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.
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 3.10 L when a 3.00-L volume is introduced. However, because variability exists both within and between spirometers, a 3-L subject could record 3.10 L on one spirometer and 2.90 L on a different spirometer, although both spirometers met minimum accuracy requirements.18,36 Some variation between spirometers may be the result of their different mechanisms for determining volume or their use of variable disposable sensors. Some flow-type spirometers measure slightly different volumes when air passes through the sensor at different speeds, whereas volume-type spirometers are less affected by the speed of air entering the spirometer. Some spirometer sensors may also be subject to changes in calibration over time. Table 4 gives one example of varying volumes recorded by a flow-type spirometer when a 3-L syringe was injected at different speeds during a calibration check, as described subsequently. Although all of the values are within the acceptable range of 2.90 to 3.10 L, this spirometer clearly records lower volumes when airflow is slower.
Minimize Changes in Spirometer Configuration
Most spirometers permit users to customize various aspects of data-saving and reporting during testing. Often, there is a choice of how many maneuvers' results should be saved and reported (users should choose “all data” and “all curves”), which spirometry measurements to save and report (users should choose FEV1, FVC, or forced expiratory volume in 6 seconds [FEV6], FEV1/FVC, or FEV1/FEV6, PEF, and forced expiratory time [FET], if available, unless other requirements apply), and which values are selected from the maneuvers attempted (users should choose maximum FEV1, maximum FVC or FEV6, maximum PEF, and not the “best curve” FEV1 and FVC values).26 It should be noted that many regulations do not permit measurement of the FEV6 in place of the FVC. Any changes in spirometer configuration over time or across locations should be documented. Changing the spirometer's configuration may change the data that are saved and reported, which will adversely affect longitudinal analysis of lung function.
Several steps help to ensure that spirometers function accurately.1 First, the ATS recommends minimum acceptable levels of accuracy and precision for spirometers.26 Second, an independent testing laboratory injects 24 standard waveforms into spirometers that are submitted by manufacturers for evaluation and analyzes the spirometer responses.26 If a spirometer passes the laboratory testing, a letter is issued stating that the spirometer completed testing following the 1994 or 2005 ATS Spirometry Update protocol for evaluating diagnostic spirometers. Users should request a copy of this letter, specifically citing the 1994 or 2005 ATS testing protocol, from their spirometer manufacturers.1
However, passing laboratory testing does not guarantee continued functioning, so the third step in ensuring that the spirometer works properly is to regularly check the calibration of the spirometer before it is used for testing.1 These checks are performed at least daily when the spirometer is in use and more frequently if many subjects are tested. Calibration checks performed at the end of the testing session confirm the status of the spirometer during the preceding tests. Calibration checks are decision-making prompts; if the spirometer fails a properly performed calibration check, the spirometer is out of calibration and should not be used for subject testing. Although the ATS recommends checking the calibration every 4 hours and whenever temperature changes occur,26 the frequency also depends on how many tests the health professional can afford to discard and repeat if a calibrated spirometer loses its calibration.
Spirometer calibration is either set or verified during the “calibration” routine; users should consult their manufacturer to determine which procedure is performed for their spirometer. If calibration is verified, the 3 L should be injected once at a moderate speed for a volume spirometer and three times at slow, medium, and fast speeds (eg, over 0.5 second, 3 seconds, and 6 seconds) for flow-type spirometers.1,26 If calibration is set, the 3-L volume should be injected at the speed specified by the manufacturer; after the calibration is set, flow-type spirometer accuracy should be verified at three speeds of injection using a manufacturer-recommended protocol. For flow-type spirometers with disposable sensors, it is prudent to perform a calibration check using the sensor that the subject will use,37 but if this is not feasible, sensors used for calibration should at least be drawn from the same batch as those used for subject testing. Technicians should avoid the incorrect practice of using one sensor for calibration checks over extended periods of time while changing the subjects' sensors.
