How do you know you have a nutrient deficiency? In the past, people had clear clinical signs of deficiency, or suffered with non-specific symptoms for years with no one knowing what the problem was. Nowadays, various biochemical tests can identify if nutrient levels are low. For each micronutrient, there appears to be an array of different testing methods that can be employed. Choice sparks controversy.
Source: Controversy in the Measurement of Vitamin B12 Levels