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Original Research: ASTHMA |

Comparison of Peak Expiratory Flow Variability Between Workers With Work-Exacerbated Asthma and Occupational Asthma* FREE TO VIEW

Samah Chiry, MD; André Cartier, MD; Jean-Luc Malo, MD; Susan M. Tarlo, MD, FCCP; Catherine Lemière, MD, MSc
Author and Funding Information

*From Hôpital du Sacré-Cœur de Montréal (Drs. Chiry, Cartier, Malo, and Lemière), Montréal, QC; and Gage Occupational and Environmental Health Unit (Dr. Tarlo), Toronto Western Hospital, Toronto, ON, Canada.

Correspondence to: Catherine Lemière, MD, MSc, Department of Chest Medicine, Sacré-Coeur Hospital, 5400 Gouin West, Montreal, QC, Canada H4J 1C5; e-mail: catherine.lemiere@umontreal.ca



Chest. 2007;132(2):483-488. doi:10.1378/chest.07-0460
Text Size: A A A
Published online

Background: Peak expiratory flow (PEF) monitoring is frequently used to diagnose occupational asthma (OA). The variability of PEF between periods at work and away from work has not been described in workers with work-exacerbated asthma (WEA). We sought to assess and compare the diurnal variability of PEF during periods at and away from work between subjects with OA and WEA.

Methods: Workers referred for work-related asthma underwent PEF monitoring for 2 weeks at and away from work. The diagnostic of OA or WEA was subsequently made according to the respective positivity or negativity of the specific inhalation challenges. PEF mean diurnal variability was calculated during periods at and away from work. PEF graphs were also interpreted using direct visual analysis by five observers and using a computer program (Oasys-2, Expert System ) [available at: http://www.occupationalasthma.com].

Results: Thirty-four subjects were investigated (WEA, n = 15; OA, n = 19). There was a greater variability of PEF at work than away from work in both OA (19.8 ± 8.7% vs 10.7 ± 6.3%, p < 0.001) and WEA (14.2 ± 4.8% vs 10.6 ± 5.6%, p = 0.02). However, the magnitude of the variability was higher in OA than in WEA (p = 0.02). The visual interpretation of PEF or the Oasys-2 program failed to distinguish WEA from OA.

Conclusion: Although workers with OA showed a higher PEF variability than workers with WEA when at work, clinicians were unable to reliably differentiate OA from WEA using the visual interpretation of PEF graphs or the computerized analysis.

The workplace can trigger or induce asthma leading to “work-related asthma.”1 Work-related asthma can be classified into occupational asthma (OA) and work-exacerbated asthma (WEA). OA is defined as “a disease characterized by variable airflow limitation and/or hyperresponsiveness and/or inflammation due to causes and conditions attributable to a particular occupational environment and not to stimuli encountered outside the workplace.”1 In contrast, WEA can be defined as preexisting or coincident asthma worsened by workplace exposures,1implying that the workplace triggers the asthma but does not induce it. OA and WEA are often difficult to distinguish in clinical practice because in both conditions the workers complain of a deterioration of their asthma when working. In some settings, OA and WEA have been defined according to the respective positivity or negativity of specific inhalation challenge (SIC).23 However, although SIC with the suspected agent is still acknowledged as the reference method for diagnosing OA,4 this test is only available in a few specialized centers worldwide. Peak expiratory flow (PEF) monitoring during periods at and away from work is an alternative method commonly used to investigate OA.

Diagnosing WEA entails the demonstration of a worsening of asthma when at work. However, the criteria necessary to diagnose WEA are far less established than those used for diagnosing OA. Since workers with WEA complain of increased asthma symptoms when at work, these workers may show an increased variability in PEF when working, compared to periods away from work as observed in uncontrolled asthma. However and to the best of our knowledge, the magnitude of PEF variability between periods at and away from work in subjects with WEA has not been described or compared to the changes of PEF found in workers with OA. Therefore, the aim of the present study was to assess and compare PEF variability during periods at and away from work in subjects with OA and WEA.

