Pleural fluid volume estimation: a chest radiograph prediction rule.

Rationale And Objectives: We devised a prediction rule for estimating pleural effusion volume on the basis of posteroanterior and lateral chest radiographs.

Methods: A prediction rule was devised for estimating pleural effusion volume on the basis of the presence or absence of a meniscus on chest radiographs. The rule was tested and validated using separate data sets obtained from a retrospective review of patients having both a chest radiograph and computed tomography (CT) scan (the gold standard) within 24 hr of each other. The accuracy of the prediction rule and the degree of interobserver agreement between the two independent readers were determined.

Results: For the test and validation sets, the weighted accuracies of the prediction rule were 86% and 85%, respectively. The respective weighted interobserver agreements were 97% and 88%. Pleural effusions became visible as a meniscus on the lateral chest radiograph at a volume of approximately 50 ml; at a volume of 200 ml, the meniscus could be identified on the posteroanterior radiograph. At a volume of about 500 ml, the meniscus obscured the hemidiaphragm.

Conclusion: The volume of a pleural effusion can be estimated from the chest radiograph appearance with a reasonable degree of accuracy.