A 47 year old man is brought to the ER for evaluation of left knee pain for three days. Do you know the best, next step in evaluating his condition? Let’s find out!
A 47-year-old man presents to the emergency department with left knee pain for 3 days. He reports no recent trauma or injury. His medical history includes obesity, hypertension, gout, and diabetes mellitus. Temperature is 38°C (100.4°F), blood pressure is 145/70 mmHg, pulse is 111/min, respiratory rate is 16/min, and oxygen saturation is 98% on room air. Physical examination demonstrates diffuse swelling, erythema, warmth, and tenderness of the left knee. Flexion and extension are markedly limited because of severe pain. Patellar tap testing is positive for a joint effusion.
Which of the following is the best next step in evaluation?
A. Discharge with outpatient orthopedic follow-up
B. Plain radiography of the left knee
C. Venous duplex ultrasonography of the left lower extremity
D. Arthrocentesis with synovial fluid analysis
E. MRI of the left knee
Scroll down for the correct answer!
The correct answer to today’s USMLE® Step 2 CK Question is…
D. Arthrocentesis with synovial fluid analysis
Correct: See Main Explanation.
Incorrect Answer Explanations
A. Discharge with outpatient orthopedic follow-up
Incorrect: This patient requires urgent evaluation because septic arthritis can rapidly cause permanent joint destruction and systemic infection. He should not be discharged for routine outpatient follow-up before the effusion has been evaluated.
B. Plain radiography of the left knee
Incorrect: Plain radiography may be useful when a fracture, osteoarthritis, or another bony abnormality is suspected. However, it is not the initial diagnostic test in a patient with an atraumatic, febrile knee effusion. Arthrocentesis is needed to evaluate the joint fluid.
C. Venous duplex ultrasonography of the left lower extremity
Incorrect: Venous duplex ultrasonography is indicated when deep vein thrombosis is suspected. It is not the initial test for an acutely inflamed knee with a joint effusion.
E. MRI of the left knee
Incorrect: MRI may be useful for evaluating ligamentous or meniscal injuries, osteomyelitis, or soft-tissue infection. These conditions are not the immediate priority in a patient with findings concerning for septic arthritis.
Main Explanation
This patient presents for evaluation of nontraumatic knee pain with a fever, erythema and warmth over the joint, and a joint effusion. These findings are concerning for septic arthritis but may also represent an inflammatory or crystal-induced arthritis, such as gout. Arthrocentesis with synovial fluid analysis should be performed to distinguish between these possibilities.
Patients with nontraumatic knee pain should first be assessed for an effusion and for signs of septic or inflammatory arthritis. These findings include acute joint swelling, warmth, erythema, fever, systemic symptoms, and severe pain with passive range of motion. When an effusion is accompanied by these inflammatory features, arthrocentesis should be performed. Synovial fluid analysis should include a cell count with differential, crystal analysis, Gram stain, and bacterial culture. Together, these studies help distinguish septic arthritis from crystal-induced arthropathy, such as gout or pseudogout, and from other inflammatory arthritides.
Major Takeaway
Patients with nontraumatic knee pain, a joint effusion, and other signs of septic or inflammatory arthritis should undergo arthrocentesis with synovial fluid analysis.
Want to learn more about this topic?
Watch the Osmosis video: Approach to knee pain: Clinical sciences
References
- Bunt CW, Jonas CE, Chang JG. Knee Pain in Adults and Adolescents: The Initial Evaluation. Am Fam Physician. 2018 Nov 1;98(9):576-585. PMID: 30325638.
- Calmbach WL, Hutchens M. Evaluation of patients presenting with knee pain: Part II. Differential diagnosis. Am Fam Physician. 2003 Sep 1;68(5):917-22. PMID: 13678140.
- Chan, B., Crawford, A., Kobes, P. H., Allen, H., Leake, R., Hanrahan, C. J., & Mills, M. K. (2020). Septic Arthritis: An Evidence-Based Review of Diagnosis and Image-Guided Aspiration. American Journal of Roentgenology, 215(3), 568–581. https://doi.org/10.2214/ajr.20.22773
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