The calibration syringe must be accurate; syringes can be calibrated annually and checked for leaks periodically by trying to empty the syringe with the outlet blocked.26 Store the calibration syringe near the spirometer in the testing environment, and perform calibrations and calibration checks in that environment. It is unacceptable to perform calibrations or checks in a warm environment to guarantee that the spirometer passes the calibration and then move the spirometer into a colder environment, eg, an unheated mobile testing van, for subject testing. If the testing environment can be maintained at 23°C (73°F) or above, testing errors resulting from cold temperatures will be minimized35; the ATS sets a minimum spirometer temperature at 17°C.11
Volume-type spirometers should also be checked for leaks daily and whenever breathing hoses are changed. The current ATS acceptable leak level is 0.03 l/min. If a chart recorder is used, the chart speed should be checked quarterly, along with linearity of the volume measurement.26
Finally, attention has recently been drawn to the fact that serious problems can develop during testing even after the spirometer passes its calibration checks.30,38 Particularly with flow-type spirometers, problems can develop as a result of faulty zeroing or contamination of the sensor, causing anomalous results and spirograms with unusual shapes. Therefore, even after calibration checks indicate that a spirometer is acceptably accurate, users should evaluate visual patterns in spirograms and be watchful for unlikely patterns of elevated results during testing.30,31,38 Such vigilance is particularly important when longitudinal analysis is planned because falsely elevated baselines will exaggerate the loss of function over time in many individuals, whereas falsely elevated follow- up test results will have the opposite effect.
Save Calibration Records Indefinitely
Calibration records support the accuracy of employee spirometry tests conducted on the calibration date and should be saved indefinitely.39 When contracting out to vendor(s), users should obtain and save records from all calibrations or calibration checks performed while testing is conducted at their facility. If problems with test results are discovered later, calibration records may provide the solution to the problems. Figure 2 presents 3 years of spirometry surveillance results, showing that five of six employees experienced significant declines in their FVC in April 2002. The mean FVC for the six employees declined by 1.1 L from the previous test 18 months earlier, but then returned to baseline levels on further testing 2 months later. Calibration records later revealed that the spirometer used in April 2002 was calibrated incorrectly, causing subject volumes to be grossly underrecorded and producing the apparent FVC declines. As with errors in testing technique (Fig. 1), such increased variability probably precludes meaningful evaluation of change over time.18
As airway caliber changes, spirometry measurements demonstrate diurnal (within a day) and seasonal (within a year) variability, so that time of day and year should be standardized when collecting serial measurements for long-term longitudinal analysis. Although diurnal variability, in particular, gives important information when short-term changes are evaluated, eg, as a result of asthma, these factors should be controlled when long-term change in function is the outcome of interest. Many medical surveillance programs conduct examinations on the employee's birthday so that seasonal variability is controlled.
Other factors may also affect test results and should be queried before conducting a spirometry test.32 NIOSH recommends that testing be postponed for 3 weeks if the subject has had a recent severe respiratory infection. The test should be postponed for 1 hour if the subject has had a large meal, smoked a cigarette, or used a bronchodilator within the last hour. The 1-hour postponement can sometimes be achieved by performing the spirometry test later in a physical examination. If it is not feasible to postpone a test, these factors should at least be documented on the report of test results.
What Is a ‘Significant Change' Over Time?
Because measurement variability strongly affects estimates of change in lung function over time, the expected rate of change is not as well defined as the cross-sectional “predicted” value. Definitions of “significant change” should minimize false-negatives and false-positives; deteriorating lung function should be detected early enough to permit the rate of loss to be slowed and the remaining function to be preserved, but at the same time, workers should not be labeled as having “significant loss” if they are not developing impairment. Definitions of “significant change” should be simple to apply even when practitioners do not have access to sophisticated statistical programs. The current ACOEM recommendations for evaluating change over time are summarized in Table 5.
