Persistent diarrhoea — loose or watery stools that keep returning for more than two weeks — is one of the most common reasons a clinician orders a stool test, and one of the most common reasons that test comes back unhelpful. The summer of 2026 made the point hard to ignore. A nationwide surge of Cyclospora infections linked to recalled iceberg lettuce reached 6,358 confirmed illnesses across 15 states by 5 August, with at least 278 hospitalisations and two deaths reported in Michigan. Alongside it, the CDC reminded clinicians and laboratories that a standard parasite exam does not reliably find this particular parasite unless it is requested by name.
In this article you’ll learn what actually counts as persistent diarrhoea, which stool tests exist and what each one can and cannot see, when testing earns its place, and how newer molecular panels change the odds of getting a real answer.
What counts as persistent diarrhoea
Duration is what separates a nuisance from a question worth investigating. Most acute gastroenteritis settles within a few days without any testing at all. Clinicians generally use three brackets:
- Acute: under 14 days. Usually viral, usually self-limiting.
- Persistent: 14 to 28 days. The window where infectious causes, including parasites, become more likely.
- Chronic: more than four weeks. A 2026 review in JAMA estimates this affects roughly 6% to 7% of US adults, and notes that more than nine in ten of these cases turn out to have a non-infectious cause.
That last figure matters for expectations. If loose stools have run past a month, the useful question is often no longer “which bug is it?” but “which mechanism is it?” — inflammation, malabsorption, bile acid loss, or a functional bowel disorder. This guide describes normal and abnormal stool consistency.
The 2026 Cyclospora outbreak exposed a real testing gap
The current US outbreak is a textbook illustration of why the test that gets ordered matters as much as the decision to test. Cyclospora cayetanensis is a microscopic parasite spread through contaminated food or water, not usually from person to person. It causes watery diarrhoea that can follow a remitting-relapsing pattern for weeks — exactly the profile of persistent diarrhoea.
The problem is analytical. CDC guidance issued in July 2026 states that detection of Cyclospora in stool is difficult even in clearly symptomatic patients, and that routine ova and parasite examinations might not detect it reliably. The agency asked laboratories to check that their stool protocols include a specific method — modified acid-fast staining or PCR — rather than relying on the standard microscope exam alone. A separate article analyses the Cyclospora stool test in detail.
The practical takeaway for patients is narrow but useful: a normal stool result does not always mean nothing was there. It can mean the laboratory was not asked to look for the right thing.
The stool tests a doctor can order, and what each one sees
“Stool test” is not one test. It is a menu, and the items behave very differently.
| Test | What it looks for | Usual turnaround | Main limitation |
|---|---|---|---|
| Stool culture | Bacteria that grow in a dish: Salmonella, Shigella, Campylobacter | 2 to 3 days | Blind to viruses and to most parasites |
| Ova and parasites exam | Parasite eggs and cysts seen under a microscope | 1 to 3 days | Shedding is intermittent, so several samples on different days may be needed |
| Multiplex GI PCR panel | Genetic material of 20 or more bacteria, viruses and parasites at once | Same day to 24 hours | Finds DNA or RNA, which does not always mean an active infection |
| Cyclospora-specific testing | Cyclospora oocysts, via acid-fast staining or PCR | 1 to 3 days | Usually performed only when specifically requested |
| Faecal calprotectin | A protein released when the bowel lining is inflamed | 2 to 5 days | Signals inflammation without identifying its cause |
| C. difficile toxin testing | Toxin produced by Clostridioides difficile | Same day to 2 days | A positive result can reflect carriage rather than active disease |
Our library explains the ova and parasites stool test, covers the stool culture test, and details faecal calprotectin results. Our team also describes the C. difficile toxin test.
When a stool test earns its place
Testing everyone with loose stools would waste time and money and would generate confusing results. A 2025 narrative review in the American Journal of Gastroenterology sets out a workable threshold: stool testing makes sense when the pre-test probability of an infection is genuinely high — more than three unformed bowel movements in 24 hours, symptoms lasting beyond a week, or circumstances that point to infection.
