometer and 2.90 on a different spirometer, although both spirometers minimum accuracy Some between spirometers may be the of their different for volume or their use of variable Some spirometers slightly different when the at different whereas spirometers are by the of the Some spirometer may be subject to changes in calibration over time. Table of by a spirometer when a was at different during a calibration as subsequently. Although all of the values are within the acceptable range of 2.90 to L, this spirometer records lower when is into a at Changes in spirometers users to various of and during there is a choice of many results should be and should and which spirometry measurements to and should or forced expiratory volume in or and forced expiratory time if other and which values are from the maneuvers should FVC or and not the and FVC should be that many regulations do not of the in of the changes in spirometer over time or across locations should be documented. the may change the data that are and which will affect longitudinal analysis of lung function. to ensure that spirometers function the ATS recommends minimum acceptable levels of accuracy and for an testing into spirometers that are by for evaluation and the spirometer a spirometer the a is stating that the spirometer testing the or ATS Spirometry protocol for evaluating should a of this the or ATS testing protocol, from their spirometer However, testing does not continued so the in that the spirometer is to the calibration of the spirometer before it is used for checks are performed at least when the spirometer is in use and more if many are checks performed at the of the testing the of the spirometer during the checks are if the spirometer a performed calibration the spirometer is out of calibration and should not be used for subject Although the ATS recommends the calibration and whenever changes the on many tests the health can to and if a spirometer its calibration is or during the users should their to determine which is performed for their calibration is the should be at a for a volume spirometer and at and over 0.5 and for calibration is the volume should be at the specified by the the calibration is spirometer accuracy should be at of using a For spirometers with it is to a calibration using the that the subject will but if this is not feasible, used for calibration should at least be from the as those used for subject should the practice of using for calibration checks over of time the The calibration must be can be and for periodically by to the with the the calibration the spirometer in the testing and calibrations and calibration checks in that is to calibrations or checks in a to that the spirometer the calibration and then the spirometer into a eg, an testing for subject the testing can be at or testing errors resulting from temperatures will be the ATS sets a minimum spirometer at spirometers should be for and whenever are The current ATS acceptable level is a is used, the should be with of the volume has been to the that serious problems can develop during testing the spirometer its calibration with problems can develop as a of or of the results and spirograms with Therefore, calibration checks indicate that a spirometer is users should evaluate in spirograms and be for of elevated results during Such is particularly important when longitudinal analysis is because falsely elevated will the loss of function over time in many whereas falsely elevated test results will have the records the accuracy of employee spirometry tests conducted on the calibration and should be When out to users should and records from all calibrations or calibration checks performed testing is conducted at their problems with test results are calibration records may provide the to the Figure of spirometry surveillance that of experienced significant declines in their FVC in The FVC for the by from the previous test but then returned to baseline levels on further testing records that the spirometer used in was subject to be and the FVC As with errors in testing technique such increased variability probably precludes meaningful evaluation of change over time.18 Variability As spirometry measurements a and a variability, so that time of and should be standardized when collecting serial measurements for longitudinal Although variability, in important when changes are eg, as a of these factors should be when change in function is the outcome of interest. Many medical surveillance programs on the employee's so that variability is Other factors may affect test results and should be before conducting a spirometry NIOSH recommends that testing be for if the subject has had a recent respiratory The test should be for 1 if the subject has had a large a or used a within the The can be by the spirometry test in a it is not to a these factors should at least be on the of test results. a Over Time? Because variability of change in lung function over time, the expected rate of change is not as well as the “predicted” Definitions of should minimize and deteriorating lung function should be to the rate of loss to be and the function to be but at the time, workers should not be as if they are not Definitions of should be simple to when practitioners do not have to programs. The current ACOEM recommendations for evaluating change over time are summarized in Table Change Over of and of Testing of individual rate of change more as time and only large of function can be over time eg, longer in an individual's or measurements should be made over at least to using standardized equipment and testing is by than by of follow but periodic measurements are needed to detect workers declines in pulmonary function and to detect between over ACOEM recommends that spirometry should be conducted 1 to when because of exposures, specified by regulations or The of testing may vary with age and of exposure as in the National examination protocol, which recommended spirometry testing for age for to and for and Evaluating and of or FVC over time can be by evaluating the between measurements at points in time or by