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• We established age- and ethnicity-related reference intervals (RIs) for serum B 12 test. • We used a modified indirect method based on the Hoffmann approach to establish RIs. • We have shown significant changes in serum B 12 concentrations across the life span by ethnicity and sex. • We have shown that Black patients and children have significantly higher B 12 concentrations. • We suggest that using universal reference RIs leads to frequent diagnostic dilemmas, especially in geographical areas with a diverse ethnic population. Age and ethnicity are known to influence serum vitamin B 12 (B 12) concentration, yet universal reference intervals (RIs) are typically applied by laboratories. Both lower and upper RI limits for B 12 are clinically relevant. Low values suggest deficiency leading to anemia and/or neurological impairment, while high values are not always an innocuous consequence of high B 12 intake but are associated with some cancers, autoimmune, liver, and renal diseases. This work aimed to establish age- and ethnicity-related RIs for B 12 using a modified indirect method based on Hoffmann's approach. A total of 72,091 anonymized B 12 results (Jan 2018-Nov 2019) were analyzed from an ethnically-diverse South-East London general practice patient population. Patients belonged to five ethnic groups: Asian, Black, White, Mixed, or Other. Multiple records for the same patient and results with missing ethnicity were excluded from the analysis of adult RIs. B 12 analyses were performed using ARCHITECT® (Abbott Diagnostics). B 12 was significantly higher in Black compared with Asian and White adults. There were no differences in B 12 between Asian and White adults. Children (all ethnicities) between 2 and 5 years old had the highest B 12. Because of the small number of children (up to the age of 13) in each ethnic-related age category, all ethnic groups were combined to obtain age-related RIs. The children's RIs ranged from 159 to 1025 pmol/L for 0–1-year-olds to 276–1102 pmol/L for 2–5-year-olds. The RIs for Black and White/Asian people >13 years of age were 166–805 pmol/L and 134–511 pmol/L respectively. The application of age- and ethnicity-appropriate RIs into diagnostic practice will provide a more accurate evaluation of B 12 status when using the B 12 test alone or in combination with other markers. [ABSTRACT FROM AUTHOR]