In a woman in her mid‑30s with intermittent diarrhea, mouth ulcers, occasional constipation, dermatitis‑herpetiformis–type skin lesions, on metformin, and iron‑deficiency anemia with hemoglobin 10 g/dL, should the next step be to order tissue transglutaminase immunoglobulin A (IgA) antibody testing with total serum IgA level, or proceed directly to upper endoscopy and colonoscopy?

Order Tissue Transglutaminase IgA Antibodies and Serum IgA First

In a woman in her mid-30s with iron deficiency anemia (hemoglobin 10 g/dL), intermittent diarrhea, mouth ulcers, dermatitis herpetiformis-type lesions, and on metformin, you should order tissue transglutaminase IgA antibody testing with total serum IgA level before proceeding to endoscopy.

Clinical Reasoning

This patient's presentation strongly suggests celiac disease with dermatitis herpetiformis. The constellation of symptoms—iron deficiency anemia, gastrointestinal symptoms, mouth ulcers, and particularly the dermatitis herpetiformis-type skin lesions—creates a high pretest probability for celiac disease that warrants serological screening first.

Why Serology First in This Younger Patient

The 2021 British Society of Gastroenterology guidelines explicitly recommend an age-stratified approach to investigating iron deficiency anemia 1. In younger patients with IDA, where the sensitivity of celiac serology is high and the risk of serious pathology (particularly GI malignancy) is low, checking celiac serology may be all that is required initially, with gastroscopy and biopsy only if the result is positive 1. The guidelines note that in older subjects, the sensitivity of serology for celiac disease is lower and the probability of GI malignancy is much higher, making bidirectional endoscopy more appropriate 1.

Specific Testing Recommendations

• Order tissue transglutaminase IgA (tTG IgA) with total serum IgA level 1
• The tTG IgA test has both sensitivity and specificity of 90.9% with a negative predictive value of 99.6% 1
• Total IgA measurement is necessary because IgA deficiency occurs in approximately 2% of celiac patients, which could yield false-negative results 1

The Dermatitis Herpetiformis Factor

The dermatitis herpetiformis-type lesions are particularly significant. Dermatitis herpetiformis is the cutaneous manifestation of celiac disease, and approximately 75% of patients with dermatitis herpetiformis have villous atrophy in the small bowel 2, 3. IgA antibodies against tissue transglutaminase are detected in 95% of dermatitis herpetiformis patients 4. This further elevates the pretest probability of celiac disease and supports serological testing as the initial step.

Celiac Disease Prevalence in IDA

Celiac disease is found in 3-5% of patients investigated for unexplained iron deficiency anemia 1. The guidelines strongly recommend routine serological screening for celiac disease in all patients with IDA 1.

Metformin Consideration

While metformin can cause vitamin B12 deficiency and has been associated with anemia risk 5, 6, this typically occurs with longer-term use and presents differently. Metformin-associated anemia shows an early drop in hemoglobin within 6 months but doesn't fully explain the constellation of symptoms this patient presents with 6. The B12 deficiency from metformin is also rapidly reversible with supplementation 5.

Algorithmic Approach

Step 1: Order tTG IgA and total serum IgA immediately

Step 2: Interpret results:

• If tTG IgA positive: Proceed to upper endoscopy with duodenal biopsies to confirm celiac disease 1
• If tTG IgA negative with normal total IgA: Consider colonoscopy if there are other risk factors (family history of colorectal cancer, persistent symptoms despite iron replacement) 1
• If total IgA is deficient: Order IgG-based tests (IgG tTG or IgG deamidated gliadin peptide) 1, 7

Step 3: If serology is positive, also consider skin biopsy of perilesional skin for direct immunofluorescence to confirm dermatitis herpetiformis (looking for granular IgA deposits in papillary dermis) 2, 3

Step 4: Start iron replacement therapy immediately—do not defer while awaiting investigations 1

Important Caveats

• Do not wait for endoscopy to start iron replacement therapy unless colonoscopy is imminent 1
• The patient must remain on a gluten-containing diet during testing, as both serology and biopsy require ongoing gluten exposure for accuracy 8, 9
• If serology is negative but clinical suspicion remains very high (given the dermatitis herpetiformis), proceed to endoscopy with duodenal biopsies, as seronegative celiac disease can occur in approximately 2-10% of cases 1
• In premenopausal women with hemoglobin <10 g/dL, the presence of upper GI symptoms, and absence of heavy menstrual bleeding history increases the likelihood of finding significant GI lesions 10, 11

The cost-effectiveness and diagnostic efficiency of starting with serology in this younger patient with high pretest probability for celiac disease makes it the most appropriate next step, reserving endoscopy for confirmation if serology is positive or if symptoms persist despite negative serology and iron replacement.