Acute diverticulitis diagnosis and severity

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Acute diverticulitis is inflammation centred on one or more diverticula, usually in the sigmoid colon. The clinical pattern is constant focal pain and tenderness, often with fever and raised inflammatory markers, but clinical features alone do not establish the anatomy. Severity is defined by physiology and complication: uncomplicated disease lacks abscess, free perforation, fistula and obstruction.

Contrast CT in secondary care confirms pericolic inflammation, identifies drainable or operative disease and exposes mimics such as cancer, colitis and urinary pathology. Treatment then follows the actual CT findings and host risk. A selective no-antibiotic strategy applies only to systemically well uncomplicated disease; it must never be carried into a septic, immunosuppressed or complicated presentation.

Key points

Suspect acute diverticulitis when pain becomes constant in the left lower quadrant with focal tenderness, fever or bowel-habit change.
Assess physiology and the abdomen before assigning severity; peritonism, sepsis, vomiting, distension or fistula symptoms suggest complicated disease.
In secondary care, raised inflammatory markers should prompt contrast CT within 24 hours to confirm the diagnosis and define complications.
CT should report the inflamed segment, abscess dimensions, free gas or fluid, obstruction and any suspected fistula or malignant alternative.
A systemically well person with CT-confirmed uncomplicated disease can be considered for no antibiotics; immune suppression, systemic illness or major comorbidity favours treatment.
Worsening pain, fever or organ function after treatment requires repeat examination and possible re-imaging for an uncontrolled collection or perforation.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Diverticular neck obstruction

Faecal material or mucosal oedema can block a diverticular neck, promoting local bacterial proliferation and inflammatory injury.

Host and drug modifiers

Immune suppression, smoking, obesity and NSAID exposure can increase susceptibility or reduce the ability to contain a microperforation.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Focal mural inflammation
Inflammation around a diverticulum produces bowel-wall thickening and pericolic fat stranding, causing constant local pain and tenderness.
2
Contained microperforation
Mesentery and adjacent structures may seal a small leak, limiting contamination to phlegmon or a local collection.
3
Loss of containment
Escape of pus, gas or faecal material into the wider peritoneum causes diffuse inflammation, sepsis and organ dysfunction.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Clinical inflammatory pattern

Persistent focal pain with tenderness, fever and raised inflammatory markers supports diverticulitis over chronic diverticular discomfort.

Complicated features

Abscess, perforation, obstruction and fistula create local or systemic consequences beyond mural inflammation.

Sepsis physiology

Tachycardia, hypotension, altered mentation, oliguria or rising lactate requires resuscitation while source control is planned.

Red flags requiring action

Generalised guarding or rigidity indicates possible uncontained perforation and must not be managed as uncomplicated disease.
Hypotension, confusion, oliguria or rising lactate signifies organ hypoperfusion and requires parallel sepsis resuscitation and source-control planning.
A tender mass, faecaluria, persistent vomiting or marked distension suggests abscess, fistula or obstruction requiring urgent imaging and specialist assessment.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Contrast CT abdomen pelvisFirst step
Why
confirm disease and identify complications.
Interpretation and limitations
Location, abscess size, free gas and obstruction guide antibiotics, drainage and surgery.
02
Full blood count CRP renal profile
Why
measure inflammatory burden and organ effect.
Interpretation and limitations
Trends support severity assessment but cannot distinguish every mimic.
03
Urinalysis and pregnancy testing where relevant
Why
evaluate urinary or pregnancy-related alternatives.
Interpretation and limitations
Inflammation adjacent to bladder can cause sterile urinary abnormalities.
04
Serial abdominal examination
Why
detect progression from focal tenderness to peritonism.
Interpretation and limitations
Analgesia should improve comfort without preventing repeated assessment.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Colorectal cancer

An obstructing or perforated tumour may resemble diverticulitis; eccentric thickening, nodes or incomplete recovery should prompt colonic evaluation.

Colitis and ischaemia

Diarrhoea, bleeding and a longer inflamed segment may indicate infectious, inflammatory or ischaemic colitis rather than focal diverticular inflammation.

Urinary or pelvic disease

Ureteric calculus, pyelonephritis, ectopic pregnancy and adnexal pathology can all produce lower abdominal pain and inflammatory change.

Additional chapter-specific clues

Alternative diagnosis

Cancer, colitis, appendicitis, urinary infection and gynaecological disease can produce overlapping pain and laboratory findings.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Worked case: CT defines uncomplicated diseaseConvert clinical suspicion into a severity decisionFirst stepA 70-year-old has 48 hours of constant left iliac fossa pain, temperature 38.2°C, focal tenderness without generalised guarding, blood pressure 132/76 mmHg and CRP 146 mg/L.

1Record full observations, examine for peritonism and assess hydration, renal function, immune status, comorbidity and ability to take oral treatment.
2Arrange contrast CT within 24 hours because the inflammatory markers are raised and the patient is being assessed in secondary care.
3CT shows short-segment sigmoid wall thickening and pericolic fat stranding with no abscess, obstruction, fistula, free gas or free fluid, establishing uncomplicated diverticulitis.
4Because fever makes the patient systemically unwell, give oral co-amoxiclav 500/125 mg three times daily for five days, with renal and allergy checks, plus analgesia and hydration advice.
5At 48-hour review temperature is 36.8°C, pain is improving, oral intake is normal and CRP has fallen to 72 mg/L; complete the five-day course and retain return instructions.

