The Clinical and Phenotypic Significance of Sub-Threshold JAK2 V617F Signals in Patients Classified as JAK2 V617F-Negative

Background: This study investigates the relationship between clinical and hematological parameters and sub-threshold positive findings in cases with suspected hematological malignancies subjected to Janus Kinase 2 (JAK2) V617F qPCR testing. Methods: Adult patients referred to the Medical Genetics Clinic from the Erciyes University Faculty of Medicine Hematology Department between 2022 and 2024 for JAK2 V617F analysis due to suspected hematological malignancy were evaluated in this retrospective, cross-sectional study. Registered data from 2394 patients were analyzed, and 173 cases with a JAK2 V617F allele burden below 0.1% but above the negative control value were included in the study. SPSS version 25.0 software was used for statistical analysis. Results: More than half of the patients (56.6%) had no definitive hematological diagnosis at initial testing, while 28.9% developed a hematological disorder during follow-up, including overt MPN in 13.3%. Significant differences across VAF groups were observed for leukocytosis (p = 0.004), neutrophilia (p = 0.030), and monocytosis (p = 0.024), with the highest frequencies in Group 3 (0.041–0.099%). ROC analyses identified closely clustered optimal cut-offs around 0.027–0.029%, whereas no independent predictor of higher sub-threshold allele burden emerged in multivariate analysis (p > 0.05). Conclusion: Even sub-threshold levels of the JAK2 V617F mutation may exhibit specific clinical manifestations and may be associated with constitutional symptoms and leukocytosis in particular. The study findings also showed a lower awareness of the JAK2 test in clinical practice, and that it is significantly employed in off-label cases. In order to accurately interpret low-allele-burden results, molecular findings must be evaluated in conjunction with hematological parameters and appropriate clinical indications. The findings underscore the importance of applying the JAK2 V617F PCR test selectively and based on clinical suspicion.