Subjects

Workers who were referred for possible work-related asthma at the occupational clinic of Sacre-Cœur Hospital and had asthma as defined by the American Thoracic Society criteria5 were included in the present study. Subjects with reactive airway dysfunction syndrome or irritant-induced asthma were not included in this study. All subjects were ≥ 18 years of age and were complaining of a worsening of their symptoms of asthma when at work.

All procedures used in this study were in accordance with the recommendations found in the Helsinki Declaration of 1975. The study was approved by the research ethics committee of Sacre-Coeur Hospital. Appropriate written informed consent was obtained from all subjects.

Study Design

The study was of prospective crossover design with periods of 2 weeks at work and 2 weeks away from work. Data originating from the same study but addressing a different research question were previously published elsewhere.6 The subjects were seen within 48 h after the end of each period or whenever there was an exacerbation of symptoms. On the first visit, demographic and clinical characteristics of the subjects were recorded. Atopy status was assessed by performing skin-prick tests with common inhalants. After each period at and away from work, spirometry and methacholine challenge were performed. PEF was monitored serially during a 2-week period at work and a 2-week period away from work. All workers were asked to maintain their antiasthma controller medication unchanged throughout the study, but long-acting β2-agonists were stopped 72 h before the methacholine challenge and SIC testing. Short-acting inhaled β2-agonists were used on an as-needed basis. Subjects exposed to seasonal common inhalants to which they were sensitized during the study period and within the 4 weeks prior to the first visit were not enrolled.

SICs were performed to confirm the diagnosis of OA or WEA. Subjects with a worsening of their asthma symptoms when at work who showed a positive SIC result were defined as OA, whereas subjects with worsening of their asthma symptoms at work with a negative SIC were defined as WEA.

Procedures

Spirometry was performed according to American Thoracic Society standards.7The methacholine inhalation challenge test was performed using a Wright nebulizer (Roxon Medi-Tech; Montreal, QC, Canada) as previously described.8On all occasions, the subjects were asked to score their respiratory symptoms on a Borg scale from 0 (no symptoms) to 10 (worst symptoms ever).9Subjects underwent skin-prick tests with a battery of 12 common inhaled allergens.10 Atopy was defined by the presence of at least one positive skin-prick test result with a wheal diameter ≥ 3 mm.

SICs were performed as previously described11in the laboratory or at the workplace. PEF was monitored serially using a Mini-Wright peak flowmeter (Clement Clarke International; Harlow, UK) during a 2-week period at work and a 2-work period away from work.12

Subjects were asked to record their PEF rates in a diary every 2 h. On each occasion, they were asked to blow three times into the peak flowmeter and to record all readings. The best of the three attempts was kept for analysis. Days at work and days away from work were identified.

PEF graphs were drawn and interpreted independently using direct visual analysis by five experts in a double-blind fashion as previously described.13The observers did not receive any guidance to interpret PEF graphs. They were asked to interpret the graphs as they would do it in their clinical practice. We also employed a computerized approach to analyze PEF monitoring (Oasys-2, Expert System) [available at: http://www.occupationalasthma.com].14

Data Analysis

Results were expressed as mean and SDs, except for provocative concentration of methacholine inducing a 20% fall in FEV1 (PC20), which was expressed as a geometric mean with minimum and maximum values. Quantitative and qualitative approaches were used to analyze the PEF data of the subjects with OA and WEA.

Quantitative Approach

PEF variability was calculated for each day during the two periods at and away from work. PEF variability was expressed as the diurnal PEF variation and calculated for the two periods at and away from work using the following indexes: index 1, amplitude percentage highest (highest − lowest/highest × 100); index 2, amplitude percentage mean (highest − lowest/mean × 100).14 These indexes were selected from previous studies13,15 because they showed the best diagnostic performances among other calculated indexes.

The means of PEF variability were calculated for days spent at work and away from work. The 24-h periods were defined by calendar days. A paired t test was used to compare the differences of PEF variability between periods at work and away from work. A Student t test was used to compare the PEF variability between subjects with OA and WEA.

