High BMI can lead to false-positive diagnosis of asbestos-related outcomes

Clinicians should use caution when evaluating radiographs of younger obese patients for asbestos-related pleural plaque, especially in populations anticipated to have low or background prevalence of localized pleural thickening (LPT), according to a study published in the January 2014 issue of Academic Radiology.

LPT, the most common health outcomes associated with inhalation exposure to asbestos, can be difficult to discern from subpleural fat on chest radiographs, as increased body mass index (BMI) has been correlated with pleural thickening.

Vermiculite mining and processing occurred for much of the 20th century in Libby, Mont., leading to widespread asbestos exposure. “Consistent with pervasive asbestos exposure,” wrote the study’s lead author, Theodore C. Larson, MS, of the Agency for Toxic Substances and Disease Registry in Atlanta, and colleagues, “radiographic surveys have found pleural abnormalities among these vermiculite workers and their families and among other Libby residents.”

Larson and colleagues used patient data from Libby to examine the influence of BMI on measures of radiograph reader performance for the detection of LPT, including sensitivity, specificity, and predictive values. They also aimed to model the association between BMI and false-positive and false-negative radiographic readings.

The study’s participants were patients undergoing screening or treatment for asbestos-related health outcomes. Each subject underwent a film chest radiograph, a digital chest radiograph, and a high-resolution computed tomography (HRCT) chest scan. All radiographs were independently read and scored by readers using the International Labour Office system. The HRCT scans, perceived as the gold standard for the presence of LPT, were read by three experienced thoracic radiologists. The researchers calculated measures of radiograph reader performance for each image modality. 

The study’s results revealed that the proportion of false-positive readings correlated with BMI and that the probability of a false-positive result increased with increasing BMI category, younger age, lack of pleural calcification, and among subjects who did not report occupational or household contact asbestos exposure.

“These data indicate that the probability of false-positive detection of LPT correlates with BMI. This is likely because of BMI acting as a surrogate for increased extrapleural fat, which can be misinterpreted as pleural plaque on chest radiographs,” wrote the authors. “Clinicians should be cautious when diagnosing asbestos-related pleural plaque in patients who are obese, especially among those that may be younger than the age when plaque is apt to be apparent on radiograph,” they concluded.

Charmith S. Rakapaske, PhD, of the University of Pennsylvania in Philadelphia and Gregory Chang, MD, of the New York University Langone Medical Center, wrote in an associated guest editorial: “The role of BMI in radiology could very well be a more general issue not necessarily limited to radiographs. Large body habitus can degrade image quality in all medical imaging modalities, sometimes making it difficult to obtain accurate clinical interpretations.”

While experienced radiologists can internally adjust their interpretations given the relative size of patients, if a patient is so obese that the contrast is skewed, the ability to accurately interpret images will be negatively affected, they wrote.

“Ultimately, as radiologists and imaging scientists, it is important that we closely evaluate the diagnostic performance/comparative effectiveness of imaging technologies in the context of varying body habitus. In the future, this should permit us to take an evidence-based approach/rationale for choosing the most appropriate imaging modality for the detection and monitoring of disease,” surmised Rakapaske and Chang.