Measurement of a **pleural effusion volume **with point-of-care ultrasonography may be a useful tool for intensivists and is an active area of research in critical care ^{7}.

In controlled settings ultrasound may detect constitutive pleural fluid, can reliably detect effusions >20 mL in clinical settings, and may approach the sensitivity and specificity of computed tomography. Assessment of pleural effusions by ultrasound has historically been semi-quantitative, a gestalt of whether the effusion appears minimal, mild, moderate, or severe ^{6}.

#### Measurement

Some intensivists and emergency physicians also use ultrasonography to directly quantify the volume of a given pleural effusion, incorporating this measurement with an assessment of gas exchange in deciding whether to drain the fluid collection in question ^{1}.

To this end, three formulae have enjoyed robust support in the literature, and are used commonly in clinical practice:

- Balik formula
^{ 4}- patient supine with mild (15°) trunk elevation, transducer perpendicular to the dorsolateral chest wall, measurements taken at end-expiration
- operator measures the maximum distance (in millimeters) between the visceral and parietal pleura
- Pleural effusion volume (mL) = (measured distance) x 20

- Eibenberger formula
^{3}- patient supine, transducer perpendicular to the chest wall, measurements taken at maximum inspiration
- operator measures the maximum distance (in millimeters) between the lung and posterior chest wall
- pleural effusion volume (mL) = (47.6 x distance) - 837
- Note: Due to the subtraction in the above equation the Eibenberger formula returns a spurious negative value if the distance between the pleural layers is small (<18 mm). For very thin effusions other (e.g. the Balik) formulae should be used.

- Goecke formula
^{5}- two popular variants exist, which are both performed in the erect position with the transducer on the dorsolateral chest wall
- the index marker is directed cephalad (a longitudinal orientation) with distance measurements (cm) taken at end-expiration
- the first Goecke formula uses the craniocaudal extent (lateral height) of the effusion
- one caliper is placed in the near field in the costophrenic angle
- the subsequent caliper is placed in the far-field at the lung base, constituting a maximum distance between lung and diaphragm
- pleural effusion volume (mL) = distance (cm) x 90

- the second Goecke formula measures the distance between the lung base and the mid-diaphragm (the subpulmonary height)
- height of the dome of the diaphragm is connected to the lung base, the line being perpendicular to one's field of insonation

There exists no consensus on a best or preferred formula; patient positioning factors and ease with calculation of a specific formula typically dictate one's choice.