Impact of body mass index on the detection of radiographic localized pleural thickening

Our plain-English summary of Impact of body mass index on the detection of radiographic localized pleural thickening, by T.C. Larson, A. Franzblau, M. Lewin, et al (Academic Radiology, 2014).

The study

• Authors: T.C. Larson, A. Franzblau, M. Lewin, A.B. Goodman, V.C. Antao
• Journal: Academic Radiology
• Published: 2014;21(1):3-10
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Rationale And Objectives: Subpleural fat can be difficult to distinguish from localized pleural thickening (LPT), a marker of asbestos exposure, on chest radiographs. The aims of this study were to examine the influence of body mass index (BMI) on the performance of radiograph readers when classifying LPT and to model the risk of false test results with varying BMI.

Materials And Methods: Subjects (n = 200) were patients being screened or treated for asbestos-related health outcomes. A film chest radiograph, a digital chest radiograph, and a high-resolution computed tomography (HRCT) chest scan were collected from each subject. All radiographs were independently read by seven B readers and scored using the International Labour Office system. HRCT scans, read by three experienced thoracic radiologists, served as the gold standard for the presence of LPT. We calculated measures of radiograph reader performance, including sensitivity and specificity, for each image modality. We also used logistic regression to estimate the probability of a false-positive and a false-negative result while controlling for covariates.

Results: The proportion of false-positive readings correlated with BMI. While controlling for covariates, regression modeling showed the probability of a false-positive result increased with increasing BMI category, younger age, not having pleural calcification, and among subjects not reporting occupational or household contact asbestos exposure.

Conclusions: Clinicians should be cautious when evaluating radiographs of younger obese persons for the presence of asbestos-related pleural plaque, particularly in populations having an anticipated low or background prevalence of LPT.

Published by Elsevier Inc.

In plain English

This study asked whether body mass index (BMI, weight relative to height) affects how accurately chest X-rays show localized pleural thickening (LPT), a marker of asbestos exposure. Subjects were 200 Libby, Montana, patients screened or treated for asbestos-related health outcomes. Each had film and digital chest X-rays, read by seven B readers, and a high-resolution CT scan read by three thoracic radiologists as the gold standard. On film X-rays, the unadjusted share of positive LPT readings not confirmed by CT (false positives) was 0.28 (95% CI 0.12 to 0.51) with normal BMI and 0.65 (0.37 to 0.86) with morbid obesity. Adjusted for age, exposure category and pleural calcification, false-positive odds in morbidly obese subjects were 6.7 times (95% CI 1.6 to 27.7) those with normal BMI, and in obese subjects 4.4 times (1.4 to 14.0). The overweight result was not statistically significant. Younger subjects, those without pleural calcification, and those reporting only residential exposure had more false positives. The authors concluded that false-positive LPT detection correlates with BMI, urged caution with obese patients, and called for studies in other populations.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Diagnostic accuracy study comparing chest radiograph readings with HRCT as the gold standard, using generalized estimating equations (logistic regression) to model false-positive and false-negative results
• Who was studied: 200 consecutive patients being treated or screened for asbestos-related health outcomes in Libby, Montana, who had an HRCT scan within 24 months of enrollment. 143 (72%) were male, 48% were obese and 9% morbidly obese. The analysis used 400 hemithoraces and 2800 hemithorax readings per radiograph type.
• Exposure: The main factor studied was BMI, calculated from measured height and weight. Self-reported asbestos exposure was recorded: occupational (46), household contact (51) or residential only (103). The background states that Libby vermiculite contained asbestiform amphiboles including winchite, richterite and tremolite asbestos.
• Main results:
  • On film radiographs, without adjustment for other factors, the proportion of false-positive LPT readings (among positive hemithorax readings) was 0.48 (0.40 to 0.56) overall. By BMI category it was 0.28 (0.12 to 0.51) for normal, 0.42 (0.28 to 0.57) for overweight, 0.53 (0.41 to 0.64) for obese and 0.65 (0.37 to 0.86) for morbidly obese subjects. Film and digital results were similar.
  • In the categorical model, which also included age, exposure category and pleural calcification, the odds ratios for a false-positive result on film, compared with normal BMI, were 6.7 (1.6 to 27.7) for morbidly obese subjects, 4.4 (1.4 to 14.0) for obese subjects and 2.6 (0.8 to 9.3, P = .13) for overweight subjects. Only the obese and morbidly obese results were statistically significant.
  • With BMI treated as a continuous measure, the odds of a false-positive result rose 6% for every 1 kg/m2 increase in BMI.
  • Older age was associated with lower odds of a false positive. Being a worker or household contact (OR 0.5, 0.2 to 1.0) or having pleural calcification (OR 0.4, 0.2 to 0.7) was significantly protective against a false-positive result.
  • In model estimates for subjects aged 63 with occupational or household contact exposure, the probability of a false positive ranged from 0.21 (normal BMI) to 0.65 (morbidly obese). For subjects with normal BMI and residential exposure, it ranged from 0.49 at age 53 to 0.17 at age 83.
  • For false negatives, subjects with normal BMI had a higher estimated probability than the other categories (0.52 vs. about 0.3), but this difference was not statistically significant.
  • For diffuse pleural thickening, radiograph and HRCT readings agreed in 93% (2608/2800 readings). Because only 5% of subjects (13/200) had diffuse pleural thickening on HRCT, the regression models for it did not converge.
  • In one obese subject (BMI 39 kg/m2), all seven B readers judged LPT present on the radiograph, but HRCT showed the structures were extrapleural fat.
• The authors' conclusion: The authors concluded that the probability of false-positive radiographic detection of LPT correlates with BMI, likely because BMI acts as a surrogate for extrapleural fat. They advised clinicians to be cautious when diagnosing asbestos-related pleural plaque in obese and younger patients. They stated that further studies are needed to see whether the findings generalize to other populations.
• Limitations the authors state:
  • HRCT scans were collected up to 24 months before the radiographs, so disease status may have been misclassified. The median interval was 30 days.
  • HRCT readers were not instructed on how to tell fat tissue from plaque.
  • BMI does not measure fat or its distribution directly, so using it as a stand-in for extrapleural fat may have misclassified some subjects.
  • Requiring a recent chest CT for enrollment suggests subjects may have had equivocal LPT findings on radiograph, which may have increased the likelihood of false positives.
  • Results could only be compared by hemithorax, not by specific structure. This could have caused misclassification in an unknown direction.
  • Because the subjects had unique characteristics, the findings may not generalize to other populations.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Libby vermiculite mine, Montana

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis, Pleural plaques and thickening.

Medical terms in this study, explained in our Lexicon: Pleural Thickening, Asbestos Exposure, Chest X-Ray, High-Resolution CT (HRCT), Pleural Calcification, Pleural Plaques.

Related studies

• Increased body mass index is related to apparent circumscribed pleural thickening on plain chest radiographs — Y.C. Lee, C.K. Runnion, et al. (2001)
• Overstating the consequences of radiographic abnormalities — J.J. Flynn, S. Kardos, et al. (2004)
• Scoring CT/HRCT findings among asbestos-exposed workers: effects of patient's age, body mass index and common laboratory test results — T. Vehmas, L. Kivisaari, et al. (2005)
• Low levels of exposure to libby amphibole asbestos and localized pleural thickening — K.Y. Christensen, T.F. Bateson, et al. (2013)
• Radiographic abnormalities and asbestos exposure: Libby, Montana — B. Price (2004)
• Asbestos-related radiographic findings among household contacts of workers exposed to Libby vermiculite: impact of workers' personal hygiene practices — T.J. Hilbert, A. Franzblau, et al. (2013)

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