Periprosthetic Joint Infection Diagnosis
On this page
Direct answer
Periprosthetic joint infection is diagnosed by a layered algorithm, not a single test: two major criteria — a communicating sinus tract, or two positive cultures of the same organism — settle the question alone, while everything else is scored as minor criteria (elevated CRP and ESR, synovial white cell count, alpha-defensin, leukocyte esterase, histology, purulence, positive cultures). The numbers to carry are the synovial thresholds: above 3000 cells per microlitre with a polymorph fraction above 80 percent for the chronic knee, rising toward 10,000 in the acute postoperative period. Antibiotics before aspiration, and cultures held too briefly, are the two preventable errors that wreck the diagnosis.
What you must remember
- Major criteria (either one diagnoses): a sinus tract communicating with the prosthesis, or two positive periprosthetic cultures with the same organism.
- Minor criteria (ICM/MSIS scoring): elevated CRP AND ESR; elevated synovial WBC count with raised polymorph percentage; positive alpha-defensin; positive leukocyte esterase; positive histology (≥5 PMNs per high-power field); purulence at surgery; single positive culture — scored to infected, inconclusive or not infected.
- The synovial numbers: chronic knee — WBC above 3000 cells/μL with PMN fraction above 80 percent; hip commonly quoted at similar or slightly higher thresholds; acute postoperative infection uses higher counts (around 10,000) because early inflammation is physiological.
- Serology first: CRP is the more sensitive early marker, ESR slower and longer-lasting; both elevated pushes straight to aspiration of the joint for cell count, differential and culture.
- Alpha-defensin: among the most accurate single synovial tests for chronic PJI, and less affected by prior antibiotics than culture — the modern workhorse when cell counts are equivocal.
- Culture discipline: hold cultures longer than routine (up to 14 days), because Cutibacterium acnes — the shoulder's classic pathogen — needs weeks; stop antibiotics for about two weeks before aspiration when clinically safe; at surgery, three to five periprosthetic tissue samples and implant sonication improve yield over swabs.
- Differential mimics: aseptic loosening, crystal arthropathy flares, haemarthrosis, arthrofibrosis, and metal allergy (ALVAL — a lymphocyte-dominated response); in India add chronic low-grade tuberculosis, which presents with granulomas rather than pus.
- Indian practice reality: synovial cell counters and alpha-defensin assays may be unavailable outside large centres — there, paired blood and synovial cultures taken before any antibiotic, held appropriately, and frozen-section histology carry the diagnosis.
Building the diagnosis stepwise
A painful knee replacement of four years, now warm: radiographs show a loose tibia — pain explained, warmth not — so the infection workup begins. CRP is 38 and ESR 52 — both raised, so the joint is aspirated under asepsis, with antibiotics held. The synovial fluid returns 5,800 cells with 85 percent polymorphs: above 3000, above 80 percent — the chronic-knee threshold is crossed, and cultures are sent with instructions to hold them long. One culture grows coagulase-negative staphylococcus; the major criterion needs two, so the case is not closed — the decision leans on the full picture of warmth, raised serology and histology at surgery before a two-stage revision is committed. Run the ladder in reverse for the traps: a crystal flare resolves with a low cell count; a sinus tract needs no tests at all, only courage to declare the joint infected.
The same-organism rule
Why does one positive culture not diagnose? Because coagulase-negative staphylococci are skin flora and the commonest contaminants, while a single genuine culture can reflect a tiny biofilm release — the International Consensus Meeting therefore demands two positive cultures of the same organism to satisfy the major criterion. The viva then asks the discriminating questions: same organism twice, antibiotic sensitivities consistent, cultures from separate samples taken at the same sitting — a contaminant rarely repeats itself. The second favourite trap is the acute threshold: applying the 3000-cell chronic cutoff to a knee three weeks after surgery convicts healthy joints, because early postoperative inflammation legitimately runs counts near 10,000 — thresholds move with the calendar. The third is the antibiotic-before-aspiration error, which starves cultures for weeks and converts a curable single-stage candidate into an unsolved mystery.
Frequently asked questions
What are the two major criteria for periprosthetic joint infection?
A sinus tract communicating with the prosthesis, or two positive periprosthetic cultures growing the same organism.
What are the synovial fluid thresholds for chronic knee PJI?
More than 3000 white cells per microlitre with a polymorphonuclear fraction above 80 percent; acute postoperative infection uses higher thresholds, around 10,000 cells.
Why is alpha-defensin valuable in the PJI workup?
It is among the most accurate synovial markers for chronic infection and, unlike culture, is little affected by prior antibiotic exposure.
Why must cultures be held longer than routine?
Slow-growing organisms — Cutibacterium acnes classically — need extended incubation, and premature reporting at 48 hours misses them.
What does a single positive culture for coagulase-negative staphylococcus mean?
Nothing diagnostic by itself — the organism is a common contaminant; the major criterion needs the same organism from two cultures, judged alongside serology and histology.