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Difficulty: HardOrdering Laboratory Tests

A 28-year-old woman comes to the clinic reporting a 4-month history of progressively worsening fatigue, intermittent abdominal bloating, and loose, foul-smelling stools. She has a past medical history of type 1 diabetes mellitus, which is well managed with a basal-bolus insulin regimen. Her vital signs are within normal limits. Physical examination reveals conjunctival pallor and mild abdominal distension without focal tenderness. Laboratory studies show a hemoglobin of 10.2 g/dL, a mean corpuscular volume of 74 fL, and a serum ferritin of 8 ng/mL. Which of the following is the most appropriate next step in laboratory evaluation?

  1. Measurement of tissue transglutaminase IgA and total serum IgAAnswer
  2. B
    Measurement of tissue transglutaminase IgA and anti-endomysial IgA
  3. C
    Measurement of deamidated gliadin peptide IgG and tissue transglutaminase IgG
  4. D
    Human leukocyte antigen DQ2 and DQ8 genotyping
  5. E
    Measurement of fecal calprotectin and qualitative stool fat

Answer

Measurement of tissue transglutaminase IgA and total serum IgA
The correct sequence for establishing a diagnosis of celiac disease begins with recognizing the high clinical suspicion in this patient with type 1 diabetes, chronic diarrhea, and iron deficiency anemia. The most appropriate initial laboratory step is testing for tissue transglutaminase (tTG) IgA antibodies, which offer excellent sensitivity and specificity. However, it is a critical requirement to concurrently measure total serum IgA levels. Patients with celiac disease have a significantly higher prevalence of selective IgA deficiency compared to the general population. If total IgA is not measured, an IgA-deficient patient will yield a false-negative tTG IgA result, leading to a missed diagnosis. If total IgA is found to be deficient, the laboratory evaluation must pivot to IgG-based testing, such as deamidated gliadin peptide (DGP) IgG.

Step-by-Step Solution

1
Synthesize the clinical presentation.
The combination of chronic diarrhea, bloating, and microcytic iron deficiency anemia in a patient with an existing autoimmune disease (type 1 diabetes mellitus) strongly suggests celiac disease.
Patients with one autoimmune condition are at a markedly increased risk for others, and iron deficiency is a classic presentation of duodenal malabsorption seen in celiac disease.
2
Determine the preferred initial diagnostic serology.
Serologic testing with tissue transglutaminase (tTG) IgA is the most sensitive and specific initial screening laboratory test.
tTG IgA has high diagnostic accuracy and is cost-effective compared to direct endoscopic biopsy.
3
Identify potential laboratory pitfalls requiring concurrent testing.
Total serum IgA must be measured alongside tTG IgA.
Selective IgA deficiency is prevalent in patients with celiac disease (up to 2-3%). If total IgA is profoundly low, the tTG IgA will be falsely negative, necessitating the use of IgG-based tests instead.

Key Concept

Initial laboratory screening for celiac disease requires measurement of tissue transglutaminase (tTG) IgA alongside total serum IgA to prevent false-negative interpretation in patients with concurrent selective IgA deficiency.
Estimated Time:1m 15s
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