Patient demographic data
The clinical characteristics of the PJI and aseptic groups are presented in Table 2. We included a total of 26 PJI cases and 26 cases without infection, including 27 (52%) hip and 25 (48%) knee prostheses in our study. There were no significant differences in sex, age or BMI between the PJI group and the aseptic group (p > 0.05). All patients have undergone revision surgeries.
Comparison of indicators between the PJI group and the aseptic group
We compared the values of coagulation-related indicators, ESR, serum D-lactate, synovial D-lactate, SF WBC count and SF PMN%, and the results are shown in Table 3. There were significant increases in prothrombin time (PT), international normalized ratio (INR), activated partial thromboplastin time (APTT), FIB, ESR, serum D-lactate, synovial D-lactate, SF WBC count and SF PMN% (p < 0.05), while there was no significant difference in the coagulation biomarker of thrombin time (TT) between the PJI group and the aseptic group (p > 0.05).
Performance of serum D-lactate
The AUC of serum D-lactate was 0.80 when 1.14 mmol/L was chosen as the optimal cutoff value. The sensitivity and specificity of the D-lactate test were 88.46% and 73.08%, respectively. In 7 cases of aseptic failure, D-lactate concentrations increased above the cutoff, including 5 aseptic cases with ESR under the threshold, and the other 2 cases were also false positive in synovial D-lactate and ESR. Three patients were diagnosed with PJI according to the applied definition criteria and showed negative results for D-lactate. Of these, one case was diagnosed with a sinus tract, the second was confirmed by finding Streptococcus sanguis in the prosthesis ultrasonic vibration fluid, and the last was based on multiple fulfilled criteria (p < 0.001) (Table 4) (Fig 1 A and B).
Comparison of serum D-lactate with ESR and coagulation biomarkers
Based on our data, the ROC curves showed that FIB, ESR and serum D-lactate had the highest AUC, at 0.80, indicating that they had good performance in the diagnosis of PJI (Table 4) (Fig 1 A and B). Although the AUC values for FIB, ESR and serum D-lactate were the same, serum D-lactate had the highest sensitivity and NPV of 88.46% and 86.40% at the optimal threshold of 1.14 mmol/L, followed by FIB and ESR, which had a sensitivity and NPV of 80.77% and 79.17% at the optimal threshold of 3.27 g/L and 73.08% and 74.07% at the optimal threshold of 32.00 mmol/h, respectively. In addition, no significant differences in specificity or PPV were observed in FIB, ESR and serum D-lactate, the distribution of which is depicted in Table 4 and Fig 1C, D and E.
The other coagulation biomarkers of PT, INR, APTT, FDP and D-dimer did not have ideal AUCs, at 0.69, 0.68, 0.71, 0.69 and 0.67, respectively, indicating that they are of limited value for diagnosing PJI. In particular, for the novel biomarker D-dimer, the optimal threshold was 265.00 ng/ml, with a sensitivity, specificity, PPV, and NPV of 76.00%, 60.00%, 65.52% and 69.57%, respectively. For FDP, the optimal threshold was 2.46 µg/ml, which resulted in a sensitivity, specificity, PPV, and NPV of 76.00%, 64.00%, 67.86% and 70.83%, respectively. (Table 4).
Comparison of synovial D-lactate with leukocyte count and polymorphonuclear neutrophil percentage
Synovial markers of synovial D-lactate, SF WBC count and SF PMN% are depicted in Table 5 and Fig 2. The AUC curves for synovial D-lactate, SF WBC count and SF PMN% were 0.87, 0.80, and 0.88, respectively. The optimal predictive cutoff of synovial D-lactate for the diagnosis of PJI was 1.56 mmol/L (sensitivity 95.65%, specificity 68.00%, PPV% 73.33 and 81.82%), whereas the optimal predictive cutoffs for SF WBC count and SF PMN% were 9972.00 and 89.00%, respectively, demonstrating a sensitivity, specificity, NPV and PPV of 70.00%, 87.50%, 90.00% and 75.00% and 80.00%, 100.00%, 100.00% and 81.25%, respectively.
Comparison of serum D-lactate and synovial D-lactate
In the PJI group, the level of synovial D-lactate was obviously higher than that of serum D-dimer (p < 0.05), while the ROC curve showed that synovial D-lactate also had a greater AUC, at 0.88; thus, 1.56 mm/L was chosen as the optimal threshold. When using the optimal threshold to detect synovial D-lactate, the sensitivity of 95.65% was apparently higher than that of serum D-dimer, which was 88.46%, in contrast with its lower specificity, PPV and NPV (68.00%, 73.33% and 81.82%, respectively) than those of serum D-dimer (73.08%, 76.70% and 86.40%, respectively); However both struggle to be highly effective in predicting PJI, especially in terms of high sensitivity (Fig 3 A, B and C).