a an individual's periodic Although studies often use this statement on simple to use when evaluating individual method 1 results >100% change in Pred or FVC Pred over and method results change in measured or FVC over time. 1 is important because it provides a simple and more definition of for with baseline lung function. However, if a medical program to only method for all workers, ACOEM recommends method 2, as recommended in the previous ACOEM 1 for >100% Change in Pred 1 provides a simple longitudinal normal limit for Pred and Pred for individuals baseline results Pred. The should workers with lung function they remain in the traditional normal range. employee is expected to remain above the as or the current of the baseline Pred is by to the that the of reference values must be used for baseline and all Table and Figure FVC results for a tested periodically from age to baseline FVC was L, or on the National Health and prediction age FVC was Pred. When each test was compared with the traditional normal range, all of measured were above the traditional lower limit of normal and appeared to be a in of vital capacity but in the traditional normal evaluating results relative to own baseline to a different For baseline FVC of the is Pred. As shown in Table each of tests above Pred age when below the this is by a should be results remain within the traditional normal illustrates the of repeatedly comparing periodic test results with the traditional normal range, particularly for with levels of pulmonary function. When baseline values lung function must before test results will fall below the traditional normal range. However, longitudinal evaluation using a will be more to lung function for Change in Values ACOEM recommends method to a longitudinal normal limit particularly for with baseline results Pred personal However, some medical programs may to only method for all In that case, method can be to workers with >100% because both methods the results for this only method will be used for all workers, ACOEM recommends method to the A should 1) variability, at and 2) the expected which can be as the between the baseline and predicted factors are used to determine the for test results. employee is expected to remain above the as or that the of reference values must be used for the baseline and all Table and Figure results for a tested from age to baseline was L, or on the prediction Although a was for each test only the age results are As in Table the at age is L, ie, the drop to at age as a of variability and Because the age test is below L, the may be and the subject should be if the is by a that the below the for all although it not fall below the traditional normal range at age subject's deteriorating lung function was by longitudinal evaluation than it have been by comparisons with the traditional normal expiratory volume in 1 second by age and in the traditional normal range age a is as lung function, ATS recommends a definition for evaluating of because both the measured and the of predicted are than for the healthy individuals discussed In the statement on and ATS and the Society recommend interpreting a loss of or more of the measured baseline at least as a to to ie, a significant if the change is by changes in capacity or the baseline In an from the measured baseline of or more at least is interpreted as a significant if the change is by changes in capacity or and is for within a to Changes than of measured baseline for within a to indicate pulmonary a Periodic a of lung function measurements on test more than but can be or on a on reviews of the longitudinal spirometry the previous ACOEM spirometry statement recommended that an or FVC decrease of to over at least to should further medical evaluation of pulmonary Although this of current longitudinal predicted values have been recommended for the evaluation of individual of change over time in occupational or clinical As summarized in Table longitudinal evaluation of pulmonary function should be particularly in the occupational setting, because many workers have levels of pulmonary function >100% Pred). Such levels of lung function can from the top to the bottom of the normal range below the normal range. loss of function may not be by the practice of whether each year's results fall within the traditional normal range. this ACOEM recommends simple methods for comparing an employee's periodic spirometry results with a longitudinal normal limit specific for that with an individual's baseline lung function the the results that be expected for his or lung function during follow as a of normal and results below the may indicate significant of pulmonary function. However, to such spirometry data must be standardized The rate of will be if test variability is not minimized quality assurance standardized testing procedures, and the of equipment. on current ACOEM recommends methods to a 1 with baseline results >100% a simple for the Pred or FVC Pred using Pred serial test can be compared to the to determine whether the pulmonary function has relative to his or own baseline approach is shown in in Table and Figure with baseline results a for the measured or FVC using baseline measured predicted serial test can be compared with the to determine whether the pulmonary function has relative to his or measured baseline approach is shown in in Table and Figure if a medical program to only method for all workers, ACOEM recommends method to the methods the results if a baseline but only method should be used for those with lower baseline a test below the longitudinal using it should be by a medical evaluation is recommended, if the test results remain in the traditional normal range. if multiple measurements are available over to or more a of lung function measurements over time can be ACOEM recommends that that are than to should be as significant of function, if the test results remain in the normal range. ACOEM was developed by and of the ACOEM Occupational and Environmental Lung Disorder Committee the of the on The was by the Committee and and by and was approved by the ACOEM of on