Length of Follow Up and Frequency of Testing
Estimates of individual rate of change become more precise as follow-up time increases, and only large losses of function can be reliably detected over short time periods, eg, <2 years. To estimate longer term trends in an individual's FEV1 or FVC, spirometric measurements should be made over at least 4 to 6 years using standardized equipment and testing techniques.1,20–24,40 Precision is less affected by measurement frequency than by length of follow up,1,20,21,39 but periodic measurements are needed to detect workers experiencing rapid declines in pulmonary function and to detect systematic differences between examinations over time.1,21,23,40
ACOEM recommends that spirometry should be conducted every 1 to 2 years when indicated because of workplace exposures, unless otherwise specified by applicable regulations or recommendations.1 The frequency of testing may vary with age and length of exposure as in the 2000 National Fire Protection Association (NFPA) examination protocol, which recommended spirometry testing every 3 years for firefighters under age 30, every 2 years for ages 30 to 39, and annually for ages 40 and above.41
Evaluating and Defining ‘Significant Change'
Loss of FEV1 or FVC over time can be estimated simply by evaluating the difference between measurements at two points in time or by fitting a least-squares “slope” through an individual's periodic measurements.1 Although epidemiologic studies often use complex statistical methods, this statement focuses on two simple approaches to use when evaluating individual workers: method 1 (for BASELINE results >100% Pred) evaluates change in FEV1 % Pred or FVC % Pred over time; and method 2 (for BASELINE results ≤100% Pred) evaluates change in measured FEV1 or FVC over time. Method 1 is important because it provides a simple and more sensitive definition of abnormality for employees with above-average baseline lung function. However, if a medical program wishes to adopt only one method for all workers, ACOEM recommends choosing method 2, as recommended in the previous ACOEM Statement.1
Method 1 for BASELINES >100% Pred: Evaluate Change in % Pred
Method 1 provides a simple longitudinal normal limit (LNL) for FEV1% Pred and FVC% Pred for individuals whose baseline results exceed 100% Pred. The LNL should identify workers with accelerated lung function decline even though they remain in the traditional normal range. An employee is expected to remain above the LNL as he or she ages. Using the current estimate of 15% year-to-year measurement variability,11 the baseline % Pred is multiplied by 0.85 to obtain the LNL. Note that the same set of reference values (prediction equations) must be used for baseline and all follow-up tests.
Table 6 and Figure 3 present FVC results for a 66-inch tall white woman tested periodically from age 30 to 50 years. Her baseline FVC was 4.39 L, or 109% Pred, based on the National Health and Nutrition Examination Survey (NHANES) prediction equations.12 At age 50, her FVC was 84% Pred. When each test was simply compared with the traditional normal range, all of her measured FVCs were above the traditional lower limit of normal and she appeared to be “normal.”
However, evaluating her results relative to her own baseline value leads to a different conclusion. For her baseline FVC of 109% Pred, the LNL is [0.85 × 109%] = 93% Pred. As shown in Table 6, each of her tests remained above 93% Pred until age 50, when her result fell below the LNL. If this low value is confirmed by a retest, she should be medically evaluated, even though her results remain within the traditional normal range.17,42
This example illustrates the insensitivity of repeatedly comparing periodic test results with the traditional normal range, particularly for employees with above-average levels of pulmonary function. When baseline values exceed 100% Pred, lung function must decline dramatically before test results will fall below the traditional normal range. However, longitudinal evaluation using a LNL will be more sensitive to possible accelerated lung function decline.
Method 2 for BASELINES ≤100% Pred: Evaluate Change in Measured Values
ACOEM recommends method 2 to calculate a longitudinal normal limit (LNL) particularly for employees with baseline results ≤100% Pred (Townsend MC, personal communication, 2003). However, some medical programs may want to adopt only one LNL method for all workers. In that case, method 2 can also be applied to workers with baselines >100% Pred, because both methods give the same results for this group. If only one method will be used for all workers, ACOEM recommends choosing method 2 to compute the LNL.1
A “significant” decline should exceed both: 1) year-to-year measurement variability, currently estimated at 15%11; and 2) the expected age-related decline, which can be calculated as the difference between the baseline and follow-up predicted values.1,39,42 These factors are used subsequently to determine the LNL for follow-up test results. An employee is expected to remain above the LNL as he or she ages. Note that the same set of reference values (prediction equations) must be used for the baseline and all follow-up tests.
Table 7 and Figure 4 present FEV1 results for a 65-inch tall white woman tested annually from age 67 to 73 years. Her baseline FEV1 was 2.42 L, or 97% Pred, based on the NHANES prediction equations.12 Although a LNL was computed for each test date, only the age 69 results are evaluated here. As illustrated in Table 7, the LNL at age 69 is 2.00 L, ie, the FEV1 could drop to 2.00 L at age 69 as a result of measurement variability and aging alone. Because the age 69 test result is below 2.00 L, the FEV1 decline may be “significant” and the subject should be medically evaluated if the low result is confirmed by a retest.17,41 Figure 4 shows that the FEV1 remained below the LNL for all subsequent tests, although it did not fall below the traditional normal range until several years later, at age 73. This subject's deteriorating lung function was identified by longitudinal evaluation 4 years earlier than it would have been detected by comparisons with the traditional normal range.
Initial Identification versus Progression.
