# Timepoint of aspiration can impact diagnostic of PJI - synovial fluid analyses exhibit a high intraindividual variation in periprosthetic joint infections of the knee

## TL;DR

This study shows that synovial fluid tests for diagnosing knee joint infections can vary significantly over time, leading to potential false-negative results.

## Contribution

The study is the first to investigate time-dependent variability in synovial fluid parameters in periprosthetic joint infections.

## Key findings

- Synovial leukocyte count (LC) can vary by up to 58% over time, risking false-negative PJI diagnoses.
- PMN% is less variable (11%) and less affected by pathogen virulence compared to LC.
- Low-virulence infections show higher variability in both LC and PMN% compared to high-virulence infections.

## Abstract

Synovial leukocyte cell count (LC) and polymorphonuclear percentage (PMN%) are key parameters in the diagnostic workup of periprosthetic joint infection (PJI). Despite ongoing debates regarding optimal thresholds—particularly for reliably excluding low-grade infections—no study has investigated potential confounders such as intra-individual, time-dependent variability of LC and PMN% in patients with PJI following total knee arthroplasty (TKA).

We conducted a retrospective, double-centre study using prospectively collected data from 35 consecutive patients with confirmed knee PJI, each of whom underwent two joint aspirations at different time points (t1 and t2; mean interval 33 ± 31 days, range 0–111 days; total 70 samples). We analysed absolute and percentage changes in LC and PMN% between t1 and t2 and identified the number of “false-negative” results potentially caused by this variability. A subset analysis was performed on cases exhibiting more than 20% variability to minimize methodological bias. Additionally, we compared LC and PMN% levels between infections caused by low- vs. high-virulence pathogens.

Mean LC among all samples was 19.969 n/µl ± 27.035 n/µl [300–135.000 n/µl], mean PMN% was 65%±28% [6-97%]. Seven (20%) of the LC results and four (11%) of the PMN% results exhibited discrepant values, either falling below or exceeding the threshold depending on the timepoint of aspiration. Regarding the time-dependent variability, there was a trend to higher percentual change for LC compared to PMN% (58% ± vs. 11%; p = 0.062). Synovial analyses of low-virulent bacteria showed significantly decreased overall LC- and PMN%-values compared to high virulent pathogen associated PJI (p < 0.001).

LC values exhibit significant intra-individual variability of up to 58% over time, which could lead to false-negative exclusion of PJI in up to 20% of cases if only a single aspiration were performed. In contrast, PMN% is less affected by timing, showing a variability of just 11%. Notably, in PJI caused by low-virulence pathogens, both LC and PMN% show approximately 80% variability, whereas PMN% variability in high-virulence infections is only 22%. LC, in particular, demonstrates a high degree of independent variability. These findings suggest that synovial LC—especially—is less reliable diagnostically than previously assumed. PMN%, on the other hand, appears to be less influenced by timing and pathogen virulence, and may therefore be the more robust parameter. In addition to timing, future research should explore other potential confounders that may impact the diagnostic reliability of both LC and PMN%.

III.

The online version contains supplementary material available at 10.1007/s00402-025-05888-8.