The Effect of Oral Antibiotics on Synovial Fluid and Differential for the Diagnosis of Infection

Periprosthetic joint infection (PJI) following total hip or knee arthroplasty is a rare but
potentially devastating complication. Accurate diagnosis of these infections remains one of
the most challenging undertakings in orthopaedics. Clinical presentation of PJI may be subtle
and distinguishing between infection versus aseptic issues can be difficult. Currently no
diagnostic approach has been developed that accurately and unequivocally diagnoses PJI.

Multiple studies have shown the high diagnostic accuracy of synovial fluid white blood cell
count (WBC) and neutrophil percentage (%PMNs) in detecting PJI. This has led to incorporation
of these two parameters into criteria for the diagnosis of PJI. WBC and %PMN cutoffs have
been published for prosthetic hips and knees in both the acute and chronic setting.
Meanwhile, synovial fluid cell counts are believed to be of particular value when patients
present on systemic antibiotics, which have been shown to compromise intraarticular cultures
by causing false negative results.

However, the effect of antibiotics on synovial fluid cell count and differential has not been
well delineated. One prospective study by Trampuz et al. of 133 synovial fluid specimens
prior to total knee revisions noted that patients receiving antimicrobial agents had lower
leukocyte counts than did those who were not receiving antimicrobial agents. To the contrary,
a recent animal study examined intra-articular administration of the antibiotic amikacin in
horses and reported a statistically significant increase in the synovial nucleated cell
count. The effect of antibiotics on synovial fluid WBC and %PMN thus remains unclear.

Furthermore, to reduce false-negative culture results, it is recommended that patients be off
of antibiotics for a minimum of two weeks prior to obtaining samples for culture.
Investigators have shown reduced false-negative culture rates in patients not taking
antibiotics prior to surgery compared to those taking antibiotics at the time of surgery.
However, the two-week time interval is relatively arbitrary and adequate supporting data do
not exist.

The primary aim of this study is to evaluate how antibiotics affect synovial fluid leukocyte
and differential counts. A secondary aim is to assess how long patients need to be off of
antibiotic therapy to generate accurate synovial fluid cultures. Further examination of the
effects of systemic antibiotics on synovial fluid composition will provide valuable
information for clinicians caring for patients with possible PJI.

Inclusion Criteria:

1. Patient>18 years old.

2. Patient with a prosthetic hip or knee in place.

3. Patient with PJI of the hip or knee based on MSIS criteria3(Table 1).

4. Patient off of antibiotics for a minimum of two weeks prior to preoperative joint
aspiration.

5. Patient with a culture-positive preoperative joint aspiration

Exclusion Criteria:

1. Inadequate preoperative or intraoperative synovial fluid sample to perform synovial
fluid WBC, %PMN and aerobic/anaerobic cultures.

2. Pregnant women.