B12 Multidomain Assessment Toolkit: A structured guide for recognising, characterising and following suspected vitamin B12 deficiency in clinical practice, audit and research

The B12 Multidomain Assessment Toolkit is an expert-derived development framework for the structured clinical assessment and longitudinal follow-up of people aged 16 years and over with suspected or established vitamin B12 deficiency. The toolkit integrates seven complementary domains: clinical phenotype; risk factors and potential cause; biochemical evidence; objective clinical findings; alternative and coexisting explanations; functional impact; and longitudinal treatment response. It is designed to support transparent clinical reasoning without reducing complex evidence to a single diagnostic score. The toolkit also introduces the Multisystem Symptom Burden Index—development version, a 12-category exploratory longitudinal symptom and function measure scored from 0 to 36. The score has no validated diagnostic threshold or established minimal clinically important difference and must not be used to diagnose or exclude vitamin B12 deficiency. The framework incorporates relevant recommendations from NICE NG239 and selected published literature, alongside the author's clinical experience. It explicitly addresses limitations of isolated biochemical testing, prior supplementation, competing and coexisting diagnoses, functional outcomes, autoimmune gastritis, neurological urgency and longitudinal response to treatment. This version has not undergone formal content validation, reliability testing or prospective clinical validation. No patient-level data were used to derive or validate it. It is released primarily as a free clinician-education and development framework and is not a validated diagnostic, predictive or treatment-selection instrumen