Ultrasound assessment of pleural effusion volume in clinical practice: a narrative review.

BACKGROUND AND OBJECTIVE: Pleural effusion (PLE) is a common clinical condition with a broad spectrum of underlying etiologies. Accurate estimation of pleural fluid (PF) volume can support clinical decision-making, particularly in critically ill patients, by guiding invasive procedures, monitoring treatment response, and informing prognosis. This review aims to provide an overview of the main ultrasound (US) techniques for estimating PLE volume and to discuss their limitations in routine practice. METHODS: A narrative review of the literature was conducted, synthesizing the most relevant evidence identified through structured manual searches and consultation of authoritative sources. KEY CONTENT AND FINDINGS: Computed tomography (CT) is considered the reference standard for volumetric assessment, but it is costly, exposes patients to ionizing radiation, and often requires transfer, which may carry additional risks. Thoracic US has emerged as a reliable bedside alternative, offering high sensitivity and specificity for the detection of even small effusions and showing good correlation with CT measurements. Multiple sonographic approaches are available for estimating PLE volume, including linear methods based on interpleural distances and more complex multiplanar models. Linear formulas are rapid and easy to apply at the bedside, with reported correlation coefficients (R2) ranging from 0.68 to 0.90 when compared with drained volumes, whereas multiplanar models achieve higher precision (R2 up to 0.98 versus CT) but require greater technical expertise. Accuracy is influenced by patient-related factors, including body position and mechanical ventilation, as well as operator experience and the presence of associated lung pathology such as consolidations or loculated effusions, which may lead to underestimation of true volume. Standardized protocols and adequate training are therefore essential to improve reproducibility. CONCLUSIONS: Although US does not replace CT or other imaging modalities, it provides a non-invasive, rapid, and generally reliable estimate of PF volume that complements bedside clinical judgment.