What initial blood tests should I order to investigate suspected vitamin B12 deficiency in a patient presenting with neurological symptoms?

Order either a serum total B12 (cobalamin) or active B12 (holotranscobalamin) as the initial test for suspected vitamin B12 deficiency, unless during pregnancy or if recreational nitrous oxide use is suspected as the cause of deficiency ¹.

Use active B12 as the initial test during pregnancy ¹.

If recreational nitrous oxide use is suspected, consider plasma homocysteine or serum methylmalonic acid (MMA) as the initial test, referring to secondary care for plasma homocysteine testing ¹.

Take blood samples for diagnostic tests before starting vitamin B12 replacement ¹.