Theordinary light microscope: anappropriate tool forprovisional detection andidentification ofcrystals insynovial fluid

SUMMARY Todetermine iftheordinary light microscope isa useful toolinthedetection and identification ofcrystals insynovial fluid 63fluids (13withmonosodium urate(MSU),14with calcium pyrophosphate dihydrate (CPPD), one withbothtypesofcrystal, and35without crystals) were examined blindly bytwoobservers, first with an ordinary light microscope, then witha polarising light microscope. Tentative identification ofthecrystals bytheir shape was attempted. Thesensitivity oftheordinary light microscope forthedetection ofcrystals was 96-2%and100%,withspecificity of100%and97-1%forobservers 1and2,bothwith respect to thepolarising light microscope. Theconcordance between observers fortheordinary light microscope was x=090(0.897, 0.902) andforthepolarising light microscope x=0 96(0-958, 0.961). Thespecificity foridentification ofMSU andCPPDcrystals bytheir shape with respect to thepolarising light microscope was 92-3%and85-7%forobservers 1and2.Whena polarising light microscope isnotavailable examination ofsynovial fluid byan ordinary light microscope may allow a provisional diagnosis; definitive identification ofthecrystals requires a polarising light microscope. Thefinding byDrJoseph Hollander ofcrystals in thesynovial fluid obtained frompatients withgout bymeansofanordinary light microscope prompted himtoshowonesuchpreparation toDr Daniel McCarty. 'DrMcCarty's subsequent workidentifying theacicular monosodium uratecrystals (MSU)and describing theacicular, rhomboidal, andparallelepipedal calciumpyrophosphate dihydrate crystals (CPPD)iswidely known2 3;theoptical properties ofbothtypesofcrystals alloweasy identification, andthecompensated polarising light microscope hassince becomethestandard toolfor crystal identification inallrheumatological units." Unfortunately, apolarising light microscope isnot always available andwhenitisnotitmaybeuseful toknowthesensitivity andspecificity ofthewidely usedordinary light microscope-with whichthe original observation wasmade-fordetection and identification ofcrystals. To determine sensitivity andspecificity twodifferent observers analysed