Investigations

• Blood count
  • haemoglobin  ^a
  • mean cell volume  ^a
  • red cell distribution width ^a

• Blood film

Why?

A low MCV makes iron-deficiency more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability
anaemia a (pre-test probability: 36%)	iron-deficiency anaemia (bone marrow aspiration)	MCV ≤ 70 µm³	12 (6.1 to 19)	87%
		70 - 75 µm³	3.3 (2.0 to 4.7)	65%
		75 - 80 µm³	1.0 (0.69 to 1.3)	36%
		80 - 85 µm³	0.91 (0.71 to 1.1)	34%
		85 - 90 µm³	0.76 (0.56 to 0.96)	30%
		≥ 90 µm³	0.29 (0.21 to 0.37)	14%

Macrocytosis and hypersegmented neutrophils make cobalamin deficiency more likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
low-serum cobalamin c (pre-test probability: 58%)	cobalamin deficiency (response to cobalamin)	macrocytosis (MCV > 100 fl)	5.4 (2.6 to 11)	89%	0.41 (0.30 to 0.55)	37%
		normal white cell count	13 (4.9 to 33)	95%	0.15 (0.088 to 0.25)	18%
		abnormal blood smear (macrocytosis and hypersegmented neutrophils)	25 (3.6 to 180)	98%	0.34 (0.24 to 0.49)	36%

Note:

• Patients with significant cobalamin deficiency may have neither significant anaemia nor macrocytosis. ^c