Situations that justify testing
- Diarrhoea that has lasted more than seven to fourteen days without improving.
- Blood or visible mucus in the stool.
- Fever, severe abdominal pain, or unintended weight loss.
- A weakened immune system, from illness or from medication.
- Recent travel, or a known food recall or outbreak in your area.
- Recent antibiotics, which raise the question of C. difficile.
Signs that need care the same day
Some situations are about hydration rather than diagnosis. Seek prompt medical attention for signs of significant fluid loss: very little urine, dizziness on standing, a dry mouth, confusion, or an inability to keep fluids down. Blood in the stool with fever also warrants same-day assessment rather than a posted sample.
Latest scientific advances
Three findings from the past two years are quietly reshaping how stool testing is done. All are worth reading with the usual caution: these are individual studies and expert reviews, not settled consensus.
Molecular panels find more, and faster. A 2025 multicentre study of 267 hospitalised adults compared a multiplex PCR panel — a single test that screens for many pathogens at once by reading their genetic material — with traditional culture and microscopy. The molecular approach identified a cause in roughly three of every four samples, against about four in ten with the older methods, and it picked up mixed infections that conventional testing missed. What this means for you: if your first round of stool tests came back empty and symptoms persist, asking whether a molecular panel is available is a reasonable question.
Faster results changed prescribing. In the same study, patients tested with the molecular panel were markedly less likely to receive an antibiotic they did not need, and those who did receive one were taken off it sooner. What this means for you: a test that comes back the same day is not only about reassurance — it can spare you a course of antibiotics.
Newer does not mean the old tests are obsolete. A 2026 prospective study in a paediatric emergency department found that the molecular panel expanded the picture but did not replace culture: more than a quarter of the bacterial infections were found by culture alone, despite a negative molecular result for the same organism. A 2024 intensive-care study reached a similar conclusion for one parasite, Cryptosporidium, where traditional microscopy outperformed the panel. What this means for you: if a laboratory runs both, that is thoroughness, not duplication.
Blood tests that complete the picture
A stool sample answers “what is in the gut”. Blood work answers “what has it cost the body”, and sometimes points to a non-infectious cause. Clinicians commonly add an electrolyte panel when diarrhoea has been heavy, because sodium, potassium and bicarbonate shift quickly with fluid loss. Our library reviews the C-reactive protein inflammation marker, which rises with inflammatory and invasive infections.
When diarrhoea has crossed the four-week line, the 2026 JAMA review recommends serological screening for coeliac disease — tissue transglutaminase IgA together with total IgA — alongside faecal calprotectin to look for inflammatory bowel disease. We examine the tTG-IgA test used in coeliac diagnosis. That same review notes that microscopic colitis, which endoscopy can miss without biopsies, accounts for about 13% of chronic diarrhoea cases.
Glossary
| Term | Definition |
|---|---|
| Cyclosporiasis | Intestinal illness caused by the parasite Cyclospora cayetanensis, usually acquired from contaminated fresh produce or water. |
| Ova and parasites (O&P) exam | A microscope examination of stool looking for parasite eggs and cysts. |
| Multiplex PCR panel | A single laboratory run that searches stool for the genetic material of many pathogens simultaneously. |
| Oocyst | The resistant, egg-like stage in which some parasites are shed in stool and passed on. |
| Acid-fast stain | A dye technique that makes certain parasites, including Cyclospora, visible under a microscope. |
| Faecal calprotectin | A protein measured in stool that rises when the bowel lining is inflamed. |
| Microscopic colitis | Bowel inflammation visible only on biopsy, with a normal-looking lining at endoscopy. |
| Bile acid diarrhoea | Watery diarrhoea caused by excess bile acids reaching the colon instead of being reabsorbed. |
| tTG-IgA | Tissue transglutaminase immunoglobulin A, the first-line blood antibody test for coeliac disease. |
Frequently asked questions
How many stool samples do I need to give?
