Execution
- 1Position the patient supine with the knee extended and quadriceps relaxed.
- 2Milk fluid from the suprapatellar pouch distally toward the patella.
- 3Press the patella posteriorly toward the femur with a quick tap.
- 4Feel for the patella striking the femur and floating back up through the fluid.
- 5Grade the finding and compare with the opposite side if needed.
Positive outcome
A floating patella that taps against the femur and rebounds is positive. The test usually requires a moderate to large effusion, commonly around 30 mL or more, so it is less useful for small effusions. A negative test does not exclude a small joint effusion.
Studies
| Study | Reliability | Sn | Sp | LR+ | LR− |
|---|---|---|---|---|---|
| Fritz et al. (1998), cited in Sturgill 2009 | kappa 0.21 (poor interrater agreement) | NA | NA | NA | NA |
| Sturgill et al. (2009) | reviews patellar tap reliability vs the bulge sign | NA | NA | NA | NA |
CommentMagee lists patellar tap for moderate swelling. Published interrater reliability is poor (Fritz 1998: kappa 0.21, fluctuation test kappa 0.37), making patellar tap less reliable than the bulge/stroke test (Sturgill 2009: kappa 0.61). It is more specific to a sizable effusion than the bulge sign but less sensitive for small fluid volumes. Effusion should be interpreted with injury timing, warmth, range loss, and red flags.
Low Clinical Value