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Common Test-Ordering Errors
Part 5: Overordered Tests

Plasma Thiamin
Serum Folate
Zinc Protoporphyrin (ZPP)


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Simplified Versions of Vitamin-Dependent Pathways

Slide 9

August 2010

This next slide shows simplified versions of 2 vitamin-dependent pathways. The first involves both folate and B12; and the second involves B12 only. I show these, not only because disturbances in these pathways are central to the pathologic changes that occur in folate and B12 deficiencies, but also to demonstrate how homocysteine and methylmalonic acid levels can help determine the cause of megaloblastic anemia.

In B12 deficiency, both homocysteine and methylmalonic acid concentrations are increased; whereas in folate deficiency, only homocysteine is increased and MMA is normal.

Simplified Versions of Pathways


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