It depends on the test. A culture or a molecular panel usually needs one sample. A parasite examination often needs two or three collected on different days, because parasites are shed intermittently and a single sample can miss them. If your laboratory asks for repeat samples, that is normal practice rather than a sign something was done wrong.
My stool test was negative but I still feel unwell. What now?
A negative result narrows the field without closing the case. It may mean the sample was collected on a day with no shedding, that the pathogen was not on the test menu, or that the cause is not infectious at all. Go back to the clinician who ordered it, describe how long symptoms have run, and ask which tests were actually included.
Should I ask for a Cyclospora test specifically?
If you have had watery diarrhoea for more than a week during the May to August season, or you ate a product covered by a recall, mentioning it is reasonable. The CDC has explicitly asked clinicians to request Cyclospora testing when it is clinically suspected, because standard parasite exams may not detect it.
Can I collect a stool sample at home?
Usually yes. Laboratories provide a container, sometimes with a preservative already inside. The main rules are to avoid contamination with urine or toilet water, to fill to the marked line, and to return the sample within the time window written on the kit. Refrigerate it in between unless the instructions say otherwise.
Do I need antibiotics for persistent diarrhoea?
Often not. Many prolonged cases are viral or non-infectious, and antibiotics taken without a confirmed bacterial cause can prolong symptoms and disturb the gut flora. Treatment decisions belong with your doctor and are best made once the cause is known.
How long is too long before seeing a doctor?
Seven days of unimproving diarrhoea is a sensible trigger for a consultation, and sooner if there is blood, fever, severe pain, weight loss, or signs of dehydration. Anyone who is pregnant, over 65, immunocompromised, or caring for a young child should seek advice earlier.
Sources
- Centers for Disease Control and Prevention — Investigation Update: Cyclospora Outbreak, July 2026 (updated 5 August 2026) — cdc.gov
- MedlinePlus, National Library of Medicine — Stool culture — medlineplus.gov
- National Institute of Diabetes and Digestive and Kidney Diseases — Diagnosis of Diarrhoea — niddk.nih.gov
- Singh P, Lee A, Sheth NM, Chey WD — Chronic, Noninfectious Diarrhoea: A Review — JAMA, 2026 — doi.org/10.1001/jama.2026.0872
- Camilleri M — When and What to Test for Diarrhoea: Focus on Stool Testing — American Journal of Gastroenterology, 2025 — doi.org/10.14309/ajg.0000000000003175
- Fischer M, Vaughn BP, Peery AF, Kelly CR — AGA Clinical Practice Update on Management of Clostridioides difficile Infection in Adults — Clinical Gastroenterology and Hepatology, 2026 — doi.org/10.1016/j.cgh.2026.05.007
- Boeriu A et al. — PCR-Based Versus Conventional Stool Testing in Hospitalized Patients with Diarrhoea: Diagnostic Yield, Clinical Impact, and Stewardship Implications — Microorganisms, 2025
- Seçilmiş Y et al. — Etiologic Diversity and Diagnostic Yield of Multiplex PCR in Children With Bloody Diarrhoea — Pediatric Emergency Care, 2026
- Ahmed A et al. — Evaluation of a multiplex polymerase chain reaction for the diagnosis of infectious diarrhoea in intensive care unit patients in Upper Egypt — The Egyptian Journal of Immunology, 2024
Further reading
- Faecal lactoferrin test
- Fatty stool: causes, symptoms and treatment
- Irritable bowel syndrome
- Crohn’s disease
- Rectal bleeding: causes and when to seek help
Understand your lab results with AI DiagMe
A stool report full of pathogen names and reference ranges is hard to read when you are the patient. AI DiagMe turns lab results — stool, urine and blood — into plain language, so you can see what a stool culture, a parasite exam, a faecal calprotectin value or an electrolyte panel is actually saying before your next appointment. It helps you understand your results and prepare better questions; it does not make a diagnosis and does not replace your doctor.
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