Blood Tests for Bloating and Digestive Issues
Persistent bloating, gas, and gut problems? A targeted blood test panel can uncover coeliac disease, nutrient malabsorption, inflammation, and other hidden causes that imaging alone cannot detect.
Why Blood Tests for Gut Problems?
When most people think of digestive investigations, they think of endoscopy and colonoscopy. But blood tests are often the crucial first step that determines which invasive investigations (if any) are actually needed.
Blood tests reveal what is happening because of your gut problem, not just what the gut looks like. Coeliac disease damages the small intestine so gradually that symptoms can be subtle for years — but the nutrient deficiencies it causes (iron, folate, vitamin D, calcium) show up on blood tests long before the damage is visible on imaging.
A well-chosen panel of blood tests can distinguish inflammatory conditions (Crohn's, ulcerative colitis) from non-inflammatory ones (IBS, coeliac disease), identify specific nutrient deficiencies that point to the location of the problem, and screen for coeliac disease with 95% accuracy — all from a single blood draw.
The Essential Digestive Health Panel
These are the blood tests your doctor should consider when investigating persistent bloating, gas, diarrhoea, constipation, or abdominal discomfort. Each one reveals a different piece of the puzzle.
Coeliac Screen (tTG-IgA + Total IgA)
What it reveals: Tissue transglutaminase IgA (tTG-IgA) is the single most important digestive blood test. It is approximately 95% sensitive and 95% specific for coeliac disease. However, 2-3% of coeliac patients are IgA deficient, which produces a false-negative tTG-IgA result. This is why Total IgA must always be tested alongside it — if IgA is low, your doctor should order IgG-based tests instead.
Clinical note: 1 in 70 Australians have coeliac disease, yet 80% remain undiagnosed. The average time to diagnosis is 11.7 years. CRITICAL: You must be eating gluten-containing foods for at least 6 weeks before this test, otherwise the antibodies disappear and you will get a false-negative result.
Iron Studies (Ferritin, Serum Iron, TIBC)
What it reveals: Unexplained iron deficiency is frequently the only sign of coeliac disease or other malabsorption conditions. Iron is absorbed in the duodenum — the exact section of the small intestine damaged by coeliac disease. Persistent iron deficiency that does not respond to supplements is a strong indication for coeliac testing.
Clinical note: Low ferritin with normal or low serum iron and elevated TIBC is the classic malabsorption pattern. If iron supplements are not raising your ferritin after 3 months, malabsorption should be investigated.
Vitamin B12
What it reveals: B12 is absorbed in the terminal ileum — the end of the small intestine. Low B12 levels suggest malabsorption in this area, which occurs in Crohn's disease, bacterial overgrowth (SIBO), and pernicious anaemia. B12 deficiency can also occur in coeliac disease, though folate and iron deficiencies are more common.
Clinical note: Symptoms of B12 deficiency include tingling in hands and feet, difficulty concentrating, and a smooth tongue. If B12 is low alongside bloating, consider SIBO testing or investigation for ileal disease.
Folate
What it reveals: Folate is absorbed in the jejunum — the upper small intestine — which is the primary site of damage in coeliac disease. Low folate is one of the earliest and most common nutrient deficiencies in undiagnosed coeliac disease, often appearing alongside iron deficiency.
Clinical note: The combination of low iron AND low folate with normal B12 is highly suggestive of coeliac disease. Red cell folate reflects longer-term stores (2-3 months) and is more reliable than serum folate.
Vitamin D (25-Hydroxyvitamin D)
What it reveals: Vitamin D is a fat-soluble vitamin, meaning any condition that impairs fat absorption will lower vitamin D levels. This includes coeliac disease, Crohn's disease, pancreatic insufficiency, and bile acid malabsorption. Vitamin D is also essential for maintaining gut barrier integrity and regulating the gut immune system.
Clinical note: Low vitamin D alongside other malabsorption markers (iron, folate, calcium) strengthens the case for a malabsorptive condition. Vitamin D deficiency itself worsens gut permeability, creating a vicious cycle.
Calcium
What it reveals: Calcium absorption depends on vitamin D and occurs primarily in the duodenum and jejunum. Malabsorption from coeliac disease or Crohn's disease can deplete calcium stores, leading to osteopenia and osteoporosis. The body compensates by releasing parathyroid hormone (PTH), which pulls calcium from bones.
Clinical note: Always check calcium alongside vitamin D and PTH. Low calcium with elevated PTH (secondary hyperparathyroidism) is a hallmark of chronic malabsorption. A DEXA bone scan may be warranted.
Full Blood Count (FBC)
What it reveals: The FBC reveals the pattern of anaemia, which helps localise where malabsorption is occurring. Microcytic anaemia (low MCV) points to iron deficiency, suggesting upper small bowel disease. Macrocytic anaemia (high MCV) points to B12 or folate deficiency. A dimorphic pattern (both large and small red cells) can occur when multiple nutrients are malabsorbed simultaneously.
Clinical note: Elevated white blood cell count may indicate infection or active inflammatory bowel disease. Low platelets can occur with severe liver involvement. Elevated eosinophils may suggest food allergy or parasitic infection.
