Vitamin B12 and folate are primarily requested to help diagnose the cause of macrocytic anaemia. They can be requested as follow-up tests when large red cells and a decreased haemoglobin concentration are found during a FBC test. Folate and vitamin B12 may be used to help evaluate the nutritional status of a patient with signs of significant malnutrition or malabsorption. This may include those with alcoholism and those with disorders associated with malabsorption such as coeliac disease, Crohn's disease, and cystic fibrosis. B12 and folate may also be used to help diagnose the cause of mental or behavioural changes, especially in the elderly.

In patients with known vitamin B12 and folate deficiencies, these tests may be used occasionally to help monitor the effectiveness of treatment. This is especially true in patients who cannot absorb vitamin B12 and/or folate and must have lifelong treatment. Either a serum or RBC folate test may be used to help detect a deficiency. Some doctors feel that the RBC folate test is more clinically relevant than serum folate but there is no widespread agreement on this.

Vitamin B12 and folate are primarily measured when a FBC, done routinely or as part of an evaluation of anaemia, indicates the presence of large red RBCs.

When a person, especially an elderly person, exhibits mental or behavioural changes such as irritability, confusion, depression and/or paranoia, vitamin B12 and folate may be requested to help diagnose the underlying cause. They may also be used when a patient has physical symptoms that suggest B12 or folate deficiency, including dizziness, weakness, fatigue, or a sore mouth or tongue.

When a patient has symptoms suggesting nerve damage or impairment, such as, tingling, burning, or numbness in their hands, arms, legs, and or/feet, a vitamin B12 test may be requested to help diagnose the cause and to detect the presence of a vitamin B12 deficiency.

B12 and folate may sometimes be requested when a patient shows signs of malnutrition or malabsorption or is known to have a disorder that affects nutrient absorption. When a breastfed infant has a vitamin B12 or folate deficiency, the mother may also be tested to see if she has a deficiency that is affecting both her and her child.

When a patient is being treated for B12 or folate deficiency, they may occasionally be monitored to evaluate the effectiveness of the treatment. In a person with a nutritional deficiency, this may be done as a follow-up to treatment. In a person with a condition causing a chronic deficiency, this may be part of a long-term treatment plan.

The doctor is looking for vitamin B12 and/or folate deficiency. If a symptomatic patient has decreased concentrations of vitamin B12 and/or folate, then it is likely that he has some degree of deficiency. The test results indicate the presence of deficiency, but do not necessarily reflect the severity of the anaemia or neuropathy associated with the deficiency.

If a patient with a vitamin B12 or folate deficiency is being treated with supplements (or with B12 injections), then normal or elevated results indicate a response to treatment.

High levels of B12 and folate are not usually monitored. Increased B12 may be seen in conditions such as leukemia or liver dysfunction. Increased folate may be seen with pernicious anaemia, vegetarian diets, or with a condition called bacterial overgrowth syndrome where bacteria multiply in the upper bowels.

If a patient is deficient in both vitamin B12 and folate, but only takes folic acid supplements, the vitamin B12 deficiency may be masked. The anaemia associated with both may be resolved, but the underlying neuropathy (nerve damage) will persist and may deteriorate.

The Schilling test was once used fairly routinely to confirm a diagnosis of pernicious anaemia as the cause of a vitamin B12 deficiency. It is still requested occasionally but has fallen from favour because it involves giving the patient radioactive vitamin B12. The Schilling test has been replaced, in part, by the measurement of intrinsic factor binding antibodies and parietal cell antibodies.

Drugs that can decrease B12 and folic acid levels include oral contraceptives, oestrogens, alcohol and some antibiotics.