Sonographic evaluation of pleural fluid in a large group of adult healthy individuals--end trial results.

This partly prospective, partly retrospective trial was performed in adult healthy volunteers to confirm the prevalence of sonographically visible physiological pleural and to establish possible individual variations of the presence and amount of pleural fluid over time as well as to asses relations in between pleural fluid and individual characteristics like age, sex, body mass index, smoking history and hormone therapy (in woman). A reliable threshold between normal and pathological pleural fluid findings was determined. Prospective chest sonography of both pleural spaces was performed with 3-12 MHz transducer in 71 randomly selected healthy adults and presence of pleural fluid was evaluated and measured as an anechoic layer at least 2 mm in thickness. Each individual was reexamined three times in two to four months intervals. Another 86 individuals were prospectively re-examined for the third time, 24 to 36 months after completed previous trials while the data on the baseline and follow up examination were retrospectively acquired. Maximum thickness of the pleural fluid was measured in the elbow position after five minutes leaning in lateral decubitus position. Examinees served as their own controls, with the quantitative measurement of the fluid layer over time. The fluid layer was visible in at least one pleural space in 51/157 (32.5%) subjects, whereas 35 (22.3%) examinees had a positive finding on all three examinations. Consistency of this finding was high between each pair of examinations over time (Cohen's Kappa > or = 0.8, p < 0.001). The maximum thickness of fluid layer ranged from 2.0 to 5.2 mm, with a mean of 2.9 mm independently of left or right pleural space and unilateral or bilateral presence. Regression models indicate that odds for observing pleural fluid in an individual decrease with age ( p = 0.013) and that if observed, the fluid tends to be thicker in women ( p = 0.017) and in subjects with higher BMI ( p = 0.028). Sonography detected small amounts of pleural fluid in 32.5% of healthy individuals. If present, maximum thickness of the fluid layer doesn't exceed the threshold value of 5.2 mm with mean values around 3 mm. The amount of physiological pleural fluid is relative stable over time and is very likely an individual characteristic with lower frequencies in elders while the frequencies of sonographically detected pleural fluid in healthy adults does not correlate with sex, obesity, smoking or hormonal therapy.