Special Sample Handling for Synovial Fluid Analysis
Synovial fluid requires treatment with hyaluronidase for accurate cell counts and cytospin preparation. The correct answer is C. Add hyaluronidase.
Why Hyaluronidase Treatment is Necessary
Synovial fluid presents unique challenges for laboratory analysis due to its viscous nature caused by hyaluronic acid. This viscosity creates several problems:
- Cell Distribution Issues: Without proper treatment, cells become unevenly distributed in the viscous fluid 1
- Cell Recovery Problems: Between 0.8-70% (median 5%) of immune cells can remain trapped in untreated synovial fluid 1
- Cell Identification Challenges: T and B cells are particularly affected, with 7-85% (median 22%) and 7-88% (median 23%) respectively remaining in suspension when untreated 1
Procedure for Proper Handling
When processing synovial fluid for cell counts and cytospin preparation:
- Add hyaluronidase (typically at 150 USP units/ml) to the synovial fluid 2
- Use a ratio of two drops of hyaluronidase to 0.25 cc of synovial fluid 2
- Incubate at 37°C for 5-30 minutes 3
- Proceed with cell counting and cytospin preparation
Benefits of Hyaluronidase Treatment
- Improved Cell Recovery: Ensures complete isolation of all cell populations 1
- Better Cell Morphology: Cytospin preparations of hyaluronidase-treated fluid provide superior morphological detail 2
- More Accurate Differential Counts: Prevents underdetection of specific cell types like monocytes 2
- Reduced Error Rates: Studies show that 31/46 untreated samples were error-flagged by automated analyzers, while 0/46 hyaluronidase-treated samples had errors 3
- Increased Precision: Intra-assay coefficients of variation are significantly lower for hyaluronidase-treated samples 3
Why Other Options Are Incorrect
- A. Dilute with acetic acid: This would be appropriate for total white blood cell counts in other body fluids but not for synovial fluid where hyaluronidase is specifically needed to break down hyaluronic acid
- B. Dilute with saline: Simple dilution does not address the fundamental issue of hyaluronic acid viscosity
- D. No special handling: This is incorrect as untreated synovial fluid yields inaccurate results due to its viscosity
Clinical Significance
Accurate cell counts in synovial fluid are crucial for diagnosing joint conditions. For example, in prosthetic joint infections, a leukocyte count >1700 cells/μL or >65% neutrophils is highly suggestive of infection 4. Additionally, polymorphonuclear leukocytes (PMN) are significantly less frequent in osteoarthritis fluids compared to other joint diseases 2, making accurate differential counts diagnostically valuable.
Without proper hyaluronidase treatment, clinicians risk missing inflammatory cells and making incorrect diagnoses based on artificially low cell counts or skewed differential percentages.