Editing Synovial fluid (section)

==Analysis==
Glucose (mg/dl) concentration in synovial fluid is nearly equal to [[Serum (blood)|serum]].

The cytological and biochemical analysis of human synovial fluid began around 1940 using cadaver-derived fluid and comparing characteristics to those of, for instance, bovine synovial fluid.<ref>{{Citation
 |last1=Ropes |first1=Marian W. |last2=Rossmeisl |first2=Elsie C. |last3=Bauer |first3=Walter |date=November 1940 |title=The Origin and Nature of Normal HUman Synovial Fluid |journal=[[J Clin Invest]] |volume=19 |issue=6 |pages=795&ndash;799 |pmid=16694795 |doi=10.1172/JCI101182 |pmc=435014}} ''and references therein''</ref>

=== Chemistry ===
The mucin clot test is a very old approach to determining if an inflammatory infiltrate is present.  In this test, acetic acid is added to the synovial fluid specimen.  In a normal specimen, this should lead to a congealing of the hyaluronic acid, forming a 'mucin clot.'  If inflammation is present, a mucin clot is not formed (the hyaluronic acid is degraded).<ref name=demais2009>{{Citation |last=De Mais |first=Daniel |year=2009 |title=Quick Compendium of Clinical Pathology |edition=2nd |publisher=[[ASCP Press]] |location=Chicago |isbn=9780891895671 |oclc=692198047 |url-access=registration |url=https://archive.org/details/quickcompendiumo0000mais }}{{pages needed|date=April 2013}}</ref>

Lactate is elevated in septic arthritis, usually above 250&nbsp;mg/dL.

Complement factors are decreased in rheumatoid arthritis and lupus arthritis.

=== Microscopy ===
Microscopic analysis of synovial fluid is performed to evaluate for cell count and crystals.  Crystals include [[monosodium urate crystals]], [[calcium pyrophosphate]], [[hydroxyapatite]] and [[corticosteroid]] crystals.<ref name=demais2009/>

Monosodium urate crystals are seen in [[gout]] or gouty arthritis and appear as needle-shaped negatively birefringent crystals varying in length from 2 to 20&nbsp;[[μm]]. With negative birefringence, the crystals appear yellow in parallel light and blue with perpendicular light.

[[Calcium pyrophosphate]] crystals are seen in [[pseudogout]] (also known as calcium pyrophosphate deposition disease or, CPPD). These crystals are rod-shaped or rhomboids varying in length from 2 to 20&nbsp;μm and with positive birefringence (blue with parallel light, yellow with perpendicular light).

[[Hydroxyapatite]] crystals are small and negatively birefringent. They are usually only detectable with an [[Alizarin Red S]] stain.

[[Corticosteroid]] crystals may be seen following therapeutic corticosteroid injection into the joint space. They appear blunt, jagged, and show variable birefringence.<ref name=demais2009/>

===Cracking joints===
{{main|Cracking joints}}
When the two articulating surfaces of a synovial joint are separated from one other, the volume within the joint capsule is increased and a negative pressure results. The volume of synovial fluid within the joint is insufficient to fill the expanding volume of the joint and gases dissolved in the synovial fluid (mostly [[carbon dioxide]]) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble.<ref>{{cite journal |vauthors=Unsworth A, Dowson D, Wright V | title = 'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint. | journal = Ann Rheum Dis | volume = 30 | issue = 4 | pages = 348–58 | year = 1971 | pmid = 5557778 | doi = 10.1136/ard.30.4.348 | pmc=1005793}}</ref> This process is known as [[cavitation]]. Cavitation in synovial joints results in a high frequency 'cracking' sound.<ref>Watson P, Kernoham WG, Mollan RAB. A study of the cracking sounds from the metacarpophalangeal joint. Proceedings of the Institute of Mechanical Engineering [H] 1989;203:109-118.</ref><ref>{{cite web|url=http://health.howstuffworks.com/question437.htm|title=What makes your knuckles pop?|date=3 August 2000|access-date=20 September 2016}}</ref>