A

33 y/o male comes to the ED for fever, knee swelling and pain.
He has a history of joint surgery because of trauma, and prothesis, dating 4 years before.
He is HIV positive
He presents feverish (38°C), the left knee is swollen and hot.
WBC 34.000/μL CRP 109 mg/L PCT 0,05 ng/mL

Are history, clinical examination, serum test enough to rule out or in a septic arthritis?

Conclusion

No finding from the history significantly decreases or increase the probability of septic arthritis (SA), included hystory of HIV infection; only recent joint surgery (<3 months ago) or a joint prosthesis with skin infection, significantly increases the risk of septic arthritis, but attention, the absence does not rule out SA!
Data about physical examination and hystory are inconcludent, the presence or absence of classical clinical signs (pain with motion, limited motion, tender swelling, joint effusion, increased heat redness, fever > 37.5°C, axial load pain) is not useful to rule in or rule out septic arthritis.
Serum test like WBC, ESR, CRP are not useful, instead Procalcitonin is generally specific with very poor sensitivity.
…..So what?

Bibliography

C. R. Carpenter et al;
Evidence-based Diagnostics: Adult Septic Arthritis
Acad Emerg Med Aug 2011, vol 18 n 8.