Periprosthetic infections are rare, but there is evidence to suggest that
their frequency may be underestimated.
No single laboratory test has perfect sensitivity and specificity for
diagnosing infection. Most tests have better specificity when they are
performed for patients in whom infection is suspected clinically rather than
when they are used as screening tests.
Screening test results that may suggest the possibility of infection
include elevation of the erythrocyte sedimentation rate and/or serum
C-reactive protein level more than three months after an arthroplasty. Most
serologic tests are difficult to interpret when the patient has an underlying
Cultures of aspirated joint fluid can be especially helpful for patients
who have symptoms suggestive of infection, but their results are best
interpreted two weeks after administration of antibiotics has been
discontinued. Joint fluid cell counts may also be helpful, but Gram stains of
joint fluid have poor sensitivity and specificity.
Criteria for diagnosing infection on the basis of frozen sections of
implant membranes have not yet been standardized, but in many laboratories
more than five neutrophils per high-power field in five or more fields
(excluding surface fibrin) has been found to be suggestive of infection.
Most polymerase chain reactions that detect the universal 16S rRNA
bacterial gene have problems with false-positive results, but combining a
universal polymerase chain reaction with subsequent bacterial sequencing can
help improve specificity. Polymerase chain reactions can detect necrotic
bacteria, so the clinical importance of positive results of this analysis in
the absence of other features of infection remains to be determined.