We analyzed serum and synovial biomarkers of 69 patients. 31 of them suffered from a periprosthetic joint infection (PJI) and 38 from aseptic arthralgia after total joint arthroplasty. We used Receiver-Operating-Characteristic-curves to calculate the Area-under-the-curve (AUC), cutoff-values, positive (+LR), negative (−LR) and interval-Likelihood-Ratios (iLR) for predicting a PJI. The most significant parameter was synovial interleukin-6 (IL-6) (cutoff-value ≥ 30,750 pg/ml, AUC = 0.959, SE = 90.0%, SP = 94.7%, +LR = 17.27), followed by synovial lactate (cutoff-value ≥ 8.3 mmol/l, AUC = 0.844, SE = 71.4%, SP = 88.0%, +LR = 5.95), and synovial glucose (cutoff-value ≤ 44 mg/dl, AUC = 0.829, SE = 79.2%, SP = 78.6%, +LR = 3.69). IL-6 ≥30,750 pg/ml and lactate ≥10 mmol/l make a PJI very likely, IL-6 <10,000 pg/ml or lactate <4.3 mmol/l makes a PJI very unlikely. If none of these thresholds are met, physicians should use the iLR of IL-6, glucose and lactate to estimate the likelihood of PJI.