Investigations

Take the following tests before giving a blood transfusion:

^{Macrocytic anaemia}

• serum folate ^d
• serum B ₁₂ (cobalamin) ^b
• urine methylmalonic acid (MMA) ^b or serum MMA ^b  

Why?

A normal serum cobalamin makes cobalamin deficiency unlikely.

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
anaemia macrocytosis, dementia or neuropathy b (pre-test probability: 1.2%)	cobalamin deficiency (Schilling test)	serum B 12 (cobalamin)< 133 pmol/l	2.2 (1.8 to 2.6)	2%	0.00 (0.00 to 0.33)	0%

A raised urinary or serum MMA makes cobalamin deficiency likely.

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
anaemia macrocytosis, dementia or neuropathy b (pre-test probability: 1.2%)	cobalamin deficiency ( Schilling test )	urinary MMA > 5 µg/ mg creatinine	47 (12 to 190)	32%	0.19 (0.070 to 0.53)	0.2%
suspected cobalamin deficiency b (pre-test probability: 74%)	cobalamin deficiency ( Schilling test, bone marrow )	serum MMA > 0.34 µmol/	11 (1.6 to 69)	97%	0.036 (0.0051 to 0.25)	9%