Serum LDH and NLR as Diagnostic Biomarkers for Drooling in Parkinson's Disease

Objective Drooling, a prevalent non-motor symptom in Parkinson’s disease (PD), lacks standardized diagnostic biomarkers. This study investigated the association between systemic inflammatory markers—serum neutrophil-to-lymphocyte ratio (NLR), systemic immune-inflammation index (SII) and serum lactate dehydrogenase (LDH)—and drooling in PD. Materials and methods A total of 127 PD patients and 84 healthy controls (HC) were recruited. PD patients were divided into drooling (PD-DR, n = 62) and non-drooling (PD-NDR, n = 65) groups. Hematological parameters, clinical scales, and neuropsychological assessments were analyzed. Statistical analysis was performed to compare clinical characteristics, logistic regression for risk factors, receiver operating characteristic (ROC) curves for diagnostic accuracy, and restricted cubic splines (RCS) for nonlinear associations. Results The PD-DR group manifested significantly elevated levels of NLR, SII, and LDH compared to HC (p < 0.001). Furthermore, PD-DR patients also exhibited significantly elevated NLR ( p  = 0.011) and LDH ( p  = 0.004) levels when compared to PD-NDR patients. Multivariate logistic regression identified LDH levels (OR = 1.014, 95% CI 1.000-1.027, p  = 0.043) as independent risk factors for drooling in PD patients. ROC analysis indicated that a serum LDH level of 174.50 mmol/L could differentiate between PD patients with and without drooling with an AUC of 0.662, sensitivity of 58.10%, and specificity of 70.80%. RCS analysis revealed a linear relationship between LDH and drooling severity ( p  = 0.032) and a nonlinear "U"-shaped association for NLR ( p  = 0.048). Conclusions This study identifies serum LDH and NLR as novel biomarkers correlating with drooling severity in PD, highlighting LDH as an independent risk factor. Clinical relevance: Monitoring LDH/NLR dynamics offers a cost-effective strategy for early detection and management of drooling severity in Parkinson's disease.