A coin test, which is also called a bell-metal resonance test, is used by a doctor to determine whether a patient has a lung infection or a collapsed or punctured lung. The test is simple to perform and requires only two coins and a stethoscope. To perform the test, the doctor will strike the coins together and listen for the metallic sound on the other side of the patient's lung. In healthy patients, this sound will not be audible.
In order to perform a coin test, a single coin is held flat against one side of the patient's chest. Another coin is hit against this coin, causing the first coin to resonate. Using a stethoscope, the doctor listens to the same lung from the patient's back. The way the sound of the ringing coin moves through the patient's lung can give the doctor an indication of the patient's injury or illness. Though any two metallic objects can be used in this test, it gets its name because coins have been traditionally used.
The most common use for the coin test is in the diagnosis of a collapsed lung. In this condition, air or fluid fills the space in the chest cavity around the lung, preventing the lung from fully expanding and making it difficult for the patient to breathe. When a coin test is performed on a patient with a collapsed lung, the ringing sound will travel through the chest and the doctor will be able to hear a metallic sound through the stethoscope on the other side of the patient's body.
It is also possible to use a coin test to diagnose chest infections such as tuberculosis and pneumonia. The doctor administers this type of coin test in the same way as the test used to diagnose a collapsed lung and again listens for a metallic sound on the other side of the patient's body. If this sound is heard, it can be an indication of excess fluid in the lungs and chest cavity. In a healthy patient, the sound of the coins hitting one another will be muffled and will lack the metallic quality.
Though the coin test can be used to help diagnose lung conditions, modern imaging tests, such as x-rays and magnetic resonance imaging (MRI) are more reliable. Doctors use images to positively identify problems with the lungs so that patients can be given prompt treatment. The coin test is rarely used in modern medicine.