Modern methods of examination have greatly facilitated the diagnosis of pleural effusion, particularly radiography. In spite of this it is nothing unusual for experienced clinicians to be embarrassed by a “dry tap” when there is clearcut evidence of pleural effusion. Nor is the embarrassment lessened when a successful thoracentesis is performed on the patient on the same day by another clinician who elected to introduce his needle one or two interspaces higher.
In most instances “dry taps” in simple effusions are due to the eagerness of the operator to remove every ounce. The patient is placed in the sitting position regardless of his physical condition, and the needle is introduced at a low level, frequently so low that it encounters diaphragm instead of liquid. One of the chief reasons for failure in thoracentesis is that the landmarks of the effusion are not clearly established. This is particularly true of the more common type, the simple pleural effusion, which is to be distinguished from hydropneumothorax and from cases in which tumor and adhesions complicate and distort the picture.
Kaunitz J. Landmarks in Simple Pleural Effusions. JAMA. 2014;311(23):2444. doi:10.1001/jama.2013.279594