The quantitative basis of vitamin B 12 is to determine the level of vitamin B 12 in blood through blood test. This is the most commonly used and reliable method at present. By taking a certain amount of blood samples and using professional equipment and technology, the laboratory measures the concentration of vitamin B 12 in blood. This quantitative detection can provide accurate data, which is helpful to judge whether an individual lacks vitamin B 12 or suffers from related diseases. Blood examination is a widely accepted standard method in medical practice, which can provide reliable results and provide important basis for diagnosis and treatment.