tTG/DGP Fusion Peptide IgG

If you have stubborn gut symptoms or celiac disease runs in your family, a normal result on the usual celiac blood test does not always close the case. In specific situations, this test can flag some people the standard test quietly overlooks.

It is not meant to replace routine celiac screening for everyone, and major guidelines do not include it in the usual first-line workup. Its real value shows up when your body cannot make enough of the antibody class the standard test depends on.

What This Test Measures

This test measures tTG/DGP fusion peptide IgG (immunoglobulin G antibodies aimed at tissue transglutaminase and deamidated gliadin peptide). Tissue transglutaminase is one of your own enzymes; deamidated gliadin peptide is a fragment of gluten that your body has chemically altered. In celiac disease, the immune system mounts an attack that produces antibodies against both.

The routine first-line celiac test measures a different antibody class, IgA, directed at tissue transglutaminase. This test looks at IgG instead. That single difference is what lets it work in people whose IgA antibodies are missing or unreliable.

Most published research studies the deamidated gliadin peptide IgG antibody and combined tTG plus deamidated gliadin peptide assays rather than one specific fusion product. The findings below come from those closely related measurements, and where that distinction matters it is noted.

Why the Antibody Class Matters

Some people do not produce normal amounts of IgA, a quirk of the immune system that happens to be more common in celiac disease, affecting roughly 2 to 3 percent of patients. In these people, the standard IgA-based celiac test can come back falsely negative even when the disease is present.

Because this test reads IgG, it keeps working when IgA is low. In a study of people with IgA deficiency and celiac disease, every IgA-based test came back negative, while the IgG deamidated gliadin peptide test (a related marker, not the fusion assay specifically) correctly identified about 88 out of every 100 cases (88.2% sensitivity), close to the IgG anti-tissue-transglutaminase test at 91.2%.

Catching Cases the Standard Test Misses

Even in people who make normal IgA, a small share of biopsy-confirmed celiac cases test negative on the standard tTG-IgA screen but positive on a deamidated gliadin peptide antibody test. In one clinic series, about 10 out of every 100 confirmed celiac patients (10.3%) fit this pattern.

Even so, an isolated deamidated gliadin peptide positive on its own has a low chance of meaning true celiac disease, and this low predictive value is why current guidelines do not include the marker in routine diagnostic algorithms for people who make normal IgA. A thorough workup may still add an IgG-based marker in select situations, but a clean standard result rarely needs it.

Celiac Disease in Young Children

In very young children, the deamidated gliadin peptide IgG antibody tends to appear early and can rise before the standard test turns positive. One pooled analysis of children under two reported this IgG marker catching about 96 out of 100 cases (0.96 sensitivity), edging out the standard tTG-IgA test at about 93 out of 100 (0.93 sensitivity).

That finding is not the last word. Larger and more recent studies, including one of 348 children under two where tTG-IgA reached 100% sensitivity versus 89% for the IgG marker, found tTG-IgA performed as well or better even in this age group. The 2023 American College of Gastroenterology guidelines now recommend tTG-IgA as the preferred single test in children under two and no longer advise combining it with the IgG deamidated gliadin peptide marker. The IgG marker is also slightly less specific in this group, meaning more false alarms, so a lone positive needs careful interpretation.

When Two Positives Point Strongly to Celiac Disease

The clearest signal comes from combining this test with the standard one. In a multicenter study of adults, when both the standard tTG-IgA test and the IgG deamidated gliadin peptide test were positive above the normal cutoff, celiac disease was confirmed in 92.5% of those patients.

When both tests read more than ten times the normal cutoff, every single patient had celiac damage on biopsy (a positive predictive value of 100%). Two strong, concordant positives are close to a definitive answer.

Why a Single Positive Is Not a Verdict

This is not a simple good-number, bad-number test. Its meaning depends on the company it keeps, specifically your standard tTG-IgA result and your total IgA level. On its own, a positive here can mislead.

In children who tested positive on the deamidated gliadin peptide IgG test but had a normal standard test, only about 1 in 40 actually had celiac disease, a positive predictive value of just 2.5%, and the single true case was IgA deficient. Read alone, an isolated positive is often a false alarm. Read as part of a pattern, it becomes useful. That is the resolution: this marker is a piece of a picture, not a standalone conclusion.

Sources: Catassi et al., Nutrients 2021; Rubio-Tapia et al., American Journal of Gastroenterology 2023; Villalta et al., Clinical Chemistry 2010; Zingone et al., Digestive and Liver Disease 2024.

What this means for you: a positive result is a prompt to look at your standard tTG-IgA and total IgA together, not a diagnosis by itself. The strongest case for celiac disease is when this test and the standard test agree, especially at high levels.

When Results Can Be Misleading

• Testing after cutting out gluten: if you have already reduced or removed gluten, the immune reaction quiets and these antibodies fall, which can produce a falsely reassuring negative. Keep eating gluten regularly until testing is complete.
• Very young age: children under two can be positive here while negative on the standard test, and the marker is less specific in this group, so a lone positive needs careful interpretation.
• An isolated positive with normal IgA: in people who make normal IgA, a positive here with a normal standard test carries a low chance of true celiac disease and can trigger an unnecessary endoscopy if overread.
• IgA status unknown: without a total IgA level, you cannot tell whether a negative standard test is genuine or a false negative from IgA deficiency, which is exactly the situation where this test earns its keep.

Tracking Your Result Over Time

A single reading is a snapshot; the trajectory tells you more. If you are diagnosed with celiac disease and go gluten-free, these antibodies fall over months. In one study of children tracked after diagnosis, positive deamidated gliadin peptide results (measured as the IgA class, a related antibody) dropped from 11% at six months to 5% at one year to zero by two years.

One caveat for follow-up: normalized antibodies do not guarantee your intestine has healed. Blood antibody levels can look clean while damage persists, so it is a helpful trend line, not a final verdict on recovery.

A practical rhythm: get a baseline while still eating gluten, retest 6 to 12 months after a diagnosis and diet change to confirm the number is falling, then check at least yearly. If you have a family history but a negative baseline, periodic retesting makes sense, because celiac antibodies can develop years later.

What to Do With an Out-of-Pattern Result

If this test is positive, order or review two companions right away: your total IgA and your standard tTG-IgA. The combination tells you how to read the result. Two strong positives point hard toward celiac disease and warrant a gastroenterology referral to discuss biopsy or a biopsy-free diagnostic pathway.

An isolated positive with normal IgA and a normal standard test is usually a weak signal; retest and investigate rather than assume disease. If your total IgA is low or undetectable, this IgG marker becomes a primary tool rather than a backup. Do not start a gluten-free diet before your workup is finished, because doing so erases the very evidence a diagnosis depends on.