Background:Hyperemesis gravidarum (HG) is a clinical condition characterized by systemic inflammation and maternal malnutrition. However, the role of a composite index integrating both inflammatory and nutritional parameters for identifying the degree of metabolic stress, as reflected by ketonuria severity, has not been investigated. This study aimed to evaluate the association between the Naples Prognostic Score (NPS), a composite index on inflammatory and nutritional biomarkers, and ketonuria severity in patients with HG.Methods:This retrospective cross-sectional study included 241 pregnant women hospitalized with a diagnosis of HG between October 2022 and October 2025. Patients were stratified into mild-to-moderate (n = 134) and severe (n = 107) ketonuria groups based on urine ketone levels. The NPS was calculated using the neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to-monocyte ratio (LMR), serum albumin, and total cholesterol levels. Group comparisons, correlation analyses, receiver operating characteristic (ROC) curve analysis, and multivariable logistic regression analyses were performed.Results:Age, body mass index (BMI), and obstetric history were comparable between groups. Neutrophil count, NLR, and NPS were significantly higher in the severe ketonuria group. In contrast, lymphocyte count, LMR, serum albumin, and total cholesterol were significantly lower, reflecting a greater inflammatory and nutritional burden in patients in the severe ketonuria group. Among all variables tested, NPS demonstrated the strongest correlation with ketonuria severity (r = 0.845; p < 0.001). ROC curve analysis revealed that NPS had excellent discriminatory performance in identifying severe ketonuria (area under the ROC curve [AUC] = 0.939; sensitivity 90.65%, and specificity 93.28% at a cutoff value ≥3). In the multivariable logistic regression analysis, an NPS ≥3 (odds ratio [OR] = 110.261; 95% confidence interval [CI]: 42.724–284.560; p < 0.001) and total cholesterol level (OR = 0.981; p = 0.037) were independently associated with severe ketonuria. The wide CI reflects model instability and shared variance between the NPS and its components; therefore, the point estimate should not be interpreted as a stable measure of effect size.Conclusions:At admission, NPS demonstrated a strong cross-sectional association with the severity of ketonuria and excellent discriminatory performance for identifying severe ketonuria. However, because both measures were obtained simultaneously, these findings should be interpreted as reflecting a concurrent association rather than true prospective prediction.