Longitudinal Trends and Analytical Consistency of Folate and Vitamin B12 Biomarkers: Two Decades of Population-Based Data and Diagnostic Implications

Background/Objectives: Vitamin B12 (cobalamin) and folate (vitamin B9) are essential cofactors in one-carbon metabolism required for DNA synthesis, methylation, and genomic stability. Deficiencies in these nutrients can cause megaloblastic anemia, neurological dysfunction, and hyperhomocysteinemia, linking micronutrient imbalance to cardiovascular and neurocognitive outcomes. Population-based surveillance of these biomarkers provides insight into nutritional trends and supports analytical standardization. Methods: This retrospective study included all routine plasma (P) vitamin B12 and folate measurements performed at Uppsala University Hospital from 2005 to 2024 (n = 647,302 and 578,509, respectively). Data were extracted from the laboratory information system and summarized using annual medians, percentile distributions, and coefficients of variation (CV). Linear regression was used to validate the method comparison and assess the impact of the 2021 transition from the Abbott Architect to the Roche cobas platform. Descriptive statistics summarized the temporal and seasonal patterns of P-vitamin B12 and P-folate. Results: Median P-vitamin B12 concentrations remained stable (340–370 pmol/L; median CV = 4.6%), while P-folate increased from 10.5 to 15.5 nmol/L (median CV = 12.9%) from 2005 to 2024. Low P-folate (