The patellar tap is a test for fluid in the knee joint. This test is likely to be positive with moderate amounts of fluid.
Excess fluid is squeezed out of the suprapatellar pouch with the index finger and the thumb. These are advanced distally from a level about 15 cm above the knee to the level of the upper border of the patella.
Then, using the tips of the fingers of the free hand, the patella is hit squarely and with force downwards.
If the test is positive then the patella can be felt striking the femur with a click and bouncing off again.
If the effusion is small or tense then the tap test will be negative.