We also interpreted PEF records by using criteria previously described.16 We assessed the frequency of days where there was a ≥ 20% diurnal variation in PEF and determined the number of work days when this occurred, divided by the number of work days when the recordings were made. We performed the same estimation for non-work days. We considered that there was a clinically significant increase in the PEF variability between periods at and away from work if these periods differed by at least 10% in the relative frequency of days with 20% variability in PEF.

PEF records were also interpreted by using the Oasys-2 score. A total score of ≥ 2.51 by applying Oasys-2 was considered suggestive of OA.14 A Student t test was used to compare the mean of Oasys-2 scores between subjects with OA and WEA.

Qualitative Approach

Graphs of PEF after periods at and away from work were analyzed visually by five experts unaware of the results of the SIC tests. The experts did not have access either to the results of methacholine challenge or sputum induction that were performed during the investigation. They estimated the probability of OA for each subject by indicating yes, no, or doubtful. The results of visual analyses of PEF graphs were studied using the κ coefficient of agreement between the experts. Significance was accepted at the level of 95%. The analysis was performed using statistical software (SPSS, version 12.0; SPSS; Chicago, IL).

Thirty-four subjects (mean age, 41.3 ± 11.6 years [± SD]) were investigated. Nineteen subjects had a positive SIC result and were considered to have OA, whereas 15 subjects tested negative and were considered to have WEA.

Baseline Characteristics

We did not find any statistically significant differences between the subjects with OA and WEA in terms of age (p = 0.6); sex (p = 0.2); atopic status (p = 0.1); smoking habits (p = 0.4); type of agents to which they were exposed (high-molecular-weight agents vs low-molecular-weight agents) [p = 0.1]; total duration of exposure to the occupational agents (p = 0.2); and utilization of inhaled steroids (p = 0.1). Baseline characteristics of the subjects with OA and WEA are reported in Table 1 .

Comparison of the Clinical and Functional Changes Between Periods at Work and Away from Work Between Subjects With OA and WEA

There was a similar and statistically significant decrease in the symptom score and in the use of short-acting β2 agonists during periods away from work compared with periods at work in both groups (Table 2 ). There was a similar slight, although not statistically significant, improvement in FEV1 and PC20 during periods away from work compared with periods at work.

Quantitative Interpretation of PEF

Both groups showed a greater diurnal variability of PEF during periods at work compared with periods away from work according to amplitude percentage highest (index 1) and amplitude percentage mean (index 2) [Table 3 ]. However, the magnitude of the variability between periods at work and away from work was higher in subjects with OA than in subjects with WEA: 9.1 ± 7.4 vs 3.6 ± 5.1, p = 0.02 9 (index 1); and 11.4 ± 10.2 vs 4.3 ± 6.0, p = 0.02 (index 2). PEF variability was similar in OA and WEA during periods away from work according to both indexes (p = 0.9).

A diurnal variation of PEF ≥ 20% was more common when at work than away from work in 84.2% of the subjects with OA and only 53.3% of subjects with WEA (p = 0.07). Subjects with OA also had a diurnal variation in PEF ≥ 20% on a greater number of days (6.9 ± 4.0) than subjects with WEA (3.5 ± 2.9 days, p = 0.009) during periods at work.

There was no statistically significant difference between the mean score calculated by Oasys-2 for subjects with OA (2.42 ± 1.0) and WEA (2.06 ± 1.4) [p = 0.4]. Among the 19 records of subjects with OA, 6 records obtained an Oasys score ≥ 2.51, whereas 6 of the 14 records of subjects with WEA showed an Oasys score ≥ 2.51.

Qualitative Analysis of PEF Rate

There was a low agreement in the interpretation of PEF between the different experts (Cohen κ varied from 0.27 to 0.70). The results of the visual analyses of PEF records of the five experts are reported in Table 4 .

