The Right Test for the Right Question
One of the most common sources of frustration for patients is feeling that they have had 'all the tests' without an answer. Often the issue is not that tests were done, but that the wrong test was done for the clinical question at hand. Each gut investigation answers a specific question — using the wrong tool wastes time, money, and emotional energy.
The Clinical Questions
Is there inflammation in my gut? Fecal calprotectin is the first-line non-invasive test. Normal calprotectin (below 50 micrograms per gram) makes IBD very unlikely. CRP adds systemic inflammation data but is less gut-specific.
Is there an infection? Stool culture and microscopy identify bacterial and parasitic pathogens. Multiplex PCR panels detect a broader range including viruses but cannot distinguish active infection from residual nucleic acid.
Is there bacterial overgrowth? Hydrogen and methane breath testing assesses SIBO and IMO. Sensitivity is limited (30-70%) and results must be interpreted in clinical context.
Is there carbohydrate malabsorption? Lactose, fructose, and sorbitol breath tests identify specific sugar malabsorption that may be driving IBS-like symptoms.
Is there structural disease? Colonoscopy with biopsies provides direct visualisation and tissue sampling — the gold standard for diagnosing IBD, microscopic colitis, colorectal cancer, and other mucosal pathology.
Is there pancreatic insufficiency? Fecal elastase measures pancreatic exocrine function and is indicated when steatorrhoea or chronic pancreatitis is suspected.
The Decision Pathway
Step 1: Define the clinical question based on symptoms and history. Step 2: Select the test that best answers that question. Step 3: Interpret the result in context — considering pre-test probability, test characteristics, and the overall clinical picture. Step 4: If the result is inconclusive, reassess the question rather than simply adding more tests.
Common Pitfalls
Ordering calprotectin to diagnose IBS (it rules IBD in or out, it does not diagnose IBS). Performing breath testing without measuring methane (missing 15-30% of SIBO cases). Ordering commercial microbiome tests expecting diagnostic information (they provide ecological description, not diagnosis). Performing colonoscopy for typical IBS symptoms without alarm features in young patients (guidelines recommend positive diagnosis via Rome IV instead).
Shared Decision-Making
Patients have the right to understand why a specific test is being ordered and what the possible results mean. Asking your clinician three questions before any investigation improves the process: What question are we trying to answer? What will we do differently based on the result? What are the limitations of this test?