Toxic megacolon is the most feared acute complication of severe ulcerative colitis — it develops when transmural inflammation extends beyond the mucosa into the smooth muscle, causing paralysis of colonic motility and progressive gaseous distension. The diagnostic threshold: transverse colon diameter >6 cm on plain AXR (measured at its widest point). Associated features: fever >38°C, tachycardia >120, signs of peritonism (localised or diffuse tenderness with guarding), and systemic toxicity. The immediate management: nil by mouth, IV hydrocortisone 400 mg/day (100 mg QDS), IV antibiotics (ciprofloxacin 400 mg BD + metronidazole 500 mg TDS IV), NG tube decompression, and strict 6-hourly clinical review with serial AXR to monitor colonic diameter. The surgical threshold: if colonic diameter increases or fails to reduce, or systemic condition deteriorates, despite 24-72 hours of maximal medical therapy — emergency subtotal colectomy is required. GPs who suspect toxic megacolon must call 999 and must not provide oral treatment alone.

The Truelove and Witts severity criteria (1955) remain the most practically validated bedside tool for assessing UC flare severity — a study by Truelove and Witts at Oxford in 1955 was the first RCT demonstrating that corticosteroids reduced mortality in UC, and as part of that study they developed the severity classification that still underpins clinical management today. The Oxford criteria for predicting colectomy requirement at day 3 of IV hydrocortisone: stool frequency >8/day OR CRP >45 mg/L with stool frequency 3-8/day predicts an 85% probability of requiring colectomy. This allows clinicians to identify early steroid-refractory patients who need rescue therapy (infliximab or ciclosporin) before proceeding to emergency colectomy. GPs should ensure that any patient with severe UC (meeting the Truelove and Witts criteria) is admitted to hospital — primary care management of severe UC is inappropriate.

The faecal calprotectin measurement is the most practical non-invasive biomarker for distinguishing IBD activity from IBS in primary care — calprotectin is a calcium and zinc-binding protein released from neutrophils into faeces when colonic inflammation is present. A calprotectin above 250 µg/g has a high sensitivity (approximately 90%) and specificity (approximately 80%) for active IBD (UC or Crohn's). In clinical practice: calprotectin provides an objective measure of mucosal inflammation that complements clinical scoring. Calprotectin below 50 µg/g makes active IBD very unlikely (high negative predictive value) and favours IBS. In known UC patients, serial calprotectin measurements track treatment response — a falling calprotectin with treatment confirms mucosal healing; a persistently elevated or rising calprotectin despite treatment indicates persistent active disease requiring treatment escalation. GPs can request calprotectin directly — it is available in most NHS community laboratories as a first-line test for patients with suspected IBD.

The UC surveillance colonoscopy programme is a life-saving intervention that reduces mortality from colorectal cancer in high-risk UC patients by enabling early detection of dysplasia and cancer — the cumulative risk of CRC in UC is approximately 2% after 10 years, 8% after 20 years, and 18% after 30 years in extensive colitis. The NICE NG151 surveillance schedule: 1 year after a colonoscopy establishing extensive colitis + 8 years of symptoms (i.e., 8 years from IBD diagnosis, first surveillance colonoscopy at 8 years) — then every 1-3 years depending on risk stratification (chromoendoscopy findings, dysplasia history, PSC co-diagnosis, family history of CRC). GPs co-managing UC patients should: check that surveillance is occurring at the appropriate interval, flag patients approaching 8 years of extensive disease who have not received a surveillance colonoscopy invitation, and ensure PSC co-diagnosis is flagged for annual colonoscopy from time of PSC diagnosis.