This study showed that work exposures induced a significant PEF variability in workers with both OA and WEA. However, the magnitude of this variability was higher in subjects with OA than in subjects with WEA during periods at work. The visual interpretation of PEF graphs or the computerized approach using the Oasys-2 software did not allow an accurate differentiation between OA and WEA.

A median and mean spontaneous diurnal PEF variability of 5 to 8.6% has been described15,1719 in adult healthy subjects. In contrast, the diurnal PEF variability, expressed as amplitude percentage mean, observed in asthmatic subjects varies between 14.6% and 17.8%, depending on whether or not the subjects were treated with inhaled corticosteroids.18,20 PEF variability reported in the present study in subjects with OA and WEA when removed from exposure was lower than the variability reported in asthmatic subjects. Subjects with OA and WEA enrolled in the present study may have had a milder asthma than the asthmatic subjects reported in the previous studies.18,2021

The magnitude of the PEF variability was higher in subjects with OA than in subjects with WEA during work exposures, a finding that to the best of our knowledge has not previously been described. The persistent exposure to an occupational agent to which the worker is sensitized may induce a significant inflammatory response inducing a poorer asthma control than in the workers whose asthma is exacerbated by a nonspecific irritant mechanism.

The comparison of the number of days during which a 20% diurnal variation in PEF occurred between periods at and away from work has been proposed as a diagnosis criteria for OA.13,16 However, in our study although subjects with OA had a greater number of days with a 20% diurnal variation in PEF when at work compared with periods away from work, the majority (8 of 15 subjects) with WEA also showed a 20% diurnal variation in PEF in a greater number of days when at work than when away from work.

Some studies13,22 have looked at the agreement between experts in the diagnosis of OA and compared the diagnostic performance of visual assessment of PEF records with quantitative analysis. Visual analysis was considered as a satisfactory method for interpreting PEF.13,22 However, to our knowledge no study has determined whether the visual analysis performed by experts allows an accurate differentiation between OA and WEA.

In several studies,13,22 the group of subjects with negative SIC results also include subjects who have normal airway responsiveness. It is likely that the PEF variability observed in these subjects is minimal, and thus distinguishing subjects with OA from subjects without OA was probably easier than between subjects with OA and WEA. This may explain the poor performance of PEF monitoring in our study compared to the earlier studies.

Visual interpretation is based on subjective criteria and does not rely on standardized ones. Baldwin and coworkers23 showed that the objective scoring system provided by Oasys-2 software removes subjectivity in diagnosing OA. In the present study, the Oasys-2 Expert System, like the visual analysis, was not able to in differentiate OA from WEA. One may argue that some subjects with WEA may have been misclassified and had actually OA. Although, such a misclassification is always possible, the investigation performed in this study was extensive and leaves little room for missing a diagnosis of OA.

Although PEF variability was greater in subjects with OA than in subjects with WEA, PEF monitoring alone did not allow accurately differentiating OA from WEA. The investigation of OA should not be based on a single diagnostic test but on a stepwise approach in which multiple objective testing contributes to improve the quality of diagnosis. Indeed, the changes observed in FEV1 and PC20 between periods at and away from work were similar in subjects with OA and WEA. Performing these functional measurements alone without any other objective testing is unlikely to allow the distinction between OA and WEA.

SIC tests are probably the most reliable tests to distinguish OA from WEA. However, they are available in only a few centers worldwide. Therefore, an alternative method of investigation is needed. Serial PEF monitoring during periods at work and away from work is certainly an asset for confirming the relationship between the workplace and symptoms,24 but additional testing such as immunologic tests and changes in airway responsiveness or in airway inflammation maybe needed to distinguish OA from WEA.

Abbreviations: OA = occupational asthma; PC20 = provocative concentration of methacholine inducing a 20% fall in FEV1; PEF = peak expiratory flow; SIC = specific inhalation challenge; WEA = work-exacerbated asthma

This work was performed at Sacré-Coeur Hospital.

This study was funded by grant MOP-42544 from the Canadian Institutes of Health Research.

This study was registered at The Montreal Sacre-Coeur Hospital, No. CE 2003–09-175.

