Infected obstructed kidney (pyonephrosis) carries a high mortality if not drained emergently — it does not respond to antibiotics alone because the infected urine cannot drain. Temperature alone can be misleading — some patients remain apyrexial initially. AAA classically presents with loin/flank pain in older men and can mimic renal colic exactly — pulsatile mass on examination, or known AAA, requires immediate 999. Pregnant women are a special population: radiation risk means CT-KUB is avoided; ultrasound is used (MRI by specialists if inconclusive), with specialist input according to severity, infection, obstruction and renal function.

The inability to find a comfortable position is a key differentiating feature of renal colic from peritonitis (where patients lie still). The classic pain radiation pattern reflects the ureteric anatomy — upper ureter pain radiates to ipsilateral testis/labia via the genitofemoral nerve. Urine pregnancy test is a mandatory first step in women — ectopic pregnancy can present identically to renal colic and delay in diagnosis is a recognised cause of maternal death (MBRRACE UK). Haematuria may be absent in confirmed stone cases — its absence should not cause abandonment of the diagnosis if the clinical picture fits.

Stone size is the single most important predictor of spontaneous passage — this drives the management decision of conservative vs intervention. EAU 2024 guidelines (international) recommend medical expulsive therapy for distal ureteric stones over 5 mm; NICE NG118 advises considering it for distal ureteric stones under 10 mm. Stone composition, determined from CT Hounsfield units or stone analysis after passage, determines secondary prevention — uric acid stones are uniquely responsive to urinary alkalinisation, making the distinction clinically important. Struvite/infection stones will recur unless the underlying urease-producing organism is eradicated — antibiotics alone without stone removal will fail. Offer stone analysis whenever a stone is retrieved.

The examination in renal colic is as much about excluding dangerous mimics as confirming the diagnosis. The patient who cannot keep still is a positive sign — patients with peritoneal inflammation lie motionless. Palpating for an aortic aneurysm takes 10 seconds and can be lifesaving — AAA is an uncommon but important mimic — older patients with vascular risk factors warrant a low threshold for urgent assessment. CVA tenderness with fever raises the possibility of obstructed infected system even with normal BP/HR — these patients can deteriorate rapidly.

Low-dose non-contrast CT is the preferred investigation, with high sensitivity and specificity, and can identify AAA and other pathologies — NICE NG118 advises it within 24 hours for non-pregnant adults. However, it involves ionising radiation — important for young recurrent stone formers who will have multiple scans over a lifetime. NICE NG118 recommends ultrasound as first-line imaging for children and pregnant women. In recurrent or high-risk stone formers, specialist metabolic assessment (including 24-hour urine) identifies correctable abnormalities (hypercalciuria, hyperuricosuria, hypocitraturia) that allow targeted prevention.

The referral decision in renal colic is largely driven by whether there is obstruction + infection (the only true emergency) or whether the stone is likely to pass spontaneously (conservative management safe). Small distal stones usually pass without intervention — aggressive referral is not warranted. Larger and proximal stones are less likely to pass and need urology intervention planning (ESWL, ureteroscopy, or PCNL). Recurrent or high-risk stone formers benefit from systematic metabolic workup and prevention strategies, which are best coordinated through urology or specialist stone clinics.

NSAIDs are first-line analgesia for renal colic (NICE NG118) — multiple RCTs and meta-analyses confirm NSAIDs are superior to opioids for pain control and have fewer side effects. The mechanism is prostaglandin inhibition reducing ureteric spasm and renal pelvis pressure. Medical expulsive therapy (MET) with tamsulosin is supported by NICE NG118 (2019; distal ureteric stones <10 mm) and EAU 2024 (international) for distal ureteric stones — it relaxes the ureteric smooth muscle via alpha-1 blockade, increasing passage rate and reducing time to passage. The evidence for stones >10mm is weaker. Importantly, dietary calcium restriction for calcium stone formers is CONTRAINDICATED — it paradoxically increases urinary oxalate and worsens stone formation. This is a common misconception that must be explicitly corrected.

Fluid intake is the single most impactful secondary prevention intervention — reducing recurrence in RCTs. After the acute episode, advise 2.5–3 L of water a day spread through the day (NICE NG118), adjusted for heart failure, renal function and any fluid restriction. The calcium restriction myth is clinically important to correct actively — patients often self-restrict dairy based on a logical but incorrect assumption, which worsens their stone disease. The dietary calcium binds oxalate in the gut, reducing intestinal oxalate absorption. Straining urine to collect stones for composition analysis is free, non-invasive, and provides actionable information that guides targeted prevention — it should be recommended to every stone patient.

Stone recurrence is common without prevention — follow-up to implement prevention measures is as important as acute management. Persistent macroscopic haematuria after stone passage should not be automatically attributed to the stone — urothelial malignancy must be excluded, particularly in older patients and smokers (NICE NG12 haematuria pathway). Long-term stone disease can cause progressive CKD — eGFR monitoring detects this early. The 48–72h safety-net call is particularly important because infected obstruction can develop after initial presentation appears benign — a patient who was afebrile at 9am can be septic by evening.