A home visit is arranged for a 45 year old man who has been unable to attend the surgery due to severe abdominal pain. It woke him last night and radiates down the left side of his abdomen. Paracetamol and Ibuprofen have been little help. He has felt nauseous but has not vomited, has no fever and his bowels are opening as normal. He has also experienced some urinary frequency and urgency. Past medical history is unremarkable. He is a non-smoker and drinks ten units of alcohol a week.
Heart Rate: 100 bpm
Blood Pressure: 145/97 mmHg
Temperature: 36.7° C
Respiratory rate: 16 breaths per minute
Oxygen saturations: 100% on air
Examination: Mr Carter is very distressed and writhing around in pain. No significant abdominal distension or masses, tenderness to palpation in left flank and left iliac fossa, bowel sounds normal. Testicular examination unremarkable. Rectal examination refused.
According to NICE CKS guidelines what is the gold standard definitive investigation for his most likely diagnosis?