Blood Tests for Gut Health: What They Can and Can't Tell You
Blood tests can detect coeliac disease, IBD inflammation, and nutrient malabsorption — but they cannot map your microbiome or diagnose food intolerances. Understanding the difference can save you hundreds of dollars on unvalidated tests.
What Blood Tests Can Tell You About Your Gut
Standard blood tests are surprisingly useful for detecting gut problems — but indirectly. They detect the consequences of gut disease (inflammation, nutrient deficiencies, autoimmune markers) rather than directly imaging or measuring the gut itself. Think of it as reading the smoke signals rather than seeing the fire.
A pattern of iron deficiency + low B12 + low vitamin D in someone with adequate dietary intake is a strong signal of small bowel malabsorption. Elevated CRP with low albumin points to intestinal inflammation. A positive tTG-IgA antibody confirms the immune response behind coeliac disease. These are evidence-based, well-validated tests available through any Australian GP and fully bulk billed under Medicare.
However, blood tests have clear limitations. They cannot tell you what bacteria live in your gut, whether you have a “healthy” microbiome, or whether specific foods are causing your symptoms. These questions require different approaches — or in some cases, don't yet have reliable answers from any test.
4 Evidence-Based Blood Test Groups for Gut Health
Coeliac Disease Screening
What these tests measure: Coeliac disease is a common autoimmune condition where gluten triggers immune-mediated damage to the small intestinal lining, causing malabsorption. The tTG-IgA antibody test is the gold standard screening blood test — it has approximately 95% sensitivity and 98% specificity. However, about 2–3% of people have IgA deficiency, which produces false negative tTG-IgA results. That is why total IgA should always be checked alongside tTG-IgA.
How to interpret: tTG-IgA above 15 U/mL (varies by lab) is positive and warrants referral for confirmatory duodenal biopsy. Weakly positive results (15–30 U/mL) may be false positives in the context of other autoimmune conditions or liver disease. If total IgA is low (below 0.07 g/L), request DGP-IgG instead (which is IgG-based and unaffected by IgA deficiency).
Australian context: Coeliac disease affects approximately 1 in 70 Australians, but 80% are undiagnosed. The Gastroenterological Society of Australia (GESA) and Coeliac Australia recommend screening in anyone with unexplained iron deficiency, chronic diarrhoea, bloating, fatigue, unexplained weight loss, or a first-degree relative with coeliac disease. Critically, you must be eating gluten for the test to be accurate — at least 4 slices of bread daily for 6 weeks before testing.
Inflammatory Bowel Disease (IBD) Markers
What these tests measure: Blood tests cannot definitively diagnose IBD (Crohn’s disease or ulcerative colitis), but they can detect systemic inflammation and its consequences. CRP and ESR rise during active flares. Faecal calprotectin (a stool test, not a blood test) is the most specific non-invasive marker for intestinal inflammation — it distinguishes IBD from irritable bowel syndrome (IBS) with approximately 95% sensitivity.
How to interpret: CRP above 5 mg/L suggests active inflammation. ESR above 20 mm/hr adds supportive evidence. Faecal calprotectin above 50 µg/g warrants further investigation (colonoscopy). Low albumin (below 35 g/L) suggests chronic inflammation or malabsorption. Anaemia (low haemoglobin) is common in IBD due to chronic blood loss and iron malabsorption.
Australian context: IBD affects approximately 100,000 Australians, with Australia having one of the highest rates in the world. GESA recommends faecal calprotectin as the first-line investigation for suspected IBD before proceeding to colonoscopy. CRP, ESR, and FBC are bulk billed. Faecal calprotectin requires a specific request and may have a small out-of-pocket cost depending on the lab, though many pathology providers now bulk bill it.
Malabsorption Markers
What these tests measure: When the small intestine is damaged (by coeliac disease, Crohn’s disease, bacterial overgrowth, or other conditions), nutrients are not properly absorbed. The resulting deficiencies leave a distinctive pattern on blood tests — low iron, B12, folate, vitamin D, and zinc, often with low albumin indicating protein malabsorption. These deficiencies may be the first clue that a gut problem exists, especially when dietary intake is adequate.
How to interpret: Iron deficiency (ferritin below 30 µg/L) with adequate dietary intake should prompt investigation for gut malabsorption or occult GI blood loss. Low B12 (below 150 pmol/L) suggests small bowel disease (ileum absorbs B12). Low vitamin D (below 50 nmol/L) is common in malabsorption but also in the general population. Multiple simultaneous deficiencies are the red flag for gut disease rather than dietary inadequacy.
Australian context: The RCPA recommends investigating for underlying malabsorption when iron deficiency is found without an obvious cause (such as heavy periods or blood donation). Coeliac serology should be checked in all cases of unexplained iron deficiency. Iron studies, B12, folate, and vitamin D are all bulk billed under Medicare when ordered with a clinical indication.