The UC biologic drug level monitoring is an increasingly important GP and gastroenterology shared care responsibility — infliximab (anti-TNF) therapeutic drug monitoring (TDM) guides dose optimisation. Trough infliximab levels above 3-7 µg/mL are associated with sustained remission; levels below 3 µg/mL predict loss of response. Anti-drug antibodies (ADAbs) against infliximab accelerate drug clearance and predict loss of response. In a patient with UC on infliximab who is having a flare: measuring infliximab trough level + ADAbs before changing therapy determines the appropriate response: low level + low ADAbs = dose-optimise (increase dose/frequency); low level + high ADAbs = switch within class (adalimumab) or switch out of class (vedolizumab, ustekinumab); adequate level + ADAbs negative = active UC despite therapeutic infliximab = switch mechanism. GPs should NOT escalate immunosuppression in a UC patient on biologics without TDM results and gastroenterology guidance.

The TPMT (thiopurine methyltransferase) testing before azathioprine is a mandatory pharmacogenetic safety test — azathioprine is converted to its active metabolite 6-thioguanine (6-TGN) via the enzyme TPMT. Patients with TPMT deficiency (approximately 0.3% of the population are homozygous-deficient; approximately 11% are heterozygous) accumulate 6-TGN to toxic levels, causing potentially fatal myelosuppression (profound bone marrow failure, pancytopenia). The standard practice: TPMT activity or TPMT genotype testing is performed before azathioprine initiation — low TPMT activity = avoid azathioprine entirely or use at very reduced doses with intensive monitoring; heterozygous = start at reduced dose (50% of standard). Prescribing azathioprine without TPMT testing is a prescribing error with serious clinical consequences. After starting, FBC monitoring every 4 weeks for the first 3 months is mandatory (myelosuppression can develop despite normal TPMT — dose-dependent toxicity).

The ileo-anal pouch (J-pouch) procedure is the gold standard surgical option for patients with UC requiring colectomy — it provides a curative operation while avoiding a permanent ileostomy by constructing a reservoir from the terminal ileum (a J-shaped loop is folded and sutured to form a pouch) that is anastomosed to the anus. Most patients achieve functional continence with 4-8 bowel movements per day (vs 20-30+ in severe active UC). The most common long-term complication is pouchitis — inflammation of the pouch mucosa, occurring in approximately 50% of patients at some point within 10 years. Pouchitis presents as increased stool frequency, urgency, and rectal bleeding. Treatment: metronidazole 400 mg TDS x 14 days (first-line) or ciprofloxacin 500 mg BD x 14 days. Probiotic VSL#3 (a multi-strain probiotic) reduces pouchitis recurrence in RCTs. Chronic refractory pouchitis requires specialist pouch clinic management.

The smoking paradox in UC represents one of the most counterintuitive findings in gastroenterology — nicotine and other tobacco smoke constituents appear to suppress intestinal mucosal inflammation via effects on mucus secretion, immunoglobulin A secretion, eicosanoid production, and gut motility. Epidemiological data consistently show that current smokers have lower rates of UC, milder disease, and less colectomy compared to never-smokers. However, ex-smokers who develop UC after quitting often notice a significant worsening of disease, and some observational studies show that UC onset is precipitated by smoking cessation. This does not change the clinical advice: GPs must never recommend smoking as a UC treatment, as cardiovascular mortality, lung cancer, and COPD risks vastly exceed any marginal bowel benefit. When patients ask about this paradox: 'Smoking does appear to have some effect on UC symptoms, but it causes much more serious harm in other ways — we cannot recommend it.' Nicotine patches are being trialled as a UC adjunct therapy in research settings.

The mesalazine nephrotoxicity monitoring is a safety responsibility that GPs often miss in long-term UC management — aminosalicylates (mesalazine, olsalazine, balsalazide) are generally very well tolerated but can cause an idiosyncratic interstitial nephritis in approximately 0.2-0.5% of patients, usually in the first 12 months of treatment but occasionally later. The nephritis is characterised by a progressive rise in creatinine and fall in eGFR, usually without haematuria or proteinuria (unlike most other causes of CKD). The critical principle: annual renal function (eGFR + creatinine) must be checked in all patients on long-term 5-ASA therapy. If eGFR declines by >25% from baseline or falls below 60 mL/min without explanation: stop mesalazine and refer to nephrology and gastroenterology. Renal function usually stabilises or improves after stopping 5-ASA, but permanent renal damage can result from delayed diagnosis.