CRP (C-Reactive Protein)
What it reveals: CRP is the key blood test that distinguishes inflammatory bowel disease (Crohn's, ulcerative colitis) from non-inflammatory conditions like IBS and coeliac disease. In IBD, the immune system attacks the gut wall, producing systemic inflammation that CRP detects. In IBS and coeliac disease, CRP is typically normal.
Clinical note: A normal CRP with persistent gut symptoms makes IBD unlikely and favours IBS or coeliac disease. An elevated CRP with gut symptoms warrants urgent gastroenterology referral for colonoscopy.
Liver Function (ALT, AST, GGT)
What it reveals: Approximately 40% of adults with untreated coeliac disease have mildly elevated liver enzymes at diagnosis. The mechanism is not fully understood but likely involves increased intestinal permeability ("leaky gut") allowing bacterial products to reach the liver via the portal vein. These abnormalities typically normalise within 6-12 months of a gluten-free diet.
Clinical note: Low albumin suggests poor protein absorption or chronic inflammation. Elevated ALP alongside low calcium and vitamin D may indicate metabolic bone disease from malabsorption.
Thyroid (TSH)
What it reveals: Hypothyroidism is a commonly overlooked cause of constipation and bloating. Thyroid hormones regulate gut motility — when they are low, the entire digestive tract slows down, leading to constipation, bloating, and a feeling of fullness. Importantly, autoimmune thyroid disease (Hashimoto's) and coeliac disease frequently co-exist because both are autoimmune conditions.
Clinical note: If you have coeliac disease, your risk of thyroid autoimmunity is 2-5 times higher than the general population. If you have thyroid disease, ask about coeliac screening and vice versa.
Coeliac Disease: The #1 Mistake People Make
You must be eating gluten for at least 6 weeks before the blood test. This is the single biggest mistake patients make. Many people start a gluten-free diet because they feel better, then get tested — and the result comes back negative because the antibodies have already dropped. This creates a dangerous false sense of security and a missed diagnosis.
The recommended gluten challenge is eating the equivalent of 2 slices of wheat bread daily for at least 6 weeks (some guidelines say 8-12 weeks for the most reliable result). If you have already gone gluten-free, talk to your doctor about whether a gluten challenge is appropriate before testing.
Australian statistics: Coeliac disease affects approximately 1 in 70 Australians, but only 20% are diagnosed. The average time from symptom onset to diagnosis is 11.7 years. It is significantly more common in people with type 1 diabetes, thyroid disease, Down syndrome, and Turner syndrome.
IBD vs IBS: How Blood Tests Tell Them Apart
Inflammatory bowel disease (Crohn's disease and ulcerative colitis) and irritable bowel syndrome (IBS) can produce similar symptoms — bloating, abdominal pain, altered bowel habits. But they are fundamentally different conditions. IBD involves measurable inflammation and tissue damage. IBS does not. Blood tests are one of the most reliable ways to distinguish them.
| Feature | IBD (Crohn's / UC) | IBS |
|---|---|---|
| CRP / ESR | Elevated | Normal |
| Blood in stool | Common | Rare |
| Weight loss | Common | Uncommon |
| Night symptoms | Yes — wakes from sleep | No — rarely wakes from sleep |
| Fever | Sometimes | No |
| Anaemia | Common (iron deficiency) | Uncommon |
| Blood tests overall | Abnormal (anaemia, inflammation, low albumin) | Usually normal |
| Age of onset | 15-35 years most common | Any age |
| Family history | Strong genetic component | Mild familial tendency |
Nutrient Malabsorption Patterns: What Your Deficiencies Reveal
Different nutrient deficiencies point to different parts of the gut being affected. Gastroenterologists use these patterns as diagnostic clues. A single deficiency might be dietary, but a pattern of deficiencies almost always indicates malabsorption.
| Deficiency Pattern | Most Likely Cause |
|---|---|
| Iron + Folate (normal B12) | Coeliac disease (duodenum/jejunum damage) |
| B12 only (normal iron and folate) | Pernicious anaemia, ileal Crohn's disease, or SIBO |
| Fat-soluble vitamins (D, A, E, K) | Pancreatic insufficiency, bile acid malabsorption, or severe coeliac |
| Multiple nutrients (iron, folate, B12, D, calcium) | Severe coeliac disease or extensive Crohn's disease |
| Iron only (normal folate and B12) | Blood loss (peptic ulcer, colorectal), or mild coeliac |
| Low albumin + multiple deficiencies | Protein-losing enteropathy, severe IBD, or advanced coeliac |
What to Ask Your Doctor
If you have been experiencing persistent bloating, abdominal discomfort, or changes in bowel habits for more than 4 weeks, it is reasonable to request a digestive health blood panel. Here is what to ask for.
Ready-to-use script for your GP appointment:
“I have been experiencing persistent [bloating/gas/diarrhoea/constipation/abdominal pain] for [X weeks/months]. It is affecting my [eating/work/daily activities]. Could we run a blood panel to check for coeliac disease and other possible causes? I have [been eating gluten / not been avoiding gluten] for the past 6 weeks.”
Coeliac screen (tTG-IgA + Total IgA)
Full Blood Count (FBC)
Iron Studies (Ferritin, Serum Iron, TIBC)
Vitamin B12
Folate
Vitamin D (25-OH)
Calcium and PTH
CRP (inflammation)
Liver Function Tests (ALT, AST, GGT)
Thyroid Function (TSH)
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This content is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual health history.