USMLE® Step 2 Question of the Day: Arthrocentesis
Published on Sep 18, 2024. Updated on Sep 6, 2024.
A 73-year-old man presents to the emergency department for evaluation of right knee pain and swelling for the past 2 days. The patient has been unable to ambulate secondary to the pain. Past medical history includes hypertension, gout, diabetes, osteoarthritis, and obesity. Temperature is 38°C (100.4°F), blood pressure is 125/71 mmHg, pulse is 98/min, respiratory rate is 14/min, and oxygen saturation is 99% on room air. On physical examination, there is a palpable effusion, warmth, and marked swelling of the right knee. The patient is unable to bear weight without significant pain. An arthrocentesis is performed. The synovial fluid analysis results from later that day are shown below. Which of the following is the most likely diagnosis?
Laboratory value | Result |
Color | Opaque |
Culture/Gram stain | Moderate polymorphonuclear cells on Gram stain No growth on culture thus far |
Cell Count | 77,532 cells/microL, 95% polymononuclear leukocytes |
Crystal Analysis | Negative |
A. Septic arthritis
B. Osteoarthritis
C. Rheumatoid arthritis
D. Gout
E. Calcium pyrophosphate deposition disease
Scroll down for the correct answer!
The correct answer to today's USMLE® Step 2 CK Question is...
A. Septic arthritis
Before we get to the Main Explanation, let's see why the answer wasn't B, C, D, or E. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
Today's incorrect answers are...
B. Osteoarthritis
Incorrect: Synovial fluid analysis from osteoarthritic joints typically have cell counts <2000 cells/microL. This patient's synovial fluid analysis and clinical presentation are more consistent with septic arthritis.
C. Rheumatoid arthritis
Incorrect: Synovial fluid analysis in a patient with rheumatoid arthritis is typically inflammatory, with a white blood cell count >2000 cells/microL, but generally the range is 5000-50,000 cells/microL. This patient's synovial fluid analysis and clinical presentation are more consistent with septic arthritis.
D. Gout
Incorrect: Synovial fluid analysis in patients with gout is typically inflammatory, with a white blood cell count >2000 cells/microL, but generally the range is 5000-50,000 cells/microL. Patients with gouty arthritis will also have evidence of negatively birefringent crystals. This patient's synovial fluid analysis and clinical presentation are more consistent with septic arthritis.
E. Calcium pyrophosphate deposition disease
Incorrect: Synovial fluid analysis in patients with calcium pyrophosphate deposition disease (pseudogout) is typically inflammatory, with a white blood cell count >2000 cells/microL, but generally in the range of 5000-50,000 cells/microL. Patients will also have evidence of calcium pyrophosphate crystals. This patient's synovial fluid analysis and clinical presentation are more consistent with septic arthritis.
Main Explanation
This patient presents for evaluation of unilateral knee swelling and pain. Given this patient's fever, joint effusion, limited range of motion, significant pain with range of motion, and inability to bear weight on the affected joint, septic arthritis is likely. Arthrocentesis and synovial fluid analysis shows opaque fluid with a white blood cell count of 77,532 cells/microL, with 95% polymononuclear leukocytes which is consistent with septic arthritis.
The diagnosis of septic arthritis is both clinical as well as dependent on results from synovial fluid analysis. Synovial fluid analysis can be used to differentiate between non-inflammatory causes of arthritis like osteoarthritis and inflammatory causes such as rheumatoid arthritis and septic arthritis. What separates septic arthritis is that there are typically >50,000 cells/microL with >90% of those cells being polymorphonuclear cells, and gram stain and culture will be positive.
Major takeaway
Arthrocentis and synovial fluid analysis in patients with septic arthritis typically shows purulent fluid with leukocyte counts (most of which are neutrophils) of 50,000-150,000 cells/microL.
References
Nair R, Schweizer ML, Singh N. Septic Arthritis and Prosthetic Joint Infections in Older Adults. Infect Dis Clin North Am. 2017 Dec;31(4):715-729. doi: 10.1016/j.idc.2017.07.013. PMID: 29079156. https://www.sciencedirect.com/science/article/pii/S0891552017300673?via%3Dihub
Expert Panel on Musculoskeletal Imaging; Pierce JL, Perry MT, Wessell DE, Lenchik L, Ahlawat S, Baker JC, Banks J, Caracciolo JT, DeGeorge KC, Demertzis JL, Garner HW, Scott JA, Sharma A, Beaman FD. ACR Appropriateness Criteria® Suspected Osteomyelitis, Septic Arthritis, or Soft Tissue Infection (Excluding Spine and Diabetic Foot): 2022 Update. J Am Coll Radiol. 2022 Nov;19(11S):S473-S487. doi: 10.1016/j.jacr.2022.09.013. PMID: 36436971. https://www.sciencedirect.com/science/article/pii/S1546144022006470?via%3Dihub
Tigges S, Stiles RG, Roberson JR. Appearance of septic hip prostheses on plain radiographs. AJR Am J Roentgenol. 1994 Aug;163(2):377-80. doi: 10.2214/ajr.163.2.8037035. PMID: 8037035.https://www.ajronline.org/doi/10.2214/ajr.163.2.8037035
Osmon DR, Berbari EF, Berendt AR, Lew D, Zimmerli W, Steckelberg JM, Rao N, Hanssen A, Wilson WR; Infectious Diseases Society of America. Diagnosis and management of prosthetic joint infection: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2013 Jan;56(1):e1-e25. doi: 10.1093/cid/cis803. Epub 2012 Dec 6. PMID: 23223583. https://pubmed.ncbi.nlm.nih.gov/23223583/
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