Dr. Lemiere holds a scholarship from the Canadian Institutes of Health Research.

The authors have no conflicts of interest to disclose.

Table Graphic Jump Location
Table 1. Characteristics of Subjects With OA and WEA*
* 

Data are presented as No., mean ± SD, or No. (%).

Table Graphic Jump Location
Table 2. Changes in Clinical and Functional Indices Before and After Periods At Work and Off Work Periods in Subjects With OA and WEA*
* 

Data are presented as mean ± SD or geometric mean (minimum-maximum).

 

p ≤ 0.001.

 

p ≤ 0.05

Table Graphic Jump Location
Table 3. Differences in Diurnal PEF Variability During Periods At and Away From Work in Subjects With WEA and OA*

Data are expressed as mean ± SD.

 

p = 0.02.

 

p < 0.001.

Table Graphic Jump Location
Table 4. Visual Analysis of PEF by the Five Experts*
* 

Data are presented as No. (%) or No.

The authors thank Simone Chaboillez and Dr. F. Girard for collecting the data, Drs. F. E. Hargreave and J. Côté for interpreting PEF graphs, and Mr. James Hatch for reviewing the manuscript.

Bernstein, IL Chan-Yeung, M Malo, JLet al eds.Asthma in the workplace 3rd ed.2006,1-4 Marcel Dekker. New York, NY:
 
Larbanois, A, Jamart, J, Delwiche, JP, et al Socioeconomic outcome of subjects experiencing asthma symptoms at work.Eur Respir J2002;19,1107-1113. [PubMed] [CrossRef]
 
Pelissier, S, Chaboillez, S, Teolis, L, et al Outcome of subjects diagnosed with occupational asthma and work-aggravated asthma after removal from exposure.J Occup Environ Med2006;48,656-659
 
Tarlo, SM, Boulet, LP, Cartier, A, et al Canadian Thoracic Society guidelines for occupational asthma.Can Respir J1998;5,289-300. [PubMed]
 
Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease (COPD) and asthma: this official statement of the American Thoracic Society was adopted by the ATS Board of Directors, November 1986.Am Rev Respir Dis1987;136,225-244. [PubMed]
 
Girard, F, Chaboillez, S, Cartier, A, et al An effective strategy for diagnosing occupational asthma: use of induced sputum.Am J Respir Crit Care Med2004;170,845-850. [PubMed]
 
American Thoracic Society.. Standardization of spirometry, 1994 update.Am J Respir Crit Care Med1995;152,1107-1136. [PubMed]
 
Juniper, EF Histamine and methacholine inhalation test: a laboratory tidal breathing protocol 2nd ed.1994 Astra Draco. Lund, Sweden:
 
Borg, GA Psychophysical bases of perceived exertion.Med Sci Sports Exerc1982;14,377-381. [PubMed]
 
Perrin, LF, Dechamp, C, Deviller, P, et al Reproducibility of skin tests: a comparative study of the Pepys prick test and the Morrow-Brown needle and their correlation with the serum IgE level.Clin Allergy1984;14,581-588. [PubMed]
 
Cartier, A Definition and diagnosis of occupational asthma.Eur Respir J1994;7,153-160. [PubMed]
 
Burge, PS, O’Brien, IM, Harries, MG Peak flow rate records in the diagnosis of occupational asthma due to colophony.Thorax1979;34,308-316. [PubMed]
 
Perrin, B, Lagier, F, L’Archeveque, J, et al Occupational asthma: validity of monitoring of peak expiratory flow rates and non-allergic bronchial responsiveness as compared to specific inhalation challenge.Eur Respir J1992;5,40-48. [PubMed]
 
Gannon, PF, Newton, DT, Belcher, J, et al Development of Oasys-2: a system for the analysis of serial measurement of peak expiratory flow in workers with suspected occupational asthma.Thorax1996;51,484-489. [PubMed]
 
Higgins, BG, Britton, JR, Chinn, S, et al The distribution of peak expiratory flow variability in a population sample.Am Rev Respir Dis1989;140,1368-1372. [PubMed]
 
