Is Light’s Criteria Still Needed in the Diagnosis of Pleural Effusion?
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Key Point
Researchers at Chest 2009 debated the necessity of using Light’s criteria to differentiate transudates from exudates in patients with pleural effusion.
SAN DIEGO—In a pro/con debate at Chest 2009, Richard W. Light, MD, Professor of Medicine at Vanderbilt University, Nashville, outlined the benefits of using his criteria—Light’s criteria—in the diagnostic work-up of patients with pleural effusion to differentiate exudates from transudates. Yun Chor Gary Lee, MBChB, Winthrop Professor of Respiratory Medicine at the University of Western Australia, posited that the idea of distinguishing between exudates and transudates may be an outdated idea and that Light’s criteria has several drawbacks.
PROVIDING A STARTING POINT
Under Light’s criteria, as outlined in a seminal study published in 1972, pleural fluid may be classified as exudate if one or more of the following criteria are met:
Pleural fluid protein divided by serum protein is greater than 0.5
Pleural fluid lactate dehydrogenase (LDH) divided by serum is greater than 0.6
Pleural fluid LDH is more than two-thirds of the upper limit of normal.
What is the rationale for separating transudates from exudates? If a patient were found to have a transudative pleural effusion—commonly due to heart failure or cirrhosis—then clinicians would presumably treat the cause of the effusion without conducting further diagnostics tests, Dr. Light explained. However, if the patient had an exudative effusion, more investigation would be needed to determine the local cause.
In one study, Spanish researchers classified 250 pleural effusions as probable exudates or transudates using radiologic studies, patient history, and physical examination, but not pleural fluid values. They were able to correctly identify 95% of exudates, Dr. Light acknowledged. “My criteria were better though—identifying approximately 99%.” However, clinicians were only able to identify 56% of transudates correctly. “My criteria weren’t great either—they only identified 75%.” To identify true transudates when exudative criteria are met, he suggested using the serum value minus the pleural fluid value, using a protein gradient (> 3.1 g) or an albumin gradient (> 1.2 g).
“Although our goal should be to definitively establish the etiology of every pleural effusion, initially classifying the effusion as a transudate or an exudate allows one to focus on systemic versus local disease,” Dr. Light concluded. “Therefore, it still seems appropriate to classify effusions, but I would certainly agree that that is not the end of the line.”
AN UNNECESSARY STEP?
“I fully agree that Light’s criteria is the best and most useful biochemical criteria separating transudates and exudates,” Dr. Lee stated. However, using Light’s criteria as the first step to assess pleural effusions has significant shortcomings, he asserted. It does not establish a diagnosis and more definitive tests are almost always necessary to identify the etiology of the disease. “So why bother with an extra step?” he asked.
Light’s criteria exclude clinical judgment and radiologic input, Dr. Lee continued. Current guidelines from the United Kingdom and the United States recommend performing an ultrasound before fluid aspiration—for safety reasons. In a study of 80 patients, investigators observed that exudates were always characterized by pleural effusion with septation or internal echogenicity, and that pleural thickening of more than 3 mm was highly suggestive of an exudate; anechoic effusion was found to be more suggestive of transudate than exudate. Another study by Qureshi and colleagues further suggested that pleural thickening, nodularity, and diaphragmatic thickening is highly suggestive of malignant pleural disease (positive predictive value, 100%). “Light’s criteria only creates extra costs and extra delays,” Dr. Lee contended.
Furthermore, pleural effusion guidelines from the British Thoracic Society, coauthored by Dr. Lee and due to be published in 2010, further suggest that aspiration should not be performed for bilateral effusion in a clinical setting strongly suggestive of cardiac failure unless patients have atypical features or fail to respond to therapy. “In other words, it is telling us that you can trust your clinical judgment,” Dr. Lee stated. The guidelines further recommend performing biochemical and microbiological tests on aspirated fluids, suggesting that it is not necessary to wait until transudates are separated from exudates before looking for the etiology of the disease, Dr. Lee added.
Another concern is the error rate for Light’s criteria. “It is no better than using clinical judgment,” according to Dr. Lee. Up to 8% of malignant effusions are transudates, he added. Furthermore, 20% to 40% of congestive heart failure effusions are misclassified as exudate by Light’s criteria—especially if the patient is taking diuretics.
More importantly, there are new, more useful disease-specific biomarkers—
including adenosine deaminase, which is widely used in endemic countries for tuberculosis pleural effusion, Dr. Lee explained. Another marker recently evaluated in malignant pleural mesothelioma is mesothelin.
“We hope that in the very near future, we can replace Light’s criteria with disease-specific markers and new diagnostic algorithms that take into consideration not only the biochemical criteria, but also clinical and imaging input.”