Once a worker is identified as having impaired lung function, ATS recommends a less conservative definition for evaluating progression of disease, because both the measured volumes and the percents of predicted are smaller than for the healthy individuals discussed previously. In the statement on “Idiopathic Pulmonary Fibrosis: Diagnosis and Treatment,” ATS and the European Respiratory Society (ERS) recommend interpreting a loss of 10% or more of the measured baseline VC (or at least 0.20 L) as a “failure to respond to therapy,” ie, a significant decline, if the change is accompanied by parallel changes in single-breath diffusing capacity or oxygen saturation 6 months after the baseline test.43
In addition, an increase from the measured baseline VC of 10% or more (or at least 0.20 L) is interpreted as a significant improvement if the change is accompanied by parallel changes in single-breath diffusing capacity or oxygen saturation and is maintained for two consecutive visits within a 3- to 6-month period.
Changes smaller than ±10% of measured baseline VC (or <0.20 L) maintained for two consecutive visits within a 3- to 6-month period indicate stable pulmonary function.43
Fitting a Least-Squares ‘Slope' Through Periodic Measurements.
Calculating a best-fit line of lung function measurements on test date requires more computational capability than calculating differences but can be programmed or computed on a calculator. Based on reviews of the longitudinal spirometry literature, the previous ACOEM spirometry statement recommended that an FEV1 or FVC decrease of 90 to 100 mL/year, calculated over at least 4 to 6 years, should trigger further medical evaluation of pulmonary function.1,21,24,44 Although this area remains one of current investigation, neither longitudinal predicted values nor fifth percentile LNLs have yet been recommended for the evaluation of individual rates of change over time in occupational or clinical settings.1,21,45
As summarized in Table 1, longitudinal evaluation of pulmonary function should be considered, particularly in the occupational setting, because many workers have above-average levels of pulmonary function (ie, >100% Pred). Such high levels of lung function can deteriorate substantially, falling from the top to the bottom of the normal range without dropping below the normal range. This loss of function may not be detected by the common practice of simply determining whether each year's results fall within the traditional normal range.
To address this problem, ACOEM recommends simple methods for comparing an employee's periodic spirometry results with a longitudinal normal limit (LNL) specific for that employee. Starting with an individual's baseline lung function level, the LNL describes the lowest results that might be expected for his or her lung function during follow up as a result of normal aging and measurement variability. Test results falling below the LNL may indicate significant deterioration of pulmonary function. However, to make such evaluations possible, spirometry data must be collected carefully, following standardized protocols. The rate of false-positives will be high if test variability is not minimized through quality assurance protocols, standardized testing procedures, and the continuity of well-maintained equipment.
Based on current recommendations, ACOEM recommends two methods to compute a worker's LNL. Method 1 (for employees with baseline results >100% Pred) computes a simple LNL for the follow-up FEV1 % Pred or FVC % Pred using [baseline % Pred × 0.85]. Each serial test can be compared to the LNL to determine whether the worker's pulmonary function has deteriorated significantly relative to his or her own baseline result. This approach is shown in detail in Table 6 and Figure 3.
Method 2 (for employees with baseline results ≤100% Pred) computes a LNL for the measured FEV1 or FVC using [0.85 × baseline measured value − (baseline predicted − follow-up predicted)]. Each serial test can be compared with the LNL to determine whether the worker's pulmonary function has deteriorated significantly relative to his or her measured baseline value. This approach is shown in detail in Table 7 and Figure 4. (In addition, if a medical program wishes to adopt only one method for all workers, ACOEM recommends choosing method 2 to calculate the LNL. Both methods give the same results if a worker's baseline exceeds 100% Pred, but only method 2 should be used for those with lower baseline values.)
If a test result falls below the longitudinal LNL calculated using either method, it should be confirmed by a retest. Once confirmed, medical evaluation is recommended, even if the test results remain in the traditional normal range.
Finally, if multiple measurements are available over 4 to 6 or more years, a slope of lung function measurements over time can be calculated. ACOEM recommends that slopes that are steeper than 90 to 100 mL/year should be flagged as significant losses of function, even if the worker's test results remain in the normal range.
This ACOEM guideline was developed by Mary C. Townsend, DrPH, and members of the ACOEM Occupational and Environmental Lung Disorder Committee under the auspices of the Council on Scientific Affairs. The guideline was peer-reviewed by the Committee and Council and by John L. Hankinson, PhD, and William L. Eschenbacher, MD. It was approved by the ACOEM Board of Directors on July 26, 2003.
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