From the Guidelines
Synovial fluid cytology can reveal several cell types, including neutrophils, lymphocytes, monocytes/macrophages, synoviocytes, and eosinophils, which are crucial for diagnosing and differentiating between various joint diseases, as supported by recent clinical practice guidelines 1.
Types of Cytology in Synovial Fluid
The types of cytology that can be seen in synovial fluid slides include:
- Neutrophils, which are typically increased in infectious and inflammatory arthritis, with counts exceeding 50,000 cells/mm³ suggesting septic arthritis
- Lymphocytes, which predominate in chronic inflammatory conditions like rheumatoid arthritis
- Monocytes and macrophages, which appear in various arthritides and may contain phagocytosed material
- Synoviocytes, which are normal joint lining cells that can appear reactive in inflammatory conditions
- Eosinophils, which may indicate parasitic infections or allergic reactions
Importance of Synovial Fluid Analysis
Synovial fluid analysis is a crucial diagnostic tool for identifying crystals such as monosodium urate (gout) and calcium pyrophosphate (pseudogout) using polarized light microscopy, as recommended by the American College of Physicians 1. Malignant cells may also be observed in cases of metastatic disease or primary joint tumors. Ragocytes (neutrophils with phagocytosed immune complexes) are characteristic of rheumatoid arthritis.
Clinical Practice Guidelines
The American College of Physicians recommends using synovial fluid analysis when clinical judgment indicates that diagnostic testing is necessary in patients with possible acute gout, as stated in their clinical practice guideline 1. This approach helps differentiate between infectious, inflammatory, crystal-induced, and neoplastic joint diseases, guiding appropriate treatment decisions.
From the Research
Types of Cytology in Synovial Fluid Slides
The examination of synovial fluid can reveal various types of cells, particles, and structures that are relevant for cytodiagnosis 2. Some of the cytological findings that can be seen in synovial fluid slides include:
- Infections, such as bacterial and fungal infections 2, 3
- Non-infectious inflammatory conditions, such as osteoarthrosis, rheumatoid arthritis, and connective tissue diseases 2, 3, 4
- Tumoral conditions, including primary and metastatic solid and hematological neoplasms 2
- Traumatic, infectious, and autoimmune arthritides 4
- Arthropathies associated with crystals, such as monosodium urate (MSU) crystals in gout 3, 5
- Neutrophil extracellular traps (NETs) and their clearance by synovial macrophages 5
Cytological Examination Techniques
The cytological examination of synovial fluid involves various techniques, including:
- Macroscopic analysis of the fluid's volume, color, clarity, viscosity, and mucin clot test 3
- Native microscopic analysis for crystals and tissue fragments 3, 4
- Counting the total nucleated cell count and semiquantitative microscopic analysis for neutrophil granulocyte percentage 3
- Examination of the native preparation in polarized light 4
- Staining with Hemacolor rapid staining or other stains 3
Diagnostic Applications
The cytological examination of synovial fluid has diagnostic applications in differentiating between inflammatory and non-inflammatory joint diseases 3, as well as in diagnosing specific conditions such as gout 3, 5. The examination can also provide information on the presence of particles, including birefringent crystals and apatite-containing particles 6.