Liss, GM, Tarlo, SM Peak expiratory flow rates in possible occupational asthma.Chest1991;100,63-69. [PubMed]
 
Hetzel, MR, Clark, TJ Comparison of normal and asthmatic circadian rhythms in peak expiratory flow rate.Thorax1980;35,732-738. [PubMed]
 
Troyanov, S, Ghezzo, H, Cartier, A, et al Comparison of circadian variations using FEV1and peak expiratory flow rates among normal and asthmatic subjects.Thorax1994;49,775-780. [PubMed]
 
Quackenboss, JJ, Lebowitz, MD, Krzyzanowski, M The normal range of diurnal changes in peak expiratory flow rates: relationship to symptoms and respiratory disease.Am Rev Respir Dis1991;143,323-330. [PubMed]
 
Douma, WR, Kerstjens, HA, Roos, CM, et al Changes in peak expiratory flow indices as a proxy for changes in bronchial hyperresponsiveness: Dutch Chronic Non-Specific Lung Disease Study Group.Eur Respir J2000;16,220-225. [PubMed]
 
Brand, PL, Postma, DS, Kerstjens, HA, et al Relationship of airway hyperresponsiveness to respiratory symptoms and diurnal peak flow variation in patients with obstructive lung disease: The Dutch CNSLD Study Group.Am Rev Respir Dis1991;143,916-921. [PubMed]
 
Cote, J, Kennedy, S, Chan-Yeung, M Sensitivity and specificity of PC20and peak expiratory flow rate in cedar asthma.J Allergy Clin Immunol1990;85,592-598. [PubMed]
 
Baldwin, DR, Gannon, P, Bright, P, et al Interpretation of occupational peak flow records: level of agreement between expert clinicians and Oasys-2.Thorax2002;57,860-864. [PubMed]
 
Moscato, G, Godnic-Cvar, J, Maestrelli, P Statement on self-monitoring of peak expiratory flows in the investigation of occupational asthma: subcommittee on Occupational Allergy of European Academy of Allergy and Clinical Immunology.J Allergy Clin Immunol1995;96,295-301. [PubMed]
 

Figures

Tables

Table Graphic Jump Location
Table 1. Characteristics of Subjects With OA and WEA*
* 

Data are presented as No., mean ± SD, or No. (%).

Table Graphic Jump Location
Table 2. Changes in Clinical and Functional Indices Before and After Periods At Work and Off Work Periods in Subjects With OA and WEA*
* 

Data are presented as mean ± SD or geometric mean (minimum-maximum).

 

p ≤ 0.001.

 

p ≤ 0.05

Table Graphic Jump Location
Table 3. Differences in Diurnal PEF Variability During Periods At and Away From Work in Subjects With WEA and OA*

Data are expressed as mean ± SD.

 

p = 0.02.

 

p < 0.001.

Table Graphic Jump Location
Table 4. Visual Analysis of PEF by the Five Experts*
* 

Data are presented as No. (%) or No.

References

Bernstein, IL Chan-Yeung, M Malo, JLet al eds.Asthma in the workplace 3rd ed.2006,1-4 Marcel Dekker. New York, NY:
 
Larbanois, A, Jamart, J, Delwiche, JP, et al Socioeconomic outcome of subjects experiencing asthma symptoms at work.Eur Respir J2002;19,1107-1113. [PubMed] [CrossRef]
 
Pelissier, S, Chaboillez, S, Teolis, L, et al Outcome of subjects diagnosed with occupational asthma and work-aggravated asthma after removal from exposure.J Occup Environ Med2006;48,656-659
 
Tarlo, SM, Boulet, LP, Cartier, A, et al Canadian Thoracic Society guidelines for occupational asthma.Can Respir J1998;5,289-300. [PubMed]
 
Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease (COPD) and asthma: this official statement of the American Thoracic Society was adopted by the ATS Board of Directors, November 1986.Am Rev Respir Dis1987;136,225-244. [PubMed]
 
