Persistent Diarrhoea: Blood and Stool Tests to Consider

Diarrhoea that lingers for weeks rather than days is worth taking seriously and investigating properly. While many causes are treatable, the symptoms can overlap, so a sensible combination of blood and stool tests often helps point to the answer. Here is how doctors tend to think it through.

What counts as persistent diarrhoea

Most bouts of diarrhoea are short-lived and settle within a few days. Persistent or chronic diarrhoea is generally loose or frequent stools that carry on for several weeks or keep coming back. Once symptoms have lasted this long, it is reasonable to look for an underlying cause rather than simply waiting for them to pass.

Common causes to consider

There are several possibilities worth keeping in mind, and they range from the very common to the less so. These include:

• Coeliac disease — an immune reaction to gluten
• Inflammatory bowel disease — Crohn's disease or ulcerative colitis
• Infections — some can linger longer than expected
• Bile acid problems — where bile is not reabsorbed properly
• Medications — a number of common drugs can loosen the bowels
• An overactive thyroid — which speeds up gut transit
• Irritable bowel syndrome (IBS) — typically a diagnosis of exclusion once other causes are ruled out

The tests that help

A focused panel can narrow things down considerably. Commonly useful tests include:

• Coeliac screen (tTG antibodies) — importantly, you need to be still eating gluten regularly for this to be accurate
• Faecal calprotectin — a stool test that flags inflammation and helps separate IBD from IBS
• Full blood count — can reveal anaemia or other clues
• CRP — a general marker of inflammation
• Thyroid function — to check for an overactive thyroid

Red flags that need prompt care

Some features suggest that a cause needs to be found without delay. Please seek medical advice promptly if you notice blood in your stool, unintended weight loss, diarrhoea that wakes you from sleep, or signs of dehydration such as marked thirst, dizziness or passing little urine. These do not necessarily mean something serious, but they do mean you should be assessed rather than waiting.

When to see a doctor

If diarrhoea has lasted more than a couple of weeks, keeps returning, or is accompanied by any of the red flags above, it is time to see a doctor. Tests are a valuable part of the picture, but they work best alongside a proper assessment of your symptoms and history. A clinician can interpret your results, arrange any further investigations, and make sure nothing important is missed.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic