The best way is from Signs and Symptoms, ie what are you suffering from? I'll come on to the blood test later. The full Protocol for Diagnosing and Treating B12 Deficiency can be downloaded by clicking on its title, but if you are just looking for the short Appendix A (many people use this to record their daily symptoms to check how soon they need their next injection) then Appendix A - 1 minute check can be dlowloaded by clicking on its title.
You are the best judge of how you feel. Your doctor might be the best judge of what to do about it, but there's no substitute for keeping a record of how you feel.
The key to identifying B12 deficiencies to trace symptoms back to the root cause. If you have a cough, and no other symptoms, you probably need to treat your throat. If it’s accompanied by headache and streaming nose, then you trace back, via catarrh, to a virus or cold infection. If your cough is painful, accompanied by difficulty swallowing, and accompanied by unexplained weight weight loss, then your doctor may refer you for a biopsy for cancer. If instead it’s accompanied by loss of sensation in your limbs, and a series of other neurological symptoms, then you may start to suspect a neuropathy-type disease such as B12 deficiency.
After 30 years of observations, we developed a checklist so that you can see how many body systems are affected, and therefore the likelihood that all traces back to B12 deficiency. The checklist can be downloaded from here.
B12 deficiency also affects the body’s ability for individual cells to detect hormones. This can have devastating consequences – infertility and menorrhagia (both heavy bleeding, and inter-menstrual bleeding – when the sex hormones don’t work), overwhelming tiredness and lack of energy (often lack of cortisol, which is also known as Addison’s disease), heart palpitations and a feeling of burning up accompanied by weight loss (malfunction of the thyroid gland and thyroxine), lack of ability to regulate glucose (diabetes due to insulin insensitivity or lack of production of insulin), and so on. In many cases, the damage to the endocrine system is an autoimmune damage, where the body’s own immune system, made up of white blood cells or phagocytes, destroys the cells of the endocrine gland.
You can download a protocol for identifying hypoadrenalism (adrenal insufficiency, Addison’s disease, low cortisol) below.