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Long-term Marijuana Use and Pulmonary Function—Reply

Long-term Marijuana Use and Pulmonary Function—Reply In Reply: As noted by Drs Patel and Khazeni, accurately measuring long-term exposure to marijuana is inherently difficult and may be made more difficult by cognitive problems with recollection caused by heavy regular marijuana use. However, we used measurements collected prospectively at the periodic study visits made during the 20-year follow-up in the CARDIA study. At each study visit, participants were asked about current/recent usage (“ . . . in the last 30 days”), and we made the assumption that use between examinations reflected use reported at each examination. While these assumptions introduce measurement error, they do not depend on long-term recollections about total usage over the course of many years. Also, the CARDIA study did not collect information about mode of inhalation (joint, bong, vaporizer, etc), so we could not conduct the subgroup analyses suggested by Patel and Khazeni. While measurement error was unavoidable, we take comfort from 2 principles: (1) that some (although not all) of the same measurement issues would be expected for tobacco smoke exposure, and despite these we found a strong relationship in the expected direction with pulmonary function (ie, it was a positive control) and (2) that we would not expect the measurement error for marijuana exposure to be differential. That is, we would not expect marijuana exposure reporting to be worse (either being too low, too high, or generally less exact) in persons with high vs low pulmonary function. With only nondifferential measurement error, we would expect only a bias toward the null; and yet we found a highly significant association between moderate marijuana exposure and higher forced expiratory volume in the first second of expiration and forced vital capacity test results. If our measurements were good enough to find a highly statistically significant positive association, we think they would have been good enough to find a significant negative association (as we did for tobacco exposure) if such an association existed. The CARDIA participants were chosen to reflect the population of 4 US communities, with oversampling as described in prior publications1,2 to achieve a relatively balanced mix of men and women, blacks and whites, and persons with higher and lower levels of education. Although overall retention was 69% at year 20, not all of these participants underwent pulmonary function testing at the year 20 visit. Only 55% (n = 2807) of the original 5115 participants contributed at the year 20 visit, but nearly all (n = 5016 or 98%) contributed at least 1 study visit with complete data on pulmonary function to our analysis. The approximately 50% of participants (2511/5016) with income greater than $50 000 per year noted in Table 1 reflects the oversampling described above, and truly describes the income distribution in the 5016 CARDIA participants included in our analysis. Back to top Article Information Conflict of Interest Disclosures: The authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Drs Pletcher and Kertesz both reported receiving a grant and travel support from the National Institute on Drug Abuse. Dr Kertesz also reported chairing a committee that advised the Drug Treatment Task Force for the Chief Justice of the State of Alabama. References 1. Hughes GH, Cutter GR, Donahue R, et al. Recruitment in the Coronary Artery Disease Risk Development in Young Adults (CARDIA) Study. Control Clin Trials. 1987;8(4):(suppl) 68S-73S3440391PubMedGoogle ScholarCrossref 2. Friedman GD, Cutter GR, Donahue RP, et al. CARDIA: study design, recruitment, and some characteristics of the examined subjects. J Clin Epidemiol. 1988;41(11):1105-11163204420PubMedGoogle ScholarCrossref http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png JAMA American Medical Association

Long-term Marijuana Use and Pulmonary Function—Reply

JAMA , Volume 307 (17) – May 2, 2012

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Publisher
American Medical Association
Copyright
Copyright © 2012 American Medical Association. All Rights Reserved.
ISSN
0098-7484
eISSN
1538-3598
DOI
10.1001/jama.2012.3607
Publisher site
See Article on Publisher Site

Abstract

In Reply: As noted by Drs Patel and Khazeni, accurately measuring long-term exposure to marijuana is inherently difficult and may be made more difficult by cognitive problems with recollection caused by heavy regular marijuana use. However, we used measurements collected prospectively at the periodic study visits made during the 20-year follow-up in the CARDIA study. At each study visit, participants were asked about current/recent usage (“ . . . in the last 30 days”), and we made the assumption that use between examinations reflected use reported at each examination. While these assumptions introduce measurement error, they do not depend on long-term recollections about total usage over the course of many years. Also, the CARDIA study did not collect information about mode of inhalation (joint, bong, vaporizer, etc), so we could not conduct the subgroup analyses suggested by Patel and Khazeni. While measurement error was unavoidable, we take comfort from 2 principles: (1) that some (although not all) of the same measurement issues would be expected for tobacco smoke exposure, and despite these we found a strong relationship in the expected direction with pulmonary function (ie, it was a positive control) and (2) that we would not expect the measurement error for marijuana exposure to be differential. That is, we would not expect marijuana exposure reporting to be worse (either being too low, too high, or generally less exact) in persons with high vs low pulmonary function. With only nondifferential measurement error, we would expect only a bias toward the null; and yet we found a highly significant association between moderate marijuana exposure and higher forced expiratory volume in the first second of expiration and forced vital capacity test results. If our measurements were good enough to find a highly statistically significant positive association, we think they would have been good enough to find a significant negative association (as we did for tobacco exposure) if such an association existed. The CARDIA participants were chosen to reflect the population of 4 US communities, with oversampling as described in prior publications1,2 to achieve a relatively balanced mix of men and women, blacks and whites, and persons with higher and lower levels of education. Although overall retention was 69% at year 20, not all of these participants underwent pulmonary function testing at the year 20 visit. Only 55% (n = 2807) of the original 5115 participants contributed at the year 20 visit, but nearly all (n = 5016 or 98%) contributed at least 1 study visit with complete data on pulmonary function to our analysis. The approximately 50% of participants (2511/5016) with income greater than $50 000 per year noted in Table 1 reflects the oversampling described above, and truly describes the income distribution in the 5016 CARDIA participants included in our analysis. Back to top Article Information Conflict of Interest Disclosures: The authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Drs Pletcher and Kertesz both reported receiving a grant and travel support from the National Institute on Drug Abuse. Dr Kertesz also reported chairing a committee that advised the Drug Treatment Task Force for the Chief Justice of the State of Alabama. References 1. Hughes GH, Cutter GR, Donahue R, et al. Recruitment in the Coronary Artery Disease Risk Development in Young Adults (CARDIA) Study. Control Clin Trials. 1987;8(4):(suppl) 68S-73S3440391PubMedGoogle ScholarCrossref 2. Friedman GD, Cutter GR, Donahue RP, et al. CARDIA: study design, recruitment, and some characteristics of the examined subjects. J Clin Epidemiol. 1988;41(11):1105-11163204420PubMedGoogle ScholarCrossref

Journal

JAMAAmerican Medical Association

Published: May 2, 2012

References