Macrocytic anemia laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Macrocytosis is the enlargement of red blood cells with near-constant haemoglobin concentration, and is defined by a mean corpuscular volume (MCV) of greater than 100 femtolitres (the precise criterion varies between laboratories).

Laboratory diagnosis

• Measurement of homocysteine and Methyl malanoic acid can help confirm and differentiate from folate deficiency.
• Both are elevated in B12 deficiency while only homocysteine is elevate in folate deficiency.
• If PA is based on low serum level, Anti-IF antibodies are confirmatory but only present in 50%.
• Anti-pareital cell Abs are more sensitive (90%) but less specific.

Schilling Test

• Dietary B12 is bound to factors that are cleaved off by acid leaving B12 free to bind to R factors secreted by the saliva and gastric juice.
• B12 bound to R factor is not absorbed, but instead requires the alkaline pancreatic secretions and proteases in the duodenum to be freed from R factor.
• Free B12 can then bind to IF where it is transported to the ileum where a specific receptor then takes up the complex. Therefore, normal B12 absorption and action are dependent of 5 things:
  • Dietary intake
  • Acid in the stomach
  • Pancreatic secretions
  • Secretion of IF by Gastric parietal cells
  • An ileum that can absorb the IF-B12 complex
• The Schilling test is designed to test the different components of this system.
• First, 1mg IM B12 is given to saturate transcobalmin.
• Radiolabeled B12 is then given orally.
• If it can be absorbed, then >9% will be excreted in the urine in 24 hours and the rest is eliminated in the feces undetected.
• Renal insufficiency causes a falsely low level.
• It may be spuriously normal in patients without gastrectomy.
• Since, acid is required to free dietary B12 from binding factors.
• Radiolabeled b12 is not bound to these factors. Therefore, persons with impaired acid and pepsin production can absorb too little dietary B12, but will absorb free B12 normally.
• Part two of the Schilling test is administering radiolabeled b12 with OF to see if the problem corrects. This should differentiate PA from those with intestinal malabsorption. Remember, B12 deficiency affects the intestinal mucosal cells and causes malabsorption. Therefore, Part II should only be done 4 weeks after replacement, giving the mucosa time to regenerate in PA.
• If Part II is abnormal, Part III is basically a repeat of Part I after a course of antibiotics/vermicides.
• If this is abnormal, an malabsorptive cause is implicated.

References

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