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Difficulty: MediumOrdering Laboratory Tests

A 72-year-old woman is evaluated in the clinic for progressive fatigue, forgetfulness, and a tingling sensation in her feet over the past 6 months. Her past medical history is notable for hypothyroidism and vitiligo. Physical examination reveals decreased vibratory sensation in the bilateral lower extremities and a smooth, beefy-red tongue. A complete blood count shows a hemoglobin of 9.4 g/dL9.4 \text{ g/dL} and a mean corpuscular volume of 114 fL114 \text{ fL}. Arrange the following laboratory investigations in the most appropriate stepwise diagnostic sequence to definitively establish the diagnosis and etiology of her anemia.

  1. 1Peripheral blood smear and reticulocyte count
  2. 2Serum vitamin B12 and folate levels
  3. 3Serum methylmalonic acid (MMA) level
  4. 4Anti-intrinsic factor antibodies

Answer

The correct diagnostic sequence begins with evaluating the peripheral smear and reticulocyte count, followed by serum B12 and folate levels, then checking methylmalonic acid for equivocal results, and finally ordering anti-intrinsic factor antibodies to determine the specific etiology.
The standard diagnostic algorithm for macrocytic anemia requires a stepwise approach to minimize unnecessary healthcare costs and avoid premature closure. The sequence starts with morphological confirmation (smear/reticulocyte count), progresses to initial biochemical screening (B12/folate), moves to confirmatory functional testing (MMA) if screening is equivocal, and concludes with etiological determination (anti-intrinsic factor antibodies) once the deficiency is proven.

Step-by-Step Solution

1
Evaluate the peripheral blood smear and reticulocyte count.
Identifies hypersegmented neutrophils (megaloblastic) or elevated reticulocytes (hemolytic).
Before pursuing nutritional assays, the clinician must confirm true megaloblastic changes and exclude brisk hemolysis, which falsely elevates the MCV due to the larger size of reticulocytes.
2
Order serum vitamin B12 and folate levels.
Provides the baseline circulating levels of these critical vitamins.
This is the standard first-line biochemical evaluation for confirmed or highly suspected megaloblastic anemia to identify the deficient nutrient.
3
Check serum methylmalonic acid (MMA) levels if necessary.
Differentiates true B12 deficiency (elevated MMA) from folate deficiency (normal MMA).
Serum B12 levels can be equivocal (200-300 pg/mL). MMA is a highly sensitive functional marker of intracellular B12 deficiency and is used as a confirmatory test.
4
Test for anti-intrinsic factor antibodies.
Confirms the diagnosis of pernicious anemia.
Once B12 deficiency is proven, determining the cause is essential for long-term management. Given the patient's autoimmune history (vitiligo, hypothyroidism), pernicious anemia is highly likely, and this specific antibody confirms the etiology.

Key Concept

Stepwise diagnostic laboratory sequencing for macrocytic anemia
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Ordering Laboratory Tests

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