02A mimic on cross-sectional imagingRevise the diagnosis when CT contradicts the labelA 45-year-old has left-sided abdominal pain and microscopic haematuria, but no fever; CRP is 4 mg/L and there is no focal peritonism.

1Keep ureteric stone, urinary infection, colitis and gynaecological pathology in the differential rather than diagnosing diverticulitis from pain location alone.
2Urinalysis shows blood without nitrites or pyuria; check renal function and choose imaging according to the suspected urinary diagnosis.
3CT demonstrates a 4 mm distal left ureteric calculus and no colonic inflammation, so antibiotics for diverticulitis are not indicated.
4Treat the ureteric-stone pathway and explain why incidental sigmoid diverticula do not account for the acute episode.
5Verify symptom resolution and ensure persistent pain, fever or urinary obstruction triggers re-evaluation.

03Deterioration to free perforationResuscitate and obtain operative source controlDuring observation, pain becomes generalised; the abdomen is rigid, blood pressure is 84/50 mmHg and lactate is 4.8 mmol/L.

1Call the colorectal, anaesthetic and critical-care teams and begin ABCDE resuscitation while repeating a focused abdominal examination.
2Give intravenous broad-spectrum antibiotics promptly, draw blood-bank samples and support persistent hypotension with anaesthetic-led vasopressor therapy.
3If transfer is safe, obtain immediate CT; free intraperitoneal gas and diffuse fluid confirm uncontained perforation, but imaging must not delay essential surgery.
4Proceed to urgent operative source control rather than applying the no-antibiotic recommendation for uncomplicated disease.
5After resection, verify control by improving haemodynamics, lactate and organ function and investigate persistent postoperative sepsis for residual contamination.

04CT-defined abscess managementUse collection size and access to plan source controlA stable inpatient with CT-confirmed sigmoid diverticulitis has a 4.2 cm pericolic abscess, temperature 38.3°C and no diffuse free gas or generalised peritonism.

1Treat this as complicated diverticulitis, admit, check allergy and renal function and start an intravenous regimen from the NICE table while observations and abdominal findings are followed.
2Because the abscess is larger than 3 cm, ask interventional radiology whether percutaneous drainage has a safe route and involve colorectal surgery in case drainage is impossible or deterioration requires an operation.
3Drain 38 mL of pus under CT guidance, culture the sample and narrow antibiotics when susceptibilities return; repeat imaging if fever, pain or CRP does not improve rather than extending treatment blindly.
4By 48 hours temperature is 37.0°C and CRP has fallen from 184 to 83 mg/L; document source control, oral step-down criteria and a later colonic assessment after recovery.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

NICE lists this regimen for people who are systemically unwell, immunosuppressed or have significant comorbidity despite uncomplicated anatomy.

Co-amoxiclav oral

For acute uncomplicated diverticulitis when antibiotics are indicated, give 500/125 mg by mouth three times daily for 5 days.

Avoid in serious penicillin allergy; adjust for renal impairment, check hepatic history and review sooner if symptoms worsen or oral treatment is not tolerated.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Diverticular abscess

A walled-off leak creates an infected cavity whose size, location and accessibility determine whether drainage is feasible.

Free perforation

Uncontained rupture causes generalised peritonitis and septic shock, requiring immediate resuscitation and emergency operative control of the bowel source.

Fistula or stricture

Recurrent transmural inflammation may erode into adjacent organs or heal with fibrosis that narrows the colonic lumen.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat pulse, blood pressure, temperature, respiratory rate and abdominal examination after analgesia because progression to peritonism changes urgency.
Trend oral intake, urine output, renal function and CRP; a single inflammatory marker cannot demonstrate that an abscess is absent or controlled.
When antibiotics are prescribed, review clinical response and adverse effects and do not prolong an uncomplicated course without a reason.
Arrange further colonic assessment after recovery when CT is atypical, symptoms persist or the colon has not been adequately evaluated for cancer.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

CT answers a management question

Imaging is most useful when it distinguishes uncomplicated inflammation from a drainable collection, obstruction or perforation.

CRP is not anatomy

A high value supports inflammation but cannot size an abscess or exclude cancer.

Pain relief is compatible with reassessment

Adequate analgesia does not erase peritonism and should not be withheld for diagnostic purity.

Complication determines urgency

Physiology and source-control needs matter more than counting previous uncomplicated attacks.

11Common pitfallsFrequent interpretation and management errors.

01
Calling chronic left-sided pain acute diverticulitis without fever, focal tenderness or inflammatory evidence leads to antibiotic overuse.
02
Discharging before CT despite guarding and raised markers can miss abscess or perforation.
03
Using a normal early white count to exclude disease ignores timing and immune status.
04
Failing to reassess a non-improving patient delays drainage or surgery.

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