Liver Function & Gut-Liver Axis
What these tests measure: The liver and gut are intimately connected via the portal vein — the gut-liver axis. Abnormal liver function tests can be caused by gut conditions: coeliac disease causes unexplained elevated transaminases in up to 40% of cases (which normalise on a gluten-free diet), alcohol excess damages both liver and gut, non-alcoholic fatty liver disease (NAFLD) shares risk factors with gut inflammation, and bacterial overgrowth can produce endotoxins that stress the liver.
How to interpret: Mildly elevated ALT/AST (1–3 times upper limit) with no obvious cause should prompt coeliac serology. GGT elevation suggests alcohol or fatty liver. ALP elevation can indicate bile duct obstruction. Isolated low albumin with normal liver enzymes suggests protein-losing enteropathy or malabsorption rather than liver disease.
Australian context: GESA guidelines recommend checking coeliac serology in anyone with unexplained abnormal liver function tests. Up to 9% of patients referred for unexplained elevated liver enzymes are found to have coeliac disease. All LFTs are bulk billed under Medicare.
What Blood Tests Cannot Tell You (Debunking Commercial Panels)
The wellness industry sells expensive “gut health panels” that are not supported by evidence-based medicine. The following tests are explicitly advised against by Australian medical authorities:
Claim: “IgG food sensitivity panels can diagnose food intolerances”
Reality: IgG antibodies to food indicate exposure, not intolerance. Everyone who eats eggs will have IgG to egg proteins. The Australasian Society of Clinical Immunology and Allergy (ASCIA) explicitly advises AGAINST IgG food testing — it leads to unnecessary dietary restriction.
Claim: “Comprehensive stool microbiome panels can guide treatment”
Reality: While the gut microbiome is important, current commercial stool panels lack validated reference ranges and reproducibility. The same sample sent to different labs often returns different results. There are no evidence-based interventions that can be reliably guided by these panels.
Claim: “Hair mineral analysis can detect nutrient deficiencies from gut issues”
Reality: Hair mineral analysis is not endorsed by any Australian medical authority. Hair mineral content is affected by shampoo, hair dye, water quality, and environmental exposure — not reliably by nutritional status.
Claim: “Leaky gut syndrome can be diagnosed by blood tests”
Reality: While intestinal permeability is a real physiological phenomenon studied in research, there is no validated clinical blood test for "leaky gut." Tests sold for this purpose (zonulin, lactulose/mannitol ratio) are research tools without established clinical cutoffs.
Claim: “Candida antibody tests diagnose gut yeast overgrowth”
Reality: Most people have Candida as normal gut flora and will have detectable antibodies. Elevated Candida IgG does not indicate pathological overgrowth. Systemic candidiasis is a serious condition in immunocompromised patients — it is diagnosed by blood cultures, not antibody panels.
When to Get Tested: Symptoms That Warrant Investigation
Script for your GP appointment:
“I've been experiencing persistent bloating / diarrhoea / abdominal pain / fatigue for more than 4 weeks. Could we screen for coeliac disease with tTG-IgA and total IgA? I'd also like to check my iron, B12, folate, and vitamin D to see if there's a malabsorption pattern. If IBD is possible, could we add CRP and faecal calprotectin?”
Persistent bloating or abdominal discomfort (>4 weeks)
Unexplained diarrhoea or change in bowel habit
Blood or mucus in stool (urgent — see GP within days)
Unexplained iron deficiency (especially without obvious cause)
Unintended weight loss
Family history of coeliac disease, IBD, or bowel cancer
Gut Health Blood Test Panel
| Test | Purpose | Cost (Australia) |
|---|---|---|
| tTG-IgA + Total IgA | Coeliac disease screening | Bulk billed |
| CRP | Systemic inflammation marker | Bulk billed |
| Faecal Calprotectin | Intestinal-specific inflammation (stool test) | Bulk billed* |
| Iron Studies | Detect malabsorption-related iron deficiency | Bulk billed |
| Vitamin B12 + Folate | Detect small bowel malabsorption | Bulk billed |
| Vitamin D | Often low in malabsorption and IBD | Bulk billed |
| Liver Function Tests | Screen for gut-liver axis conditions | Bulk billed |
| Albumin | Protein status — low suggests malabsorption or inflammation | Bulk billed |
| FBC | Detect anaemia (common in gut disease) | Bulk billed |
| Faecal Occult Blood (FOBT) | Detect hidden blood in stool (cancer screening) | Bulk billed |
* Faecal calprotectin is bulk billed by many Australian pathology providers when ordered with a clinical indication. Some labs may charge a small gap fee. Check with your pathology provider.
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Reference ranges sourced from the Royal College of Pathologists of Australasia (RCPA) and the Gastroenterological Society of Australia (GESA). SmarterBlood provides health information and AI-powered blood test analysis. It is not a substitute for professional medical advice, diagnosis, or treatment.