Why This Station Is High-Yield
Change in bowel habit is one of the most common presenting complaints in primary care and a key red flag for colorectal cancer, the fourth most common cancer in the UK. OSCE stations test your ability to characterise the symptom, exclude sinister pathology, and know the NICE 2-week wait referral criteria. Getting the red flag questions right is essential.
Characterise the Change First
Ask the patient to describe their normal bowel habit before exploring the change:
- "What were your bowels like before this started? How often did you go, and what consistency?"
- "What has changed, and when did you first notice it?"
This gives you a baseline and establishes the duration.
Diarrhoea vs Constipation vs Alternating
Characterise the type of change:
Diarrhoea
- "How many times a day do you open your bowels?"
- "What is the consistency? Watery, loose, soft?" (Bristol Stool Type 6-7)
- "Do you have urgency, needing to rush to the toilet?"
- "Have you had any accidents or soiling?" (faecal incontinence: important for IBD, overflow)
- "Any blood or mucus in your stool?"
Constipation
- "How often are you going? Less than 3 times a week is clinically constipated."
- "Is it hard and pellet-like, or difficult to pass?" (Bristol Stool Type 1-2)
- "Do you feel like you haven't fully emptied after going?" (incomplete evacuation: IBS or rectal mass)
- "Do you have to strain?"
Alternating pattern
- Alternating constipation and diarrhoea is a classic IBS pattern
- It can also be seen in colorectal cancer (constipation from the tumour mass, diarrhoea from overflow or mucus)
Blood in the Stool: Never Underestimate It
This is one of the most important questions in the history:
- "Have you noticed any blood when you open your bowels?"
- "Is the blood on the paper, coating the stool, or mixed in with it?"
- "What colour is it?"
| Blood pattern | Likely cause |
|---|---|
| Bright red blood on paper only | Haemorrhoids or anal fissure |
| Bright red blood coating the stool | Rectal polyp or rectal cancer |
| Dark red blood mixed throughout stool | Colonic tumour or IBD |
| Black tarry stool (melaena) | Upper GI bleed (oesophageal, gastric, duodenal) |
| Altered blood mixed with mucus | IBD (ulcerative colitis, Crohn's) |
⚠️ Red Flag
Never assume rectal bleeding is haemorrhoids without excluding colorectal cancer. Haemorrhoidal bleeding is a diagnosis of exclusion. Ask about all red flag features regardless of whether the patient attributes bleeding to piles.
Red Flags for Colorectal Cancer: NICE NG12 Criteria
⚠️ Red Flag
Refer on a 2-week wait pathway for colorectal cancer if:
- Rectal bleeding with unexplained weight loss or abdominal pain
- Change in bowel habit to looser stool and/or increased frequency, persisting for 6 or more weeks in a patient aged 40 or over, without rectal bleeding
- Rectal bleeding persisting for 6 or more weeks without an anal symptom, in patients aged 50 or over
- Unexplained rectal mass
- Unexplained iron deficiency anaemia in men or postmenopausal women
- Abdominal mass
Systemic Features
Always ask:
- "Have you lost any weight without trying? How much in how long?" (weight loss in GI malignancy)
- "Have you lost your appetite?"
- "Have you been feeling more tired than usual?" (anaemia from occult blood loss)
- "Any abdominal pain? Where, and does it relate to opening your bowels?"
Distinguishing the Main Differentials
Colorectal Cancer
- Age usually over 50 (but rising in younger patients)
- Change in bowel habit persisting over weeks
- Rectal bleeding, weight loss, iron deficiency anaemia
- Family history of bowel cancer or Lynch syndrome
- No improvement with dietary changes or antispasmodics
Irritable Bowel Syndrome (IBS)
- Typically young adult, more common in women
- Abdominal pain relieved by defaecation
- Bloating, distension, mucus in stool
- Symptoms worse with stress
- No weight loss, no rectal bleeding, no nocturnal symptoms
- Normal blood tests
💎 Clinical Pearl
IBS is a diagnosis that should not be made without excluding organic pathology, particularly in anyone over 40, with rectal bleeding, weight loss, or a family history of colorectal cancer. Never diagnose IBS in an OSCE without mentioning you would check bloods and consider investigation.
Inflammatory Bowel Disease (IBD)
- Younger patient (peaks 15-40 years)
- Bloody diarrhoea with mucus (ulcerative colitis)
- Abdominal pain, weight loss, systemic features (fatigue, fever)
- Extra-intestinal features: uveitis, arthritis, erythema nodosum, pyoderma gangrenosum
- Family history possible
| Feature | Ulcerative Colitis | Crohn's Disease |
|---|---|---|
| Location | Rectum always involved, continuous | Any part of GI tract, patchy ("skip lesions") |
| Depth | Mucosal only | Transmural (full thickness) |
| Key symptoms | Bloody diarrhoea, urgency | Abdominal pain, weight loss, may not have bloody stool |
| Complications | Toxic megacolon, colorectal cancer | Fistulae, strictures, abscesses, perianal disease |
| Smoking | Protective | Risk factor |
Drug History
Several drugs alter bowel habit:
- Opiates and codeine: constipation
- NSAIDs, antibiotics (especially clindamycin): diarrhoea
- Antibiotics: Clostridioides difficile colitis (especially after hospitalisation)
- Iron supplements: constipation and black stools
- Metformin: diarrhoea
- Laxative overuse: alternating pattern
Frequently Asked Questions
"What are the NICE 2-week wait criteria for colorectal cancer?"
NICE guideline NG12 states that a 2-week wait referral for suspected colorectal cancer should be made for: patients aged 40 and over with rectal bleeding AND unexplained weight loss or abdominal pain; patients aged 50 and over with rectal bleeding persisting for 6 or more weeks without an obvious anal cause; patients of any age with a rectal or abdominal mass that is unexplained; patients aged 60 and over with iron deficiency anaemia OR change in bowel habit to looser stools or increased frequency lasting 6 or more weeks, even without bleeding. Additionally, patients of any age with unexplained iron deficiency anaemia should be referred. It is essential not to dismiss rectal bleeding as haemorrhoids in older patients without a full clinical assessment.
"How do you distinguish ulcerative colitis from Crohn's disease on history?"
Ulcerative colitis (UC) is characterised by continuous mucosal inflammation starting from the rectum and extending proximally. Symptoms include bloody diarrhoea (blood and mucus mixed in stool), urgency, and tenesmus (sensation of incomplete evacuation). The rectum is always involved. Crohn's disease can affect any part of the GI tract from mouth to anus in a patchy, transmural pattern. Symptoms include colicky abdominal pain, weight loss, diarrhoea (which may or may not be bloody), perianal disease (fistulae, abscesses, skin tags), and mouth ulcers. Smoking is a risk factor for Crohn's but is protective in UC. Extra-intestinal features (joint disease, eye disease, skin disease, liver disease) occur in both. Asking about perianal symptoms, mouth ulcers, and smoking helps distinguish them.
"A 65-year-old man presents with a 3-month history of looser stools and fatigue. His FBC shows Hb 95 with MCV 72. What is your management?"
This presentation raises significant concern for colorectal cancer: an older male with a persistent change in bowel habit to looser stools, fatigue, and a microcytic anaemia suggesting iron deficiency from occult gastrointestinal blood loss. The management is urgent 2-week wait referral for suspected colorectal cancer. While waiting for colonoscopy, start iron supplementation. Further history should cover: rectal bleeding, weight loss, family history of bowel cancer, smoking history, and any previous bowel investigations. Blood tests should include a repeat FBC, ferritin, CRP, and LFTs. Consider a urine dipstick to exclude haematuria from a concurrent renal or bladder cancer as the cause of anaemia. Do not reassure this patient and treat empirically for IBS.
"What is the Rome IV criteria for IBS and when should you not make this diagnosis?"
The Rome IV criteria for IBS require recurrent abdominal pain on average at least 1 day per week in the past 3 months, associated with at least two of the following: related to defaecation; associated with a change in frequency of stool; associated with a change in form (appearance) of stool. Symptoms must have started at least 6 months before diagnosis. IBS is a clinical diagnosis made after excluding organic pathology. You should not make a diagnosis of IBS if the patient has: age over 50 with new onset symptoms; rectal bleeding; unexplained weight loss; nocturnal symptoms waking the patient; iron deficiency anaemia; elevated CRP or calprotectin; or a family history of colorectal cancer or IBD. In all these scenarios, investigation is required before attributing symptoms to IBS.