What is the likely diagnosis and management for a 64-year-old male with excessive gas, urgency to pass stools, and significant weight loss of 16kgs over the past few months, who has a history of normal Oesophagogastroduodenoscopy (OGD) and no co-morbidities, with Bristol stool chart types 6 and 4, and an active lifestyle?

Likely Diagnosis and Management

This patient requires urgent investigation for organic disease, particularly colorectal malignancy, given the significant unintentional weight loss (16kg), age over 60, and change in bowel habits—this presentation demands colonoscopy and laboratory workup before considering functional disorders like IBS.

Critical Red Flag Assessment

This 64-year-old male presents with multiple alarm features that mandate exclusion of serious pathology:

• Significant weight loss (16kg): While attributed to portion size reduction, unintentional weight loss is a red flag symptom requiring urgent gastroenterology referral 1
• Age >50 years: Patients over 50 should undergo colonoscopy regardless of symptom pattern 2
• New-onset bowel habit change: Recent onset of loose stools (Bristol 6) with urgency over 3-4 months in this age group is concerning 1

The absence of pain does NOT exclude serious pathology and should not provide false reassurance 1, 3.

Mandatory Initial Investigations

Laboratory Testing (Order Immediately)

• Complete blood count: Essential to assess for anemia, which would indicate occult bleeding or malignancy 2, 4
• C-reactive protein or ESR: To screen for inflammatory bowel disease 2
• Fecal calprotectin: Strongly recommended to exclude inflammatory bowel disease, particularly given diarrheal symptoms 2
• Celiac serology (IgA tissue transglutaminase with total IgA): Should be checked in all patients with IBS-like symptoms 2
• Stool testing for Giardia: Common parasitic cause of chronic diarrhea 2
• Fecal occult blood testing: Screening for colorectal pathology 2, 4
• Liver function tests, albumin: To identify malabsorption or hepatobiliary disease given weight loss 5

Endoscopic Evaluation

• Colonoscopy is mandatory: Age >50 with new bowel symptoms requires full colonoscopy to exclude colorectal cancer 2, 4
• Do NOT delay colonoscopy based on normal OGD—upper endoscopy does not evaluate the colon where pathology is most likely given these symptoms 2

Differential Diagnosis (In Order of Priority)

1. Colorectal Malignancy

• Weight loss + age + bowel habit change = high-risk presentation 4, 1
• Colonoscopy is both diagnostic and potentially therapeutic 4

2. Inflammatory Bowel Disease

• Loose stools with urgency can represent IBD 2
• Fecal calprotectin has high sensitivity for excluding IBD 2
• Note: 20% of Crohn's disease patients have normal CRP, so normal inflammatory markers don't exclude IBD 2

3. Bile Acid Malabsorption

• Can cause chronic diarrhea and urgency 5
• Consider SeHCAT scanning or serum 7α-hydroxy-4-cholesten-3-one if initial workup negative 2

4. Celiac Disease

• Can present with loose stools and weight loss 2
• Serology has >90% sensitivity 2

5. Chronic Pancreatitis/Pancreatic Insufficiency

• Weight loss with loose stools suggests malabsorption 5
• Consider if other causes excluded 5

6. IBS-Diarrhea (Diagnosis of Exclusion Only)

• Cannot be diagnosed until organic disease excluded 2, 3
• Requires positive symptom criteria (abdominal pain for ≥12 weeks) which this patient lacks 2
• IBS typically does NOT cause significant weight loss 3, 6

Management Algorithm

Immediate Actions (Within 1-2 Weeks)

1. Order complete laboratory panel as outlined above 2, 4
2. Refer for urgent colonoscopy (within 2-4 weeks given age and symptoms) 2, 4
3. Document exact weight loss timeline: Clarify if truly unintentional vs. deliberate dietary restriction 1
4. Assess for additional symptoms: Night sweats, fever, family history of colorectal cancer or IBD 2, 1

If Initial Workup Negative

Only after colonoscopy and laboratory tests exclude organic disease:

• Consider bile acid malabsorption testing (SeHCAT or serum C4) 2, 5
• Trial of dietary modification: Low FODMAP diet under dietitian supervision 7
• Symptomatic management: Loperamide for diarrhea and urgency 7

If Functional Disorder Confirmed

• Dietary assessment: Evaluate for lactose intolerance (consider breath testing if high dairy intake), excessive caffeine, or fermentable carbohydrates 7
• Loperamide or codeine: First-line for diarrhea-predominant symptoms 7
• Avoid restrictive diets without dietitian supervision—25% of IBS patients develop disordered eating 7

Critical Pitfalls to Avoid

• Do NOT diagnose IBS without excluding organic disease in a patient >50 with weight loss 2, 1, 3
• Do NOT attribute weight loss to voluntary portion reduction without investigation—this may represent early satiety from malignancy 1
• Do NOT rely on normal OGD to exclude significant pathology—the colon must be evaluated 2
• Do NOT assume absence of pain excludes serious disease—colorectal cancer can present with painless diarrhea and weight loss 1
• Normal inflammatory markers do NOT exclude IBD—proceed with fecal calprotectin and colonoscopy 2