Knee Special Tests: Sensitivity, Specificity & Clinical Interpretation

Before diving into individual tests, it is worth understanding what sensitivity and specificity mean in a clinical context.

Sensitivity is the ability of a test to correctly identify people who have the condition (true positive rate). A highly sensitive test is useful for ruling out a diagnosis — if the test is negative and it has high sensitivity, the condition is unlikely. The mnemonic SnNout (Sensitive test, Negative result, rules Out) is helpful here.

Specificity is the ability of a test to correctly identify people who do not have the condition (true negative rate). A highly specific test is useful for ruling in a diagnosis — if the test is positive and it has high specificity, the condition is likely. The mnemonic SpPin (Specific test, Positive result, rules In) applies.

In practice, no single test has perfect sensitivity and specificity. The skill lies in combining multiple tests and integrating them with your subjective history to form a clinical impression.

The ACL is the most commonly injured knee ligament, particularly in sports involving pivoting and cutting movements.

Lachman Test The Lachman test is the gold standard for ACL assessment. With the knee at 20–30° of flexion, the examiner stabilises the femur and applies an anterior force to the tibia. A positive test shows increased anterior tibial translation with a soft or absent endpoint. - Sensitivity: 85–87% | Specificity: 91–93% - This is the most reliable single test for ACL integrity.

Anterior Drawer Test With the knee at 90° flexion and the foot stabilised, the examiner pulls the tibia anteriorly. Less sensitive than Lachman (sensitivity ~55–62%) due to hamstring guarding at 90° flexion.

Pivot Shift Test The pivot shift test assesses anterolateral rotatory instability. It is highly specific (98%) but less sensitive (24–48%) in the clinic due to patient guarding. It is most reliable under anaesthesia.

Meniscal injuries are common, particularly in the medial meniscus, and often co-occur with ACL tears.

McMurray Test With the patient supine, the examiner flexes the knee fully, then extends it while applying a valgus or varus stress combined with rotation. A palpable or audible click with pain is a positive finding. - Sensitivity: 53–58% | Specificity: 59–77% - More specific than sensitive — a positive test is meaningful, but a negative test does not rule out a meniscal tear.

Thessaly Test The patient stands on one leg with the knee at 20° flexion and rotates the body medially and laterally three times. Joint line pain or locking is a positive finding. - Sensitivity: 66–89% | Specificity: 92–97% - The Thessaly test at 20° has better diagnostic accuracy than McMurray for medial meniscal tears.

Apley Compression Test With the patient prone and the knee at 90°, the examiner applies downward compression and rotation. Pain with compression (versus distraction) suggests meniscal pathology. - Sensitivity: 13–16% | Specificity: 80–86% - Low sensitivity limits its clinical utility as a standalone test.

Valgus Stress Test (Medial Collateral Ligament) With the knee at 0° and 30° of flexion, a valgus force is applied to the knee. Laxity at 30° suggests MCL injury; laxity at both 0° and 30° suggests a more complex injury involving the posterior capsule or cruciate ligaments. - Sensitivity: 86–91% | Specificity: 96–98%

Varus Stress Test (Lateral Collateral Ligament) A varus force is applied at 0° and 30° of flexion. Isolated LCL injuries are less common than MCL injuries and are often associated with posterolateral corner injuries. - Sensitivity: 25% | Specificity: 97% - The low sensitivity means a negative test does not reliably exclude LCL injury.

Clarke's Test (Patella Grind Test) The examiner applies a distal force to the superior patella and asks the patient to contract the quadriceps. Pain is a positive finding for patellofemoral pain syndrome. - Sensitivity: 39% | Specificity: 67% - This test has poor diagnostic accuracy and should not be used in isolation.

Patellar Apprehension Test The examiner applies a lateral force to the patella with the knee in slight flexion. Apprehension (not just pain) is a positive finding for patellar instability. - Sensitivity: 39% | Specificity: 92% - High specificity makes a positive result meaningful for patellar instability.

Patellar Tap Test With the knee extended, the examiner taps the patella downward. A ballottement sensation suggests significant joint effusion. - Sensitivity: 83% for large effusions | Specificity: variable

The diagnostic accuracy of knee special tests improves significantly when multiple tests are combined. A useful clinical rule is the Ottawa Knee Rules for fracture exclusion — if the patient is over 55, has isolated patella tenderness, has fibular head tenderness, cannot flex to 90°, or cannot weight-bear, imaging is indicated.

For ligament assessment, the combination of Lachman, anterior drawer, and pivot shift provides the highest diagnostic accuracy for ACL tears. For meniscal pathology, combining McMurray with Thessaly and joint line tenderness improves sensitivity.

Always interpret special tests in the context of your full subjective and objective assessment. A positive test in a patient with no mechanism of injury and no other supporting findings should be treated with caution.