Canis ISSN: 2398-2942

Synovial fluid: differential cell count and viscosity

Synonym(s): Joint fluid

Contributor(s): Vetstream Ltd


  • Synovial fluid is a dialysate of blood and normally has a low white cell count (<2 x 10<9/l or 100-3,000/mm3), minimal erythrocytes and high viscosity due to glycosaminoglycan content.
  • Inflammation and to a lesser extent degenerative or traumatic conditions decrease the viscosity of synovial fluid.
  • Inflammation increases cell count.
  • A differential white cell count will help distinguish inflammatory from non-inflammatory conditions.


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  • Normal synovial fluid forms a small glob on slide or in tube.
  • Subjective assessment of the length of string formed by allowing fluid to drop from the needle tip.

Cell count

  • By electronic or hemocytometer chamber methods.
  • Use isotonic saline as dilution fluid.
    Glacial acetic acid, often contained in white cell dilution fluid, can cause synovial fluid to clot and confuse the count.
  • Can estimate white cell count by comparing the average number of white cells per field with that of a blood specimen with a known count.
  • An estimate of normal, mildly, moderately or greatly elevated may be satisfactory. Normally 2 cells per x 40 field in body of smear.
  • Differential cell count compares polymorphonuclear with mononuclear counts.


  • Can be performed in practice laboratory.

Technique (intrinsic) limitations

  • When counts are moderately elevated and differentials unremarkable, may not be able definitively to separate degenerative from immune-mediated conditions.

Technician (extrinsic) limitations

  • Contamination with blood may obscure true white cell count.

Result Data

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Further Reading


Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Gibson N R, Carmichael S, Li A et al (1999) Value of direct smears of synovial fluid in the diagnosis of canine joint disease. Vet Rec 144, 463-465.