Abstract
OBJECTIVE: To determine free light chains (FLC) in cerebrospinal fluid (CSF) and serum, calculate the FLC index, and establish a model to evaluate its application value in diagnosing neurosyphilis. METHODS: This study included 87 syphilis patients, comprising 48 neurosyphilis patients (NS group) and 39 non-neurosyphilis patients (n-NS group). CSF and serum samples were collected from all patients to measure kappa free light chains (FLC-κ) and lambda free light chains (FLC-λ). The FLC index, FLC-κ index, and FLC-λ index were calculated. Receiver operating characteristic (ROC) curves were used to evaluate these indices. A neurosyphilis diagnostic model based on the FLC index was established and its visualization was verified. RESULTS: Statistically significant differences were observed between the two groups in serological syphilis screening, CSF nucleated cell count, CSF microprotein, CSF IgG, IgG synthesis rate, IgG index, and albumin quotient (P < 0.05). The areas under the ROC curves (AUC) for the FLC-κ index, FLC-λ index, and albumin quotient were 0.998 (95% CI: 0.990-1.000), 0.988 (95% CI: 0.961-1.000), and 0.635 (95% CI: 0.496-0.751), respectively. In the FLC-κ index model, the FLC-κ index (P < 0.05) significantly affected the diagnosis of neurosyphilis, while in the FLC-λ index model, only the FLC-λ index (P < 0.05) had a significant effect. The training ROC curve AUC for the diagnosis model based on the FLC-κ index was greater than 0.950. Its calibration curve showed good predictive performance when the predicted probability was close to 80%, but poor performance when the predicted probability was 40%-60%. The AUC for the diagnosis model based on the FLC-λ index was 0.926, with good predictive performance when the predicted probability was close to 85%, and poor performance when the predicted probability was 40%-70%. CONCLUSION: FLC-κ and FLC-λ have high diagnostic performance for neurosyphilis, and the FLC index has certain application value in the diagnosis of neurosyphilis.