Abstract
Literature debates whether fluid aspirates for suspected PJI should undergo prolonged incubation for cultures. We looked at sensitivity and specificity of 14-day cultures, compared to 7-days, for aspirates from prosthetic hips and knees.
Design and methods
Conducted at a quaternary referral centre for PJIs from Jan 2017 to July 2019. Suspected PJIs who underwent aspiration, incubated 14 days and later surgical intervention with minimum three tissue samples were included.
Results
176 aspirates were included. This is an increased number compared to our historic figures (average 88 Vs 48 pts/yr).
47 patients had fluid and tissue positive (true positive), 20 fluid +ive but tissue -ive (false positive), 98 fluid and tissue -ive (true negative) and 15 fluid negative but tissue +ive (False negative). Thus, sensitivity 76%, specificity 83%, positive predictive value70% and negative predictive value 87%.
Of 88 positive aspirates, only 75% were within 7-day cultures. Low virulence organisms as Propionibacterium acnes and coagulase negative staph were grown later.
Of 48 with only one tissue sample positive, 38 were culture-negative on aspiration and 6 grew different organisms on aspirate and tissues. Also, as many were cultured later, it suggests contamination.
Conclusion
Increased numbers reflect quaternary referral nature of institution and increasing PJI load.
Modest drop in sensitivity and specificity of 14-day cultures compared to 7-day(84 and 85% respectively) is due to higher false negatives. Contamination contributes to false-ive as more tissue samples become positive (there were 1076 tissue samples due to multiple sampling Vs 176 aspirates). Higher tissue yield may also be because they are more representative. Effect of antibiotic use between samples cannot be determined.
Organism profile suggest14-day culture produces more contaminant growth despite a well-equipped microbiology lab with laminar airflow for subcultures.
Caution in interpreting 14-day results in diagnosis of PJI of Hip and Knee is advised.