Girard, F, Chaboillez, S, Cartier, A, et al An effective strategy for diagnosing occupational asthma: use of induced sputum.Am J Respir Crit Care Med2004;170,845-850. [PubMed]
 
American Thoracic Society.. Standardization of spirometry, 1994 update.Am J Respir Crit Care Med1995;152,1107-1136. [PubMed]
 
Juniper, EF Histamine and methacholine inhalation test: a laboratory tidal breathing protocol 2nd ed.1994 Astra Draco. Lund, Sweden:
 
Borg, GA Psychophysical bases of perceived exertion.Med Sci Sports Exerc1982;14,377-381. [PubMed]
 
Perrin, LF, Dechamp, C, Deviller, P, et al Reproducibility of skin tests: a comparative study of the Pepys prick test and the Morrow-Brown needle and their correlation with the serum IgE level.Clin Allergy1984;14,581-588. [PubMed]
 
Cartier, A Definition and diagnosis of occupational asthma.Eur Respir J1994;7,153-160. [PubMed]
 
Burge, PS, O’Brien, IM, Harries, MG Peak flow rate records in the diagnosis of occupational asthma due to colophony.Thorax1979;34,308-316. [PubMed]
 
Perrin, B, Lagier, F, L’Archeveque, J, et al Occupational asthma: validity of monitoring of peak expiratory flow rates and non-allergic bronchial responsiveness as compared to specific inhalation challenge.Eur Respir J1992;5,40-48. [PubMed]
 
Gannon, PF, Newton, DT, Belcher, J, et al Development of Oasys-2: a system for the analysis of serial measurement of peak expiratory flow in workers with suspected occupational asthma.Thorax1996;51,484-489. [PubMed]
 
Higgins, BG, Britton, JR, Chinn, S, et al The distribution of peak expiratory flow variability in a population sample.Am Rev Respir Dis1989;140,1368-1372. [PubMed]
 
Liss, GM, Tarlo, SM Peak expiratory flow rates in possible occupational asthma.Chest1991;100,63-69. [PubMed]
 
Hetzel, MR, Clark, TJ Comparison of normal and asthmatic circadian rhythms in peak expiratory flow rate.Thorax1980;35,732-738. [PubMed]
 
Troyanov, S, Ghezzo, H, Cartier, A, et al Comparison of circadian variations using FEV1and peak expiratory flow rates among normal and asthmatic subjects.Thorax1994;49,775-780. [PubMed]
 
Quackenboss, JJ, Lebowitz, MD, Krzyzanowski, M The normal range of diurnal changes in peak expiratory flow rates: relationship to symptoms and respiratory disease.Am Rev Respir Dis1991;143,323-330. [PubMed]
 
Douma, WR, Kerstjens, HA, Roos, CM, et al Changes in peak expiratory flow indices as a proxy for changes in bronchial hyperresponsiveness: Dutch Chronic Non-Specific Lung Disease Study Group.Eur Respir J2000;16,220-225. [PubMed]
 
Brand, PL, Postma, DS, Kerstjens, HA, et al Relationship of airway hyperresponsiveness to respiratory symptoms and diurnal peak flow variation in patients with obstructive lung disease: The Dutch CNSLD Study Group.Am Rev Respir Dis1991;143,916-921. [PubMed]
 
Cote, J, Kennedy, S, Chan-Yeung, M Sensitivity and specificity of PC20and peak expiratory flow rate in cedar asthma.J Allergy Clin Immunol1990;85,592-598. [PubMed]
 
Baldwin, DR, Gannon, P, Bright, P, et al Interpretation of occupational peak flow records: level of agreement between expert clinicians and Oasys-2.Thorax2002;57,860-864. [PubMed]
 
Moscato, G, Godnic-Cvar, J, Maestrelli, P Statement on self-monitoring of peak expiratory flows in the investigation of occupational asthma: subcommittee on Occupational Allergy of European Academy of Allergy and Clinical Immunology.J Allergy Clin Immunol1995;96,295-301. [PubMed]
 
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