Is it necessary to drain a moderate pleural effusion?

A pleural effusion is an accumulation of fluid in the space between the lungs and chest wall. The decision to drain a pleural effusion is based primarily on whether the cause of the pleural effusion is already known and whether the amount of fluid is large enough to compromise lung function. Most moderate effusions would likely be drained. But there are situations when it would not be necessary. For example, a moderate effusion might not be drained if the effusion was caused by heart failure that could be treated with diuretics.