Report by: Anna O’Malley, Medical Student

Search checked by: Helene Svinos, Medical Student

Institution: University of Manchester, Manchester, UK

A short-cut review was carried out to establish whether the white cell count (WCC) in joint aspirate can rule in or rule out the presence of septic arthritis (SA). A total of 115 citations was reviewed, of which five partly answered the three-part question. The clinical bottom line is that the WCC alone of the joint aspirate does not have the sensitivity or specificity to rule out or rule in SA.

Three-part question

In [adults presenting to the ED with an acute hot joint] is [WBC of the joint aspirate] sufficiently specific and sensitive [to rule in/out septic arthritis].

Clinical scenario

A 20-year-old university student presents to the emergency department with a 2-day history of a hot, swollen joint. She reports no previous knee swelling and no recent trauma or knee surgery, illegal drug use, rash, uveitis or risky sexual behaviour. On examination, she is afebrile and has a left knee effusion. Her white blood cell count (WBC), erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are all raised. The joint aspirate reports an elevated WBC and negative Gram stain and culture. You wonder what value knowing the WBC and differential of the joint aspirate will give you in making a diagnosis and ruling out SA?

Search strategy

Multifile search using OVID—Medline (1950–2008), Embase (1980–2008), CINAL (1982–2008), Cochrane ({[exp. Infectious Arthritis OR hot joint.mp OR septic joint.mp OR septic arthritis.mp] AND [exp. Blood Cell Count, exp neutrophils, white cell count$.mp OR white blood cell count$.mp] AND [exp Synovial fluid OR aspirate$.mp OR synovial fluid$.mp]}) Limited to Humans, English Language and Adults

Search outcome

The following number of articles was identified from each of the databases: Medline 51 citations, Embase 57 citations, CINAL five citations, Cochrane two citations. Duplicates and irrelevant titles were removed from 115 articles, leaving a total of five relevant articles.

Comment(s)

From the evidence, sensitivity and specificity can neither rule out or rule in SA (Margaretten, Li 2004). The value at which these statistical tests have been based is not always noted and comparable among different subsets of patients studied. Although it seems that the likelihood ratio becomes more valuable diagnostically as the WBC increases, and in particular the polymorphonuclear cells, a cutoff value is what would be most use. The best evidence that involves receiver operator characteristic (ROC) analysis suggests that a value between 1500 and 2000 cells/mm³, namely polymorphonuclear cells, seems to be associated with maximum sensitivity (83%) and specificity (60–67%). This still may not be applicable to all patient groups, ie, immunocompromised (McCutchan). According to the area under the curve (AUC) ROC, WBC of the joint aspirate (jWBC) was considered fair, good and the best diagnostic test, ahead of WBC and ESR (Li, 2007). The combined sensitivity of jWBC, ESR and WBC is 100% despite their low combined specificity (0.24) (Li, 2004, 2007). If there is no elevation of these three tests, this may be sufficient to rule out SA when the clinical picture is uncomplicated; however, along with the other ancillary tests, WBC of the joint aspirate should be regarded as an adjunct to the diagnosis of SA (Li, 2007).

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▸ Kortekangas P. Synovial fluid in bacterial arthritis vs. reactive arthritis and rheumatoid arthritis in the adult knee. Scand J Rheumatol 1992;21:283–8.

▸ Krey PR. Synovial fluid leukocytosis: a study of extremes. Am J Med 1979;67:436–42.

▸ Shmerling RH. Synovial fluid tests: what should be ordered? JAMA 1999;26:1009–14.

▸ Margaretten ME. Does the adult patient have septic arthritis? JAMA 2007;297:1478–88.

▸ Li SF. Laboratory tests in adults with monoarticular arthritis: can they rule out a septic joint? Emerg J Med 2004;11:276–80.

▸ McCutchan HJ. Synovial leukocytosis in infectious arthritis. Clin Orthop Related Res 1990;257:226–30.

▸ Li SF. Diagnostic utility of laboratory tests in septic arthritis. Emerg J Med 2007;24:75–7.

▸ Daniel C. How sensitive is the synovial fluid white blood cell count in diagnosing septic arthritis? Am J Emerg Med 2007;25:749–52.