Which laboratory tests are recommended for diagnosing vitamin B12 defi

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Recommended laboratory tests for diagnosing vitamin B12 deficiency in primary care:

Use either total vitamin B12 (serum cobalamin) or active vitamin B12 (serum holotranscobalamin) as the initial test for suspected vitamin B12 deficiency in most cases .
Use active vitamin B12 as the initial test during pregnancy .
If vitamin B12 deficiency is suspected due to recreational nitrous oxide use, use plasma homocysteine or serum methylmalonic acid (MMA) as the initial test; plasma homocysteine testing requires referral to secondary care phlebotomy services .
Take blood samples for diagnostic tests before starting vitamin B12 replacement therapy .
Interpret total or active B12 test results using defined thresholds or local laboratory reference ranges: less than 180 ng/L (133 pmol/L) total B12 or less than 25 pmol/L active B12 indicates confirmed deficiency; intermediate values require further assessment .
Consider measuring serum MMA concentrations if symptoms/signs of deficiency are present but initial total or active B12 test results are indeterminate .
Use laboratory reference ranges to interpret serum MMA and plasma homocysteine results, considering other factors that may affect homocysteine levels such as folate deficiency .

Which laboratory tests are recommended for diagnosing vitamin B12 deficiency in primary care?