What is the preferred initial assessment test for anterior cruciate ligament (ACL) integrity in a patient presenting with suspected knee instability, comparing the Lachman test and anterior drawer test?

Lachman Test is Superior for Initial ACL Assessment

The Lachman test should be the preferred initial assessment for suspected ACL injury, demonstrating superior diagnostic accuracy with 85% sensitivity and 94% specificity compared to the anterior drawer test's poor performance, particularly in acute settings. 1

Diagnostic Performance Comparison

Lachman Test Performance

• Pooled sensitivity of 85% (95% CI: 83-87%) and specificity of 94% (95% CI: 92-95%) across all injury timeframes, making it the most valid single test for ACL rupture 1
• Maintains consistent accuracy in both acute injuries (77.7% sensitivity) and chronic injuries (84.6% sensitivity) 2
• Best negative predictive value among all ACL tests, meaning a negative Lachman test effectively rules out ACL rupture 3
• Performed at 20-30 degrees of knee flexion with the thigh stabilized and anterior tibial translation assessed 4

Anterior Drawer Test Performance

• Markedly inferior sensitivity of only 40.9% overall, with particularly poor performance in acute settings at 22.2% sensitivity 2, 1
• Improves somewhat in chronic conditions (>2 weeks post-injury) to 53.8% sensitivity, but remains suboptimal 2
• Specificity remains acceptable at 91-95%, but the poor sensitivity makes it unreliable for ruling out ACL injury 2, 1
• Pooled analysis using bivariate random effects model showed sensitivity of only 20% with specificity of 88% 3

Clinical Application Algorithm

Initial Physical Examination Sequence

1. Perform Lachman test first as the primary screening maneuver for all patients with suspected knee instability 1
2. Add pivot shift test (98% specificity) to confirm diagnosis when Lachman is positive, though pivot shift has poor sensitivity of only 24% in acute settings 1
3. Reserve anterior drawer test for chronic injuries (>2 weeks) where it shows improved but still inferior performance 2

Interpretation Guidelines

• Positive Lachman test: High probability of ACL rupture; proceed with imaging (MRI) for surgical planning 4
• Negative Lachman test: ACL rupture effectively ruled out; consider alternative diagnoses 3, 1
• Positive pivot shift test provides highest positive predictive value for confirming ACL rupture when present 3

Critical Pitfalls to Avoid

Acute Injury Considerations

• Never rely on anterior drawer test in acute settings (within 2 weeks of injury) where sensitivity drops to 22.2% due to hemarthrosis, muscle guarding, and pain limiting examination 2
• Patient guarding and effusion significantly impair anterior drawer test accuracy but affect Lachman test less 3, 2
• Consider examination under anesthesia if clinical examination is limited by pain, though Lachman remains superior even in conscious patients 5

Technical Execution Factors

• Lachman test reliability varies by examiner experience, with intratester kappa values of 0.44-0.60 and intertester values of 0.42-0.69 6
• Proper technique requires 20-30 degrees knee flexion, not the 90 degrees used in anterior drawer test 4
• The drop leg Lachman variant (leg abducted off table side) produces 1.8-2.4 mm greater translation and may be easier to perform 5

Associated Injury Assessment

• Always assess for concomitant injuries: 19.7% of ACL ruptures have posterolateral corner involvement requiring varus stress testing 7
• Evaluate for meniscal tears and bone bruises on MRI, commonly seen on posterolateral tibial plateau and anterior lateral femoral condyle 4
• MRI demonstrates 97% diagnostic accuracy for ACL tears and should follow positive clinical examination 4

Evidence Quality Considerations

The superiority of the Lachman test is supported by multiple meta-analyses pooling data from 28 studies, with consistent findings across acute and chronic injury patterns 1. The American Academy of Orthopaedic Surgeons guidelines specifically document positive Lachman testing as the standard physical examination finding in ACL-injured patients 4. The anterior drawer test's historical prominence in orthopedic teaching does not reflect its poor diagnostic performance, particularly in the acute setting